Introduction: 79cc Motorized Bicycle From Scratch

Hello again everyone! In this instructable I will be showing you how to build a 79cc, 4 stroke motorized bicycle from scratch (not a kit motor). The bicycle we will be building is far superior to kit motors in pretty much all areas, including speed (minimum 35 mph, and that's if you gear it for hills), gas mileage (100+ mpg), noise level (much quieter than a 2 stroke), pollution (meets EPA emission standards), and reliability. Let's get started!

PS: sorry for the crappy quality video, I'll upload a better one at some point once I figure out how to do editing and stuff.

Step 1: Part 0: Laws

Before you begin, it is a good idea to check the laws in your state or country, as motorized bicycles are not legal everywhere. Here are a few links to help you out:

Step 2: Part 1: Sources for Parts and Tools List

I have compiled a list of a few good sources from which to buy motorized bicycle parts.

Affordablegokarts has an entire page dedicated to aftermarket parts specifically for 79cc predator engines.

GTC makes a jackshaft which we use on our bike.

MFG makes sprockets, chains, and mini hubs, among other things.

Rebel Gears makes high quality custom motorcycle sprockets, allowing you to specify tooth count, chain size, center bore, mounting bolt pattern, color, and more.

Amazon sells motorized bicycle gas tanks, and probably lots of other useful stuff.

Custom Motored Bicycles sells a wide variety of parts for motorized bicycles, including a disc brake sprocket adapter.

Orchard Supply Hardware, Home Depot, (insert your local hardware store here)

These sell nuts, bolts, and steel shapes, as well as other useful stuff.

Step 3: Tools List

Here is a list of the bare minimum of tools you will need to have access to. Other tools may be required depending on your situation.

  • Drill
  • Band Saw (a grinder will work, but expect to go through at least a half dozen cutoff discs)
  • Grinder
  • Oxy-Acetylene Torch (only needed for bending exhaust pipe, have someone do it for you if you don't have one, or use a gas stove to heat it up)
  • Sockets
  • Spanners
  • Screwdrivers
  • Taps
  • Pliers
  • Wire Cutters
  • Utility Knife
  • Allen Keys

Step 4: Part 2: Choosing a Bike to Motorize

The first step (and one of the most important) in motorizing a bicycle is to choose a good bike to motorize. You can't just go slap a motor on any old bike and expect it to work, especially one as large and powerful as the motor we'll be using. A good bike to motorize has several key attributes, all of which must be fulfilled if the build is to be successful. I have outlined these attributes in the following steps.

Step 5: Mini Parts List

Please note that I will not be going into detail on the more advanced bicycle repair stuff, such as how to replace a front fork or install a disc brake. If you do not know how, leave it to the professionals at your local bike store.

Each mini parts list only covers one section of the instructable.


  • Steel Frame Bicycle
  • Disc Brake
  • Front Suspension Fork
  • Indestructible Wheels
  • Puncture Resistant Tires and Inserts
  • Grip Shifts
  • Brake Levers
  • Shift Cable
  • Brake Cable
  • Shift Cable Housing
  • Brake Cable Housing
  • Single Speed Conversion Kit


  • Allen Keys
  • Pliers
  • Wire Cutters
  • Grinder (optional, although you will need one later so might as well go buy one now if you don't have it already)
  • Lubricant (tri-flow, WD-40, etc.)

Step 6: The Frame

First, the bike must have a large enough frame gap to fit the engine we will be using, which is the 79cc harbor freight predator. This is a fairly large engine to be cramming into a bicycle, so as far as frame gaps go, the bigger the better. Remember that the pedals must still be able to turn after the engine is placed into the frame.

The frame must not be made of carbon fiber, as carbon fiber will crush and splinter if we try to mount the motor to it. Aluminum is not great, but it will do if you don't intend to ride it much. Be warned that if you do use an aluminum frame, the frame will eventually develop cracks and break from metal fatigue. Steel is great, by far the best.

Some specific frames that work well are the older models of the Trek 820 (main picture is of a Trek 820 frame with lots of aftermarket parts, this is one of the bikes I will be motorizing for this instructable) and the older steel frame models of the Specialized Stumpjumper and Hardrock. Look for a 21" frame (or larger if they exist).

Bear in mind that if you find a good frame with a bunch of junky parts on it, you can often swap out the other parts. (Picture #3 is of the red and black bike before I stripped it down to the frame and rebuilt everything)

Step 7: Good Brakes

One of the most important features of a motorized bicycle is good brakes. Because you will be going faster, you need more stopping power than an average bike. I'd recommend getting a disc brake for the front, however you can not have any sort of disc brake in the back, as the sprocket that drives the bike will be mounted to the disc brake mount. In order to compensate for having normal (although still good) bicycle brakes on your bike, we will be setting up the engine in such a way that letting off on the throttle makes the engine act as a secondary brake.

Pictured is the red and black bike, sporting a top-of-the-line disc brake with a 200mm rotor. The brake cost me just over $100 (not including installation, which I did myself).

Step 8: No Ram's Horns!

You will want to avoid using a bicycle with ram's horns or other handlebars with curved ends, as it will force you to use a thumb throttle instead of a twist throttle, the former of which makes your thumb get tired fairly quickly on long rides. If you have a road bike and don't want to spend the money on a new frame, however, there are workarounds. For example, I have a motorized road bike which has a thumb throttle and also has the rear shifter converted into a second throttle control that locks in place.

Step 9: Wide Rear Hub Clearance

The rear sprocket will be mounted to a disc brake mount on the rear wheel, so we need enough clearance that the chain does not rub against the frame (this will kill a bike frame very quickly).

Step 10: Crank Clearance

The cranks must be able to clear the engine and jackshaft once it is mounted in the bicycle. If you have a 21" frame, this probably won't be an issue. Don't worry about not being able to put your feet on the pedals with the engine in place, we can extend the pedals with pieces of metal if necessary.

Step 11: Nothing on Top Bar

It is helpful, although not absolutely necessary, to not have cables running along the top of the top bar, as they get in the way of mounting the gas tank. If your bike does have cables on the top bar (like the Trek 820), don't worry, we can always reroute the cables later.

Step 12: Sturdy Wheels

Because weight is not really an issue now that you won't have to propel the bike yourself, you should make sure to beef up the bike's wheels. They will be hitting bumps and rocks at much higher speeds than they normally would, and normal bike wheels won't hold up too well to prolonged riding. I recommend getting wheels with 12 gauge spokes (this will probably not happen unless you get really lucky or are willing to spend $400 on custom-built wheels), quad laced if you can find them, with as many spokes as you can get. Double-walled rims also help, but are not really necessary unless you plan to ride a lot.

UPDATE 1/8/2017: After riding with the wheels in the first picture, I found them to not be strong enough for prolonged use. Therefore, I recommend using mag wheels such as teny rims. I now use mag wheels for all my builds.

Step 13: Rear Disc Brake Hub

You will need a rear hub with a disc brake mount.

Step 14: Good Tires

Get some puncture-resistant tires. Also, try to choose thick tires with a lot of tread, as this will both reduce your risk of crashing and provide you with a more comfortable ride. Puncture-resistant inserts and tubes are also good.

Step 15: No Rear Suspension

Rear suspension makes motorizing a bicycle much more complicated. Stay away from this unless you have built these bikes already and want a challenge.

Step 16: Front Suspension Fork

If you want a smoother ride, get a good front suspension fork. Optional but recommended. Remember you'll be hitting bumps and potholes at 30-40 mph.

Step 17: Part 3: Preparing a Bicycle for Motorizing

So, you've chosen a bicycle, now you want to slap a motor on it, right? Hold on there, you still need to make several changes to the bicycle itself in order to prepare it for motorizing. Let's get started!

Step 18: Strip Everything Off the Handlebars

If your bike has anything other than grip shifts, you're going to want to take them off. If there are shifts built into the brakes, take off the brakes as well. If you have a grip shift on the left side, off it goes. Just take off everything!

The process of removing items from the handlebars should be pretty straightforward, just use hex keys of the correct sizes (usually metric) and loosen the bolts until you can slide everything off.

Protip: If you are having trouble getting the grips off, slide a flat head screwdriver under them and squirt some WD-40 in there, then remove the screwdriver and twist them in your hands and they should come off without too much trouble.

UPDATE 1/8/2017: Removed hints that I would someday make an instructable on building 2 speed transmissions, as I never actually got around to building a 79cc 2 speed bike and probably never will.

Step 19: Remove All Cables

Now you need to remove all the cables from the bike so that we can reroute them in such a way that they don't get in the way. The brake cables should be pretty self-explanatory, just loosen the brakes and take the cable out, then slide all the cable housing off and pull in the brake lever; you will be able to unhook the cable from inside pretty easily.

The shift cables would normally be harder, however we do not care about them so feel free to grab your wire cutters and destroy them while getting them off (unless you already have the required grip shifts, in which case save the cables).

Step 20: Modify Your Grip Shift

Now you need to modify your grip shift to remove the mechanism that locks the grip shift in place, as having a throttle that doesn't return automatically when you let go is a good way to kill yourself (although I will admit, one of my bikes has a secondary throttle which does exactly this, I use it as a cruise control for long rides).

Pick up your grip shift (the one that goes on the right handlebar) and twist the right half clockwise until the gear selector indicator is past the highest gear. Now pull the halves apart. Inside you should be able to see some complicated plastic shapes and the cable. You should also be able to see a little metal shape. This is the locking mechanism. Take the locking mechanism out and throw it away, then reassemble your grip shift in the same manner that you took it apart.

Step 21: Install New Brake Levers & Grip Shifts

Now you want to slide the brakes and grip shift back onto the handlebars as shown in the picture. Put the brake levers at whatever angle feels comfortable for you. Use a full length hand grip (without a grip shift) on the left side. Tighten everything to lock it in place. Again, you can use WD-40 to make it easier to slide the grips on.

Step 22: Re-Route the Rear Brake Cable and Re-Connect Brakes

Now you will want to re-install the rear brake cable. To do this, first cut a length of cable housing long enough to run from the right brake lever down along the bike frame to the rear brake. Now take the grinder and shave off any metal fragments or deformed crushed ends of the cable. Do this in quick pulses (don't hold the grinder against the cable for more than a half second at a time) to avoid melting the non-metal parts of the cable housing. Now squirt some lubricant into the cable housing. Next, cap off one end of the cable housing. Now insert the end of the brake cable back into the brake lever and thread it through the brake lever housing, then through the capped end of the cable housing until it pokes out the other side. Now re-install the cable back into whatever rear brake your bike has. Now repeat the process with the front brake. Test your brakes to ensure that they are adjusted properly.

Step 23: Swap Out the Wheels

If your bike has wheels that are anything less than bomb-proof, you will want to swap them out for sturdier ones. Even if your rear wheel is sturdy, if it does not have a disc brake hub, you will need to swap it out anyway.

A good wheel should have:

  • 12 gauge spokes
  • 36 spokes
  • Triple or Quad laced
  • Sturdy double-walled rim

12 gauge spokes and quad lacing might not be available unless you are willing to spend upwards of $200 a wheel to have them custom-built. If you can't find everything, minimum requirements are 36 spokes triple laced with a sturdy, double-walled rim.

UPDATE 1/8/2017: A better alternative is getting rid of spokes entirely and using mag wheels such as teny rims.

Things to avoid:

  • Racing Wheels (low spoke count, built for speed instead of durability, will fall apart if used for a motorized bicycle)
  • Radial spoke pattern (speaking from experience, these do not hold up very well when constantly hitting bumps at 45 mph)
  • Carbon Fiber Rims (again, they don't lend themselves well to motorizing)
  • Single-Walled Rims (too flimsy)

If you are short on cash, prioritize bulking up the rear wheel.

Remember that the rear wheel MUST have a disc brake hub.

Step 24: Remove Derailleurs

You heard me! Off they go! You won't need to be doing any gear shifting on these things. We will be converting the bike to a single speed bicycle. Removing the derailleurs is pretty self-explanatory; if you can't figure out how and aren't smart enough to Google it (hint hint), you shouldn't be trying to build one of my motorized bicycles in the first place, as you will surely fail!

It involves unscrewing a single bolt on the side of the derailleur with a hex key.

On that note, moving on...

Step 25: Convert Bicycle to Single-Speed

Follow SOME of the instructions in the single-speed conversion kit to convert your bicycle to single speed. Don't use the crappy sprocket and spacers included in the single speed conversion kit, they will make your chain skip. Instead, use your cassette and adjust the chain tensioner so it stays in one gear. I recommend gearing it for acceleration, as you will really only be using the pedals as an assist when you want to start moving.

Step 26: Ready to Motorize!

Now that that's out of the way, we can move on to actually putting a motor in your bike!

Step 27: Part 4: 79cc Predator Engine Modifications & Governor Removal

Now it's time to modify your engine so that it is both more powerful and fits inside your bike frame.

To those of you who have even less concern for your personal safety than the rest of you who are building this:

The 212cc predator engine is more than twice as powerful and will fit in some bike frames. Build with this engine at your own risk! (This engine will also greatly increase your chances of getting arrested because it is easy for a cop to tell that it's way too big for a legal motorized bicycle)

Step 28: Mini Parts List


  • 79cc Predator Engine
  • A few bolts and lock washers


  • Hammer
  • Taps
  • Drill
  • Socket Set (with deep sockets)
  • Rags
  • Screwdrivers
  • Impact Driver (optional)
  • Needle Nose Pliers

Step 29: Remove Exhaust Pipe, Cooling Tin, & Gas Tank

Pretty straightforward, you may need a deep socket to reach some of the nuts. I recommend keeping all nuts and bolts you take off the engine in neatly labeled plastic bags so you don't lose anything or use the wrong parts when you reassemble the engine. The cooling tin is that metal cover on the top of the engine.

Step 30: Remove Side Cover

There are 6 bolts holding the side cover on.

Be very careful when removing the side cover so as not to damage the gasket (although if you do, like I did in the second image, it's not the end of the world. and various people on ebay sell replacement gaskets).

Inside the engine should look like the third image.

There may be some oil left over from when the factory tested the engine, pour it out so it doesn't get everywhere.

Step 31: Remove Camshaft

It might have come out with the side cover, if not, remove it so it doesn't get in the way.

Step 32: Disconnect Throttle Rod & Springs

Remove all the rods and springs from the throttle assembly, we will be replacing them with our own.

Step 33: Remove Governor Arm

There is a metal arm that goes down into the engine block which we need to remove. To do this, get a nail and place the tip on the top of the arm, and hit it with a hammer until it goes through the hole. You may have to rotate the crankshaft to get it to go all the way through. If it still won't go all the way through, cut off the top of the arm with a grinder or something like that and then push the arm through. There is also sometimes a black washer on the inside of the engine block which you will need to remove.

Step 34: Plug Hole Left by Governor Arm

Now take a tap, 1/4" should do it (although yours may be slightly different), and thread the hole left by the governor arm. Put a short bolt, such as the one shown, into the hole. Make sure to use a lock washer. Be careful not to get metal shavings inside the engine block. I recommend putting a cloth inside the block to catch the shavings.

Step 35: Remove the Blower Housing

It has a sticker on it that says "79cc Predator" or some such, and is held on by several bolts. Removing it is pretty self-explanatory. You will also need to disconnect the kill switch to get the housing off.

Step 36: Remove the Flywheel

Under the blower housing you will see a metal cup with holes in it, a plastic fan, and the flywheel. They are all held on with a 17mm nut. Getting it off is tricky. I have found that the best way to remove the nut is with an impact driver, but if you don't have one, you can slide a long screwdriver through the holes in the cup to keep the flywheel from turning, and then use a regular socket wrench, although this is very difficult, you need a lot of strength (I'm an 18-year-old male in great shape and I still have a lot of trouble applying enough force to get it off). If you can't get it off, Harbor Freight sells a crappy impact driver for like $30 or something like that, which will get the job done. Once you've removed the nut, the cup and blower should slide right off. After that, you have to put the nut back on (not tightly, just enough to protect the threads from what we're going to do next. Now set the engine on the floor and put your foot on top of it (be careful not to crush the choke lever or something like that), and give the end of the shaft with the nut on it a good whack with a hammer to break the flywheel off it's taper. You may have to hit it a few times to get it off.

Step 37: Remove the Governor

Using the same method that we used to remove the governor arm, we will now remove the gear. Make sure to get the washer behind the gear as well. This will allow the engine to turn almost twice the RPM it would with the governor in place. Once again, you will need to plug the hole with a bolt.

Step 38: Put It All Back Together!

Reassemble the engine in the reverse order you took it apart, but leave off the gas tank, exhaust pipe, and cooling tin, and don't worry about throttle linkage yet.

Step 39: Optional: Gap Spark Plug & Set Valve Lash

If you want to go the extra mile and feel good about yourself, set the spark plug gap to .035" and the valve lash to between .004" and .006". You will need a spark plug gapper and a feeler gauge for this step. If you don't know how to set the valve lash, either look it up on the internet, or don't bother; I have enough explaining to do as it is!

Step 40: Part 5: Mounting the Engine

Now that we've modified the engine, we can finally mount it to our bicycle. Because various bicycle frames have slightly different angles and dimensions, there is no "one size fits all" mounting system. I have, however, come up with an easily customized mounting system which will fit most bicycle frames. But there is a catch... You actually have to think to complete this part. That's right folks, you can't just blindly follow step by step and come out with a workable product here. That being said, I'm confident that if you've gotten this far, you'll be able to figure it out.

I have used computer generated images in some places where I forgot to take pictures.

Step 41: Mini Parts List


  • Steel plates and shapes
  • 3/8" threaded rod
  • 3/8" nuts, bolts, and lock washers
  • Various bolts, nuts, and lock washers


  • Powerful Drill
  • 1/2", 3/8", and 1/4" cobalt drill bits (you will need at least 3 of the 3/8", as they will wear out and become dull and we are drilling a lot of holes)
  • Sockets
  • Spanners
  • Clamps (C-clamp or quick release)
  • Grinder with both grinding and cutoff discs
  • Band saw with metal cutting blade

Step 42: Construct Engine Mounting Plate

The first step is to construct a mounting plate for your engine. If you turn the engine upside-down, you will see 4 threaded holes in the bottom which you can use to attach the engine to a mounting plate. The engine comes with it's own mounting interface, but it doesn't work for our purposes. First, you'll need a 1/4" steel plate. You need to cut a rectangular piece of steel plate that is 6 1/2" long and 4 1/2" wide. Use a grinder, band saw, acetylene cutting torch, or whatever else you have available that can cut steel. Then use the mounting plate that came with the engine as a template to drill 4 3/8" holes in your plate which will line up with the mounting holes in the engine. You will need a powerful drill or drill press and a cobalt drill bit. Now, measure and draw a center line on your steel plate as if you were going to fold it in half, hotdog style. On either side of the centerline, about 5/16" from center, mark spots for 3 1/4" holes (6 holes total), evenly spaced, and offset from each other (look at the second image if that doesn't make sense).

Step 43: Attach Core Piece to Plate

Next we need to cut an 8" piece of 1" square steel tubing. Bevel one end at 45 degrees. Next, use your mounting plate as a template to copy the 6 1/4" holes onto the square tubing. Drill the holes through both the top and bottom of the tubing. Make the edge of the mounting plate line up with the 45 degree angled end of the square tubing. (the other end should stick out past the edge of the mounting plate a bit). Now, in the sides of the core piece, between where you drilled the holes through the top and bottom, drill 4 1/2" holes through both sides. Drill one at each end of the core piece, and two right next to each other in the middle. (if you're confused by the fact that the picture has 6 holes in them, I drilled some in the wrong places and had to drill extra ones, but I have highlighted the holes you're supposed to drill with notes in the second image). You may find that you have to drill holes in slightly different places as well.

After you have drilled all your holes, use 6 1/4" bolts, nuts, and lock washers to attach the mounting plate to the core piece. Remember to use threadlocker! If you want to paint your metal pieces (highly recommended to prevent rust) you will need to paint them before you bolt them together.

Step 44: Attach Plate to Engine

Now it's time to attach your mounting plate to your engine. To do this, you first need to go to the hardware store and purchase longer mounting bolts than the ones that came with your engine. I recommend bringing the engine block and mounting plate to the store with you so you can make sure you get the correct bolts. You will also need several washers to use as spacers between the plate and the engine. This is so that the bolts between the plate and the core don't touch the bottom of the engine, which would make it impossible for the engine to lay flat. I used 3 3/8" washers for each bolt. Don't forget lock washers and threadlocker!

Step 45: Cut Threaded Rod

Before you construct the plates that will mount the engine to your bike, you need a way to hold them in place. To do this, I use sections of 3/8" threaded rod. You will need about a dozen 3 1/2" sections. I recommend not using a grinder to cut the rod, as it can mess up the thread (although if it's all you have, it will do, just be ready to do some cleanup with your grinder afterwards to smooth out the cut edge). Instead, use a bandsaw, sawzall, or jigsaw with a fine-toothed metal cutting blade.

Protip from M_a_t_t: Put a nut on the threaded rod before you cut it, and when you remove the nut afterward, it will straighten the threads out.

Step 46: Cut Mounting Clamp Pieces and Mount Engine

Unfortunately, there is no "one size fits all" way to cut the mounting clamp pieces. However, the general idea remains the same for all bicycles. You have one clamp going horizontally back, one clamp going horizontally forward, and one clamp going vertically down. Look at the first two images, each shows the same general mounting configuration. The third image shows the various pieces you will need to cut. There are 3 sets of pieces, each consisting of 2 exact duplicates of each other. Note that only the position of the holes needs to be perfect, if one piece is 1/4" longer than another, it won't matter as long as the holes line up. To ensure proper hole alignment, drill the holes in the first piece, then clamp the two together and use the first as a template for the second.

To figure out the correct size for your mounting clamps, set the engine inside the bicycle frame and measure how long they will need to be. You will want some wiggle room to slide the engine around, so don't drill your hole so that the threaded rod will be touching the frame, give it a bit of clearance.

Once you have cut all the mounting pieces, use the threaded rod (and nuts, threadlocker, and lock washers) to attach the engine to the frame using the pieces you just cut. Don't tighten the mounting clamps very much, as we will need to adjust the position of the engine later. Use the images as a reference if you need help figuring out where everything goes.

Step 47: Part 6: Throttle Linkage

Now that you've mounted the engine to your bike, it's time to hook up the throttle.

Step 48: Mini Parts List


  • Coat Hanger
  • Springs
  • Various nuts and bolts
  • Small wire nut
  • Shift Cable Housing


  • Pliers
  • Wire cutters
  • Spanners
  • Drill
  • Drill bits

Step 49: Cut and Install New Throttle Rod

You will need to cut a new throttle rod, as the old one won't work anymore. I find that a metal coat hanger makes a good throttle rod. To figure out the length, push the throttle all the way forward, do the same to the place it connects to on the carburetor, and measure the distance between the two. Then cut a piece of coat hanger that's about 1 1/2" longer than that, and bend the ends to fit into the holes in such a way that they won't come out (see images). You will need to slightly enlarge both holes to accommodate the coat hanger. You will also need to fiddle with the throttle rod a bit to get the length exactly right.

Step 50: Install Throttle Cable

The first step in installing the throttle cable is to run the shift cable from the right grip shift down to the hole in the engine that I have highlighted in the first image. You will also need to install shift cable housing around the cable and a spring around the end of the cable nearest the engine to protect it from being melted by the engine.

After you have fed the cable through the hole, you will need a way to attach it to the throttle arm. To do this, drill a hole in the throttle arm about halfway out along the arm and install a small nut and bolt in the hole. Use the nut and bolt to clamp the throttle cable to the throttle arm (images 2 and 3). Make sure that the throttle is in the "idle" position (throttle arm pushed toward front of bicycle) when you secure the cable. I recommend using something to cap off the end of the cable to prevent fraying.

Step 51: Install Spring

Now install a small spring as shown in the image. This will make the throttle return when you let go. You may also need to slightly loosen the bolt highlighted in the image in order to get the throttle to return.

Step 52: Part 7: Aftermarket Air Filters & Exhaust Systems

Now it's time to install performance air filters and exhaust pipes to increase engine power.

Step 53: Air Filters

There is nothing wrong with the air filter that comes with the engine, but it's not great either. A better option is the cylindrical or cone shaped air filters found on some lawn mowers, coupled with a breather filter for the rubber hose that sticks out of the top of the engine and normally goes into the stock filter. At the very least, replace the pad inside the stock filter with a better quality one.

Step 54: Exhaust Pipes

Not only is the stock exhaust pipe for these engines positioned in such a way that it won't fit in the bike frame, but it's also absolute garbage. Better exhaust pipes can be found online at many go-kart supply websites. Just search "exhaust system predator engine" and you should be able to find something. sells a short and long exhaust pipe for the predator engine, both work well. The short one provides slightly more power but is rather loud, the long one provides slightly less power, but is nice and quiet. I recommend the long one. Note that if you buy the long pipe, you will not be able to use the stock air filter.

You may need to bend your exhaust pipe to get it to fit into your frame. To do this, heat up the section you need to bend with an acetylene torch and use vise grips to bend it while it is glowing orange-red. Repeat until it fits properly. I had to bend my long pipe in three places.

You may also need to fashion some sort of guard out of sheet metal to prevent your leg from touching the pipe and getting burned.

UPDATE 3/23/2017: Affordablegokarts no longer carries complete pre-made exhaust pipes for the 79cc predator engine. I will be adding a section on how to make your own pipe shortly.

Step 55: Part 8: Gear Ratios

In order for your engine to output enough torque to power your bicycle, we will need significant gear reduction. We will be using a jackshaft for 2-stage gear reduction in order to avoid having to use gigantic sprockets. For a bicycle with 26" tires, I suggest gear ratios anywhere from 1:11.5 (speed) to 1:15 (power). To give you a baseline, my current favorite bike is geared at 1:13 and has enough torque to go up pretty much any hill without any pedal assist, and can take off from a standing start, but can still reach speeds of 36 mph on level ground. If you gear for speed, you can hit over 40, although with an 1:11.5 ratio, you will need quite a bit of pedal assist to make it up the hills. In my opinion, 1:13 is a really good balance between speed and acceleration.

Now, on to how to calculate your gear ratios:

For a single stage ratio, simply count the number of teeth on your drive gear, and the number of teeth on your driven gear, and write them as a ratio (x:y). For example, if we have an 11t sprocket driving a 36t sprocket, our ratio is 11:36, or about 1:3.27.

Now, we are using a 2-stage gear reduction, so things get a bit more complicated. For a 2-stage reduction, the overall gear ratio is the product of the individual stages. So if you have a gear ratio of (a:b) for the first stage, and (x:y) for the second stage, your overall ratio would be (ax:by). For example, let's say we have the 11t driving the 36t from before, but then the second stage is a 9t driving a 36t. First we multiply 11 x 9 = 99 and 36 x 36 = 1,296. Then you write them as a ratio 99:1,296. Then, we simplify that ratio by dividing bot sides by 99 (divide the left side by itself, and the right side by the left side). So 99 / 99 = 1, and 1296 / 99 = 13.09. So the final ratio is 1:13.09. In case that's too complicated for you, here's a link to a Gear Ratio Calculator.

Keep in mind when ordering sprockets, the first stage uses #35 chain, and the second stage uses #41 chain.

The first sprocket will probably be an 11t, because as far as I can tell, that's the only tooth count that you can get for a max torque clutch with a 5/8" bore for a #35 chain.

If you want a 1:13 ratio, use 11t (max torque clutch, 5/8" bore, #35 chain) - 36t (first stage driven sprocket, #35 chain) - 9t (second stage drive sprocket, 5/8" bore, #41 chain) - 36t (second stage driven sprocket, attaches to rear wheel, #41 chain).

Step 56: Part 9: Installing Rear Sprocket

Now we need to install the sprocket onto our rear wheel. We are going to use a sprocket adapter to bolt the sprocket directly to the disc brake mount. This ensures perfect sprocket alignment and will not rip the spokes out of a good wheel (which is what the spoke interface sprocket mount in most kit motors does).

Step 57: Mini Parts List


  • Rear sprocket, 36t, #41 chain
  • Top Hat sprocket adapter or similar
  • Various nuts, bolts, washers, and lock washers


  • Sockets
  • Spanners
  • Grinder

Step 58: Mount and Test Fit Sprocket

To install your rear sprocket, first inspect the mounting hardware that came with the sprocket and adapter. If there is no mounting hardware included, or if the hardware is of questionable quality, go to your local hardware store and purchase new mounting bolts for both the sprocket to adapter interface and the adapter to hub interface. Bring the sprocket, adapter, and your rear wheel to ensure that you buy the correct nuts, lock washers, and bolts. Use lock washers and threadlocker on everything, but be careful not to over torque the small bolts that go into the hub, as breaking them off is a disaster.

Once you have mounted the sprocket to the rear wheel, test fit the wheel in your bicycle. You may find that the heads of some bolts hit the frame in some places (usually right next to the axle), if this is the case, use a grinder to grind down the heads until they can clear the frame.

Step 59: Part 10: Jackshaft Assembly

Theoretically, with the right bicycle you could just put a 150t sprocket on your rear wheel and bypass the jackshaft entirely, but for practical purposes a 150t sprocket isn't really feasible, so we are going to use a jackshaft for 2-stage gear reduction.

Step 60: Mini Parts List


  • GTC Jackshaft
  • 11t, 5/8" bore, #35 chain max torque clutch
  • 36t aluminum sprocket, #35 chain
  • Mini hub, 5/8" bore
  • 9t, 5/8" bore engine sprocket, #41 chain
  • Spacers for 5/8" bore shaft
  • Set screws (may be included when you order other parts)
  • Various nuts, bolts, washers, lock washers
  • 3/16" square shaft keys

Note: Tooth counts on sprockets are merely suggested values, and may vary depending on personal preference.


  • Sockets
  • Spanners
  • Hammer
  • Allen Keys

Step 61: Install GTC Jackshaft Plate

Installing the jackshaft plate itself is pretty straightforward. Simply use the included mounting hardware to bolt the jackshaft plate to the side of the engine.

Step 62: Install Max Torque Clutch

Now slide the clutch onto the engine shaft. You will need to line up the slots on the clutch and engine shaft and insert a 3/16" square key into the slot. A key may have been included with your clutch. If not, use one about 3/4" long. Do not tighten the setscrews or add the bolt and washer on the end of the shaft yet, as we will need to adjust the alignment of the chain.

Step 63: Bolt Mini Hub to Sprocket and Install Mini Hub

Now bolt your 36t sprocket to your 5/8" bore mini hub. Use threadlocker and lock nuts or lock washers. Then slide the assembled mini hub and sprocket onto the shaft. Now take a 5/8" bore spacer and cut it using a grinder or bandsaw so that it is about 1/2" long, and slide it onto the shaft between the mini hub and the jackshaft plate (you will need to take the mini hub off to do this). Slide the mini hub back on, insert another 3/16" key, and slide a 5/8" lock collar onto the end of the shaft. Tighten down all the set screws on the mini hub and lock collar (and use threadlocker).

Step 64: Install Second Stage Sprocket

Slide a spacer onto the shaft on the right side of the jackshaft plate behind the engine block. The spacer should be about 3/8" long. Take the 9t sprocket for #41 chain and slide it onto the shaft, with the sprocket end facing the left side of the bike (see image if that's confusing). Insert another 3/16" key. Then slide another lock collar onto the end of the shaft. Tighten (and threadlock) all set screws.

Step 65: Part 11: Installing Chains

Now to install the chains that will drive your bicycle.

Step 66: Mini Parts List


  • #35 chain
  • #41 chain
  • #35 chain master link
  • #41 chain master link


  • Needle Nose Pliers
  • Chain Breaker

Step 67: Install #35 Chain

Measure the length of #35 chain that you need by threading the chain around the clutch and sprocket. Once you have found the correct length, use a chain breaker to break off the correct length. Use a #35 master link to connect the two ends of chain. Use a pair of needle nose pliers to remove the clip on the master link and to aid in snapping it back on. The closed end of the clip must face the direction of chain travel.

If there is too much slack in the chain, you will need to loosen the mounting bolts on the jackshaft plate and slide the plate from side to side to take up the slack in the chain.

Next, adjust the clutch from side to side so that the chain is aligned properly and not touching the jackshaft plate. Then tighten all setscrews on the clutch and install a bolt and washer in the end of the engine shaft to prevent the shaft key from falling out in the event that a set screw becomes loose.

Step 68: Install #41 Chain

Now measure and break the #41 chain. Use a #41 master link, again facing out, to connect the ends of the chain. You may need to adjust the size of the spacer in order to get the chain alignment right. If it's off by a lot, you'll know immediately when you start riding as it will make a bunch of harsh noises and vibrate your bike and then derail. If it's off by a little, it will wear down your sprockets faster.

Remember way back when we were mounting the engine, how I said not to tighten the bolts on the mounting hardware very much? The reason for leaving them rather loose is that we have to loosen them anyway to adjust the chain tension. Loosen the mounting bolts enough that you can slide the engine around in the frame. Now slide the engine up and forward until any slack has been removed from the chain you just installed. Now hold the engine in place (ask a friend to help you or use clamps) as you tighten down all the mounting hardware again. This time tighten it down all the way, but be careful not to tighten it so much that it crushes the frame of your bike.

You will need to repeat the process of taking the slack out of your chain a few times as the chain stretches out a bit when you begin to ride.

Step 69: Part 12: Installing Gas Tank

Almost done! We just need to install the gas tank and we'll be ready to ride!

Step 70: Mini Parts List


  • Motorized Bicycle gas tank
  • Sheet Metal
  • Brass Pipe Fittings
  • Various Nuts and Lock Washers
  • Teflon Tape
  • 1/4" ID Fuel Line
  • In-Line Fuel Filter
  • Old Bike Inner Tube


  • Pliers
  • Grinder or Bandsaw
  • Spanners
  • Utility Knife
  • Drill

Step 71: Install Pipe Fittings

Take your gas tank to the hardware store and figure out what brass pipe fittings you need to install on the end of the tank. You need to get from whatever thread size they put on the tank, converted to a 1/4" barbed brass nipple. How you do it doesn't matter, just try to use as few pieces as possible. There are several different gas tanks floating around, so I can't provide a "one size fits all" solution.

Use teflon tape when you screw the pipe fittings together.

Step 72: Mount Gas Tank to Top Bar

If the mounting hardware that was included with your gas tank fits your bike frame, use it. This is extremely unlikely, so you will most likely have to make your own out of sheet metal. Cut, bend, and drill strips of sheet metal into the same shape as the included mounting hardware, just make them big enough to fit your bike frame. Then mount the tank to your top bar as shown in the image. Use a piece of bike inner tube between the tank and the top bar to prevent the tank from sliding around. As lock washers don't work very well on curved surfaces, double bolt instead.

Step 73: Install Fuel Line and Fuel Filter

Now use two pieces of 1/4" ID fuel line to connect the barbed brass nipple to the fuel filter, and the fuel filter to the barbed nipple on the carburetor.

Step 74: Part 13: Starting and Riding

We're almost ready to ride!

Step 75: Fill Engine With Oil

Pretty straightforward, fill the engine with 4 stroke small engine oil. If you put too much oil in, it will leak off the excess oil. Too little, and your engine will be destroyed. Better to put too much in if you're not sure!

Step 76: Fill the Tank With Gas

Just fill the tank with normal gas, regular is fine, although don't use high ethanol gas. Pure gas is best, but it's really hard to find these days. You do not need to mix oil with the gas. Don't fill the tank quite all the way to the top or gas may spill out when you hit bumps at high speeds.

Step 77: Ladies and Gentlemen, Start Your Engines!

Push the choke lever to the "on" position, turn the kill switch to the "on" position, and pull on the recoil starter until the engine starts. Let it run for 30 seconds or so, then flip the choke lever to the "off" position. You're ready to ride!

Take it slow at first. Don't immediately floor the accelerator, twist it slowly and use a bit of pedal assist to get going. I would tell you to wear a good helmet, but those of you who were going to wear one didn't need me to tell you that, and those of you who weren't, probably still won't!

