Introduction: Tuning Two-Stroke Engines

Picture of Tuning Two-Stroke Engines

As promised last week, here is the instructable inspired by the violent failure of this instructable

Step 1: Why It Failed

Picture of Why It Failed

The faliure of the 50 mph trike was most likely not caused by wear and tear on the engine, or even damaged parts, but by a simple mistake in the tuning process. Two stroke motors are VERY touchy in the tuning process, if they run too rich, they bog down and quit. if they are too lean, they will over-power themselves, and 90% of the time they explode violently. I'm not a certified mechanic in the least. but I do know what I am doing when it comes to re-building and tuning small engines, mainly two strokes. The reason the trike's motor blew up, was because it was running too lean, and it was a racing motor (potentially deadly combination).

Step 2: Two Stroke Theory

Picture of Two Stroke Theory

here is an overview of how two stroke motors work. Two stroke motors are more efficient that four strokes for a few reasons. 1 is the displacement, they generally have smaller combustion chambers, which means less fuel burned. 2 is the fact that they have fewer moving parts, which decreases the constant load on the engine, allowing it to burn the air-fuel mixture more efficiently, and reach much higher revolutions per minute.

Here is the process of the engine.

Intake, The fuel/air mixture is first drawn into the crankcase by the vacuum created during the upward stroke of the piston. The illustrated engine features a poppet intake valve, however many engines use a rotary value incorporated into the crankshaft. During the downward stroke the poppet valve is forced closed by the increased crankcase pressure. The fuel mixture is then compressed in the crankcase during the remainder of the stroke.

Transfer/Exhaust. Toward the end of the stroke, the piston exposes the intake port, allowing the compressed fuel/air mixture in the crankcase to escape around the piston into the main cylinder. This expels the exhaust gasses out the exhaust port, usually located on the opposite side of the cylinder. Unfortunately, some of the fresh fuel mixture is usually expelled as well. I'm going to interrupt here for a second to explain how that fresh mixture is prevented from being expelled.

so, you think an exhaust pipe is just meant to get the exhaust away from the engine, right? well in the 4 stroke application that's completely true, but in the two stroke, you must use a specially formed pipe, that uses something called backpressure, to force the expelled mixture back into the combustion chamber, this can be seen in the animation quite clearly.

Compression. The piston then rises, driven by flywheel momentum, and compresses the fuel mixture, insuring that the mixture explodes properly. (At the same time, another intake stroke is happening beneath the piston).

Power. At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.

Step 3: The Nitty Gritty

so folks, let's get down to the nitty gritty. the engine I am using is from my chainsaw, which is a Husqvarna. safety first, you will need to remove the chain, and bar from the motor. I removed the bar, because it was in the way. next, you will need to remove a plastic shroud that sits in front of the carburetor. on the body of the carb, you should see two screws, labeled L, and H. L stand for low speed (idle) and the H stands for high speed. you will want to gently turn both screws clockwise, closing off the fuel to the engine. Then , turn the L screw counter clockwise two and a half turns, opening the passage for the idle circuit. attempt to start the motor after priming it, by pressing the rubber button 3-4 times. it should at least sputter, if not, then tighten the screw 1/4 of a turn, and try again. contunue to do this until it starts. if it does not start, you will need to close it again, and turn it out two and a half again, prime it, and loosen the screw 1/4 turn at a time. once it starts, it should promptly die when you try to give it throttle. this is good. now, with the motor running, you will want to tighten the screw very slowly, and you should notice a difference in the speed of the engine. keep tightening it slowly, until it bogs down, or quits, now loosen it 1/4 of a turn, no more, no less. now, do the same with the high speed circuit, except when you do this, have it at wide open throttle. when it starts to get really powerful, turn the screw out until you notice a decrease in power.

Step 4: You Did It!!

Picture of You Did It!!

ta-da! you've just tuned a 2 stroke motor properly, so you'll never have to worry about it exploding on you. now don't you feel better about a motor running at five-thousand revolutions per minute next to your delicate organs? I know I do, infact, I'm overjoyed!


Deerbass (author)2017-11-03

Thank you Sir. I've been ------- with a chainsaw engine for over a year, off an on, trying to get it to run. Replaced rotted fuel lines, filter, plug, carb. Never thought about leaning the mixture from the start point, CLOCKWISE on the screws; I was only going CCW. Runs like a champ.

StacyF3 (author)2016-04-22

"Explode"? I don't think so- if you literally mean explode these engines would not be on the market. The problems with engines especially 2 stroke stem from pollution controls and economics. Fuel with ethanol softens the flexible rubber-like parts and causes swelling then failure. Fuel degrades quickly and if left in the tank and carb for a month you will need a new carb. Tiny carbs must work perfectly to power a tiny piston and are unforgiving. Carbs are cheaply made and expensive to replace they also siphon fuel after you have pushed the primer bulb because the needle valves are poorly made and do not seat as they should, then you have fuel pouring out the exhaust. On a four stroke the fuel is filling the crankcase ruining the oil so you may ruining without proper lube and won't be running long. It will start but no power because rings are shot. Pick up your chainsaw by the pull handle only and if the start rope feeds out from the weight of the saw you have no compression. Heat created is trapped in EPA mufflers shortening the life of everything around it. The cooling is tantamount to engine life or death. New gas powered 2 stroke equipment is already crippled by pollution controls so expect it to not start at anytime. Buy the best and correct for the job gas powered equipment. If three pulls on the rope do not start the engine it is going to start so stop pulling. Look at the spark plug immediately out of the engine, dry or wet. Dry is no fuel, wet is no spark.
Close the choke and pull once on a warm or hot engine to create the vacuum needed to draw in fuel, open choke and pull again to start it. A cold starts requires the choke to be closed but open it up as soon as it starts.
A minuscule drop of water will seal the screen of a small carb and it will not allow fuel through. Heat causes condensation in your fuel tank after running so when you are done fill the tank immediately with fuel so that does not occur.

Explosions do not occur in engines but rather controlled burns. If the controls are askew the burn does not occur. Engines do not explode unless they are drag racing engines bored out and using high octane fuel with a bearing out of specs or failing. Piston splitting the cylinder or rod coming through the oil an oil pan is as bad as it gets. Nitro fuel will cause a fireball from hell on a 4000 hp dragster but no nitro is used on a 2 stroke with 10 hamster power. Don't say "but its jet fuel!", jet fuel is kerosene...
Always keep in mind that gasoline is dangerous but it does not burn well but its vapors mixed with air will ignite from 30 feet away and engulf you in an instant. 2 stroke gas is contaminated with oil so optimum conditions are mandatory for combustion. A V8 350 w/4 barrel carb is forgiving with 7 more pistons ready