Introduction: Building a Bike: Custom Honda Hobbit Moped

Picture of Building a Bike: Custom Honda Hobbit Moped

I bought a 1981 Honda Hobbit a while back, a fun little relic from an oil crises before my time. I expected to ride it for a year or two, save a bunch of money on gas, and make it a lil faster. I quickly fell down the rabbit hole, spent the gas savings on parts, and made it a lot faster.

This is a rough list of parts and tools that could be applied to most two stroke bikes.

Tools:

  • Metric socket and wrench set (If your'e still rocking standard your'e doing it wrong)
  • Welder
  • Sand blasting equipment
  • powder coating oven and sprayer
  • Angle grinder
  • Wiring crimper/cutter
  • Dremel w/ appropriate bits
  • Honda Hobbit clutch puller (If you plan on upgrading the clutch springs or changing the crank)
  • Random hand tools: Hammer, screw driver, etc

Materials:

  • A moped, I chose a Honda Hobbit as my base
  • Engine, If you want speed your'e gonna want a kit, carb, and, a pipe. I recommend spending sometime on http://www.mopedarmy.com to learn about different engine set ups. My Cylinder, head, piston, crank, seals, and bearings are all new. The only stock part on the engine is the crankcases.
  • Powder of your choosing. There are lots of places out there to order powder from, I got Cortez Teal and Racing Raspberry since it sounded fast.
  • One of the really fun parts about building custom bikes is finding parts
    • The seat is from a Batavus top tank moped that was trashed. .
    • The tank is from an old 70's Kawasaki 100cc dirt bike
    • Handlebars came from my buddy Steve for $5
    • Fenders? My buddy Simon gave em to me, not sure what there from but welded em to fit
    • Headlight, shocks, grill, and a few other parts came from the wonderful kingdom of https://www.treatland.tv
    • Mild steel tubing for welding reinforcement bar

Step 1: Planning Out the Build

Picture of Planning Out the Build

I love the stock styling of the Honda Hobbits but the easy to weld tubular frame was giving me ideas. I saw a new bike hidden in the aged machine and had to go for it. I got it to the ugly orange and red stage for a summer and rode it like that until I rebuilt it over winter.

Frame: I wanted something that felt a little bigger and gave me better access to the carb so I chopped and flipped the original frame twice. This gave it a slightly taller stance but it feels extra stable at high speeds, especially with the extra cross bar welded in. I was doing this welding with a stick welder since it was all I had at the time, that was quite the challenge!

Engine: The bike went 25 MPH when I got it but now it can do 60 plus. In order to get there I had to throw an Athena kit, race crank, 21 PHBG carb, and a custom fabricated expansion chamber. It's very fast for being 74cc and is so small and light its easy to maneuver.

Wheels: I would have liked to have mags for my bike but they are too expensive, plus spoke you can re-true! I definitely wasn't going to go with stock tires for this build. I decided on some Michelin Gazelles since there really beefy. I wanted that extra weight on my wheels so they acted as bigger gyroscopes, keeping me more stable at high speed.

Tank: Had no idea what I was going to do for a tank since the stock hobbit one wasn't looking good with the frame. I eventually just typed "tank" into craigslist and the first one I saw was perfect, $30 is a steal for a rust free tank!

Seat: I got a nasty batavus top tank seat for free. I covered it in electrical tape and ran it for awhile. Eventually I cut out a fabric template and my buddy Craig sewed it out of some marine vinyl that cost $20ish a square yard. I got super lucky on this one since the seat and tank match up perfectly. Looks like they were made for each other.

Step 2: Powder Coating

Picture of Powder Coating

Powder coating a bike takes several steps but the end results are worth it. The finish is more durable than paint and its resistant to gas and other fluids bikes are likely to come into contact with. Michigan's winters are cold and long which gives you plenty of time to make it pretty, plus the over heats the shop space!

  • First the part must be sandblasted
  • Plug any holes you don't want powder to get into with silicone plug. Certain areas should be taped off since they are so big. Fiberglass tape works best but blue painters tape will work in a pinch
  • Hang the part from a strong wire, this will support the part and give us a way to move it without touching the powder
  • Spray the part down with prep-all/rubbing alcohol/acetone then use compressed air to blow it off
  • Spray an even amount of powder onto the part, make sure the current is going from the powder gun to the part. For some small parts it may work better to have the current disconnected and preheat the part
  • Once the oven is preheated (usually 400 degrees Fahrenheit but I run 25-50 degrees hotter if I will be moving lots of parts in and out of the oven), place the part in.
  • Once the powder begins to flow allow it to stay in the oven for the suggested cure time, normally 10 minutes but check your powder each time
  • After the item has cured take it out and allow it to cool in a safe place
  • Enjoy your really pretty parts, you earned it

Step 3: Modifying the Subframe

Picture of Modifying the Subframe

The stock Honda Hobbit subframe like to flex on turns/high speeds/under a heavy load. To beaf if up you can fabricate you're own or just reinforce the existing welds. You can use what ever welding you have access to as long as you are competent with it, for the subframe I used a MIG welder that was at the Grand Rapids Ghost Riders shop.