On the subject of helmets, normal bike helmets won't cut it. Get a good helmet. A full-face helmet is great, but if you don't want to draw attention to yourself, I recommend the S1 helmet. It is rated for multiple high speed impacts and you can get them with a face shield if you want, which is really nice. Also they only cost $55. (No, I am not sponsored by S1.)

Remember that you can use the engine to assist in slowing down simply by letting off on the throttle.

Step 78: Finished! Well, Sort Of...

Have fun with your new motorized bicycle!

Next part is for troubleshooting, maintenance, and repairs.

Step 79: General Maintenance

In order to keep your motorbike running smoothly and avoid dying a gruesome and horrible death, it is important to maintain your bike.

The following should be done every 6 hours or so of riding.

  • Oil clutch (more specific instructions are included with the clutch)

The following should be done every few weeks (more if you ride a lot), and every few days for the first 100 miles or so.

  • Check for loose nuts, bolts, and set screws
  • Check engine oil level
  • Check chain tension
  • Lubricate chain

In addition, the following should be done every few months (again, if this is going to be your transportation and not just something fun to do on the weekend, check stuff more often).

  • Check gears for signs of wear
  • Check bike frame for signs of metal fatigue (steel bike frames don't have this issue, aluminum frames are the ones to worry about)
  • Lubricate cables

Step 80: Part 14: Repairs

The following section outlines some common problems you may encounter with your motorized bicycle and how to fix them.

Causes are listed in order of most likely to least likely, so start at the top of the list and work your way down.

Step 81: Chain Derails

Cause #1: Incorrect Chain Tension

Solution: Loosen mounting clamps and slide engine up and forward to take up slack in chain, then tighten mounting clamps back down to secure engine.

Cause #2: Poor Sprocket Alignment

Solution: Sight along chain and make sure sprockets are in line with each other. If not, loosen set screws on lock collar and second stage drive sprocket, then remove spacer. Cut a new spacer of the correct size to properly align sprockets and reinstall drive sprocket and lock collar.

Cause #3: Chain is Worn Out

Solution: Replace chain.

Cause #4: Sprocket(s) are Worn Out/Broken

Solution: Replace sprocket(s). Common signs of wear include missing/damaged teeth and extremely pointy teeth, as well as obvious wear marks where chain contacts sprocket.

Step 82: Clutch Will Not Engage Fully

Cause #1: Incorrect Gear Ratio

Solution: If you've geared your bike for speed, you've geared it too steeply. You need to use a lower gear ratio to allow the clutch to engage. Swap out either the first or second stage driven sprocket with a larger one.

Cause #2: Damaged/Worn Out Clutch

Solution: Replace Clutch.

Step 83: Oil Leak

Cause #1: Too Much Oil

Solution: Wait for oil to stop leaking, you put too much in and once the extra has leaked out you'll be fine.

Cause #2: Engine Tilted Too Far Back

Solution: Loosen mounting hardware and tilt engine forward. You may need to adjust the length of your chain.

Cause #3: Oil Plug is Loose

Solution: Tighten oil plug. (it's that bolt at the bottom of your engine block on the front side)

Step 84: Fuel Leak

Cause #1: Hole in Fuel Line

Solution: This happened to me once because the fuel line was touching the engine, and the vibrations from the engine eventually wore a hole in it. Replace the fuel line and adjust the position of the gas tank so that the fuel line doesn't touch the engine.

Cause #2: Cracked Gas Tank

Solution: Sometimes the gas tank will crack around where the mounting bolts are. Replace the tank, and don't over-tighten it.

Cause #3: Brass Fittings Leak

Sometimes the mishmash of brass pipe fittings that you used for your gas tank don't work so well, and they leak. Use thread sealant that is designed for use on fuel lines to seal threads. If that doesn't work, it may be possible to use a tap and die set to cut new threads into your gas tank and eliminate the need for five different pipe fittings.

Step 85: Extreme Vibrations

Cause #1: Loose Mounting Hardware

Solution: Check and tighten mounting bolts. Be sure to check the bolts that connect the engine to the plate, the plate to the core, the core to the clamps, and the clamps to the bike.

Step 86: Engine Takes 10+ Pulls to Start

Cause #1: Not Enough Gas

Solution: Try giving it some gas by twisting the throttle while you pull. Just be ready to let go the moment the bike starts so it doesn't shoot forward and kill someone. If you stand on the left side of the bike, your left hand can twist the throttle, your right hand can pull the starter, and your right foot can be planted in front of the rear wheel as an extra precaution.

Cause #2: It's Really Cold Outside

Solution: Same as #1. Just keep trying, it will start eventually. You could also store your bike in the garage next time so that it isn't quite as cold.

Step 87: Lack of Power

Cause #1: Choke is Still On.

Solution: Turn choke off. (duh)

Cause #2: Incorrect Gear Ratio

Solution: Swap out either driven sprocket for a larger one.

Step 88: Insert Your Problem Here

Leave a comment with your issue, and I'll try to get back to you soon. I can offer advice on both building and repairs. I will add more problems and solutions as they arise.

Step 89: Finished!

That's all, folks! Again, have fun and be safe!

If this instructable were to win the full spectrum laser contest, I would use the laser cutter and 3D printer to design and prototype custom parts for my bikes, starting with a 2 speed transmission.

Full Spectrum Laser Contest 2016

Second Prize in the
Full Spectrum Laser Contest 2016