What I did was

  • Cut down the top of the air box for more clearance for the carb
  • Added one rectangular steel plate to the top and welded it in place
  • Extended the existing welds of the original air box as far as I could
  • This took the subframe down from 3/4 of an inch of flex to less than a 1/4 of an inch (With me pushing the frame in opposite directions as hard as I could)
  • Powder coat to prevent rust

Step 4: Custom Two Stroke Tuned Exhaust Pipe AKA Expansion Chamber

Picture of Custom Two Stroke Tuned Exhaust Pipe AKA Expansion Chamber

There is a lot of math involved in making two stroke pipe, Jennings two stroke tuning book is a good guide. If you know what pipe you want and have access to one you can clone it without as much calculation. You will want access to a Tig welder if you are going make your own expansion chamber, I took my existing stick welder, got a gas bottle and modified it to be a Tig welder with this kit. For on the fly amp control I hooked up a bungee, a string, and a drum pedal to the existing control knob. Cheap but effective.

  • First acquire a pipe that performs how you want it, I chose the H95 Hobbit pipe
  • Take the pipe and one at a time wrap each cone in foil, rubbing the foil over the welds to get an indentation as to where they are.
  • Cut the foil out along the indentations from the rubbing. Take this foil pattern and transfer it to paper
  • Take the paper pattern and trace it onto some mild steel, I used 18 gauge. I got it from a local scrap yard, 6 bucks for a 2x3 sheet and I only used 1/3 of it
  • Cut out along the lines, this takes precision. The better the cuts the less time you will spend welding
  • Once the metal is cut its time to roll it into cones. I hammered the cones around various diameter metal pipes until I got the shape I wanted. This part takes lots of trial but is worth spending the time to get the pipe perfectly round.
  • Take the rolled cones and tack weld the edges together, then weld the seams
  • Now that the cones are welded tack them together. Metal expands when heated so tack around the pipe so the fit up does't change when your'e welding the seems
  • To seal the end of the last cone you can weld it shut or weld a fitting with a thread on it. The threaded fitting allows you to put various bolts on with different sized holes in them. The different sized holes allow you to control the pressure in the pipe which affects its performance, yay tuning!
  • To drill out the bolts I held them with pliers in a drill press. The washer in the picture keeps it from falling through the hole in the drill press plate
  • The header piece was tricky. Each case will be different but I used an old socket which fit perfectly over the inner pipe and inside the hole on the cylinder
  • I welded on a baffle from a dented expansion chamber, its located in the middle of the widest part of the pipe to help bleed off some pressure
  • Mounting the pipe is always a challenge. I did multiple test fits before welding on the brackets and header tube. Check twice before welding, grinding it out and re-welding is no fun
  • Once I tested the pipe for a week and found no leaks I sand blasted and sprayed it with high temp oven paint

Step 5: Notching the Variator

Picture of Notching the Variator

Honda Hobbits (And a lot of Scooters) have a CVT belt transmission that controls the gear ratio and with some modifications you can get free speed! As the bike revs up the roller weights in the variator move outwards, they ride on the ramp plate and force the conical surface the belt rides on to move inward. The further inward this conical surface moves the higher the belt will ride and the greater gear ratio you will get. This mod is effective on kitted and non kitted bikes but caution need to be taken as to what model the bike is. The ramp plate on the Honda Hobbit PA50-2 rides up higher than the Honda Hobbit PA50-1, this allows you to notch the variator further. A variator notched to work with a PA50-2 plate will not work with a PA50-1 ramp plate. Since the 1 plate does not ride up as far as the 2 plate the weights can shoot out through the notches like bullets... It's okay though, your'e not in a christmas story, you won't shoot yer eye out, and this is a good idea.

Here's an existing tutorial I found on notching variators. It really only take 30-45 minutes and give you a few extra mph. Well worth it in my opinion.

http://outofcontrolmopeds.blogspot.com/2010/06/honda-hobbit-variator-notching.html

Step 6: Tuning the Transmission

Picture of Tuning the Transmission

The notched variation gives you more top end speed by improving your gear ratio but if the weights are too heavy it can shift early causing a boggy response. Think of pedaling a road bike in the highest gear from the start, its going to be very hard on your legs until you get going fairly fast. To keep it in a lower gear ratio for longer and improve your take off lighter weights and stiffer springs help a lot. The PA50-1 has the 8 gram weights which are light, the PA50-2 has heavier weights which are less desirable but can be drilled out to about 8.5 grams easily. Since the PA50-2 has the better ramp plate your better off tracking one of them down and drilling out the weights but if your'e reading this its probably too late.

  • Stiffer springs cause you to rev up higher before your clutches engage this helps you to skip some low end bog
    • The easiest way I have found to install springs is with a vice and a old piece of steel cable (see images)
    • You can use a pair of vice grips but its a little tricky
  • Ideally you can tune your springs and weights so that you launch directly into your pipes power ban

Step 7: Pull Start and Starter Clutch Mod

Picture of Pull Start and Starter Clutch Mod

The starter clutches on the Honda Hobbit are necessary for starting you bike but the are annoying since they will always be engaged and cause you to engine brake when letting up on the throttle. One way around this to make a pull start and remove the starter clutches like these rad dudes did over here http://blackpipes.tumblr.com/post/5291429387/a-cheap-effective-pull-start-for-hobbits

The only problem with the pull start is that it will wear out quickly and need to be maintained. My solution is leaving the starter clutches in but putting set screws in the bell to block them from engaging. Then when your pull start breaks you can remove the setscrews and bump start your bike.

  • First make your pull start
  • Use a center punch to mark your holes for the set screws. This is tough as there is only a small sweet zone
  • Get a tap set and drill and tap you holes
  • Insert set screw and check the fit
  • Enjoy having a pull start hobbit with the added reliability of being able to bumpstart

Step 8: Enjoy Your New Ride!

Picture of Enjoy Your New Ride!

Well you took it all apart now its time to put it back together and ride, hopefully your memory is good! Do some plug chops and make sure you tune in your new engine. Too rich and you lose performance, too lean and you'll ruin your engine. My bike seem to like a 98 main jet but every bike is different.

So far I've taken it to one moped rally and have plans for more this summer. I won the best build trophy at the rally I went to and was honored since it was my first ground up build! I'm extremely satisfied with how the build turned out and take the scenic route as much as possible to enjoy the nice weather. The bike is more new parts than old. What remains original are the rims, forks, controls/grips, brakes, cranks cases, rear transmission, and a trimmed down electrical system. I think thats pretty much it.

Thanks for reading! Post pics of any projects you have that was aided by the information in this 'ible!

I am entering this in several instructable contests, if you like what you see then please vote for me in the top right corner. Thanks!

https://www.instructables.com/contest/beforeandafte...

https://www.instructables.com/contest/outside2015/

https://www.instructables.com/contest/metal2015/

Comments

YouNeverKnowItUntil (author)2016-06-25

Where did you source your parts

kniegmann (author)2016-01-25

Props on building your own expansion chamber. Nice work!

MattTheMaker (author)kniegmann2016-01-31

Thanks! It was quite the learning curve making cones and tig welding such thin steel (18 Gauge). Since my first one I've made 4-5. It's very satisfying to put all that work in and then see the performance gains!

chrisjlionel (author)2015-08-21

WOW! You've got lot space. Add some clamps to hold additional battery or make a storage box or anything useful. Congrats for being a finalist... Hope you'll get a spot welder for your future projects.

Thanks! Its a stripped down bike so no battery needed when you hardwire it. No spot welder this time but my tig will do just fine :)

SethN1 (author)2015-07-27

So cool mate good job

TheNecromancer13 (author)2015-07-26

Cool. Reminds me of a bicycle I have that I attached a 66cc 2 stroke and it can do 45.

Nice! Two stroke engines are really cool. The shape of the exhaust really affects the performance of the engine. A tuned pipe on a two stroke engine is like turbocharging a 4 stroke engine.

loony1 (author)2015-07-26

Bada$$ little ride! The hours you spent on it are evident, as is the quality of your build. Kudos!

MattTheMaker (author)loony12015-07-26

Haha yeah a lot of hours, thanks for noticing man!

Uncle Kudzu (author)2015-07-25

Way too cool! You really went all out creatively - thanks for sharing.

MattTheMaker (author)Uncle Kudzu2015-07-26

Thanks, I'm glad you enjoyed it!

About This Instructable

8,215views

74favorites

License:

Bio: I have too many projects going on to know where all the little cuts and scratches on my hands come from. Follow me here to ... More »
More by MattTheMaker:Hand Forged Wrench Knife Multi-tool: 10mm Carb Adjustment for MotorbikesDuo: The Multipurpose Desk and Coffee TableBuilding a Bike: Custom Honda Hobbit Moped
Add instructable to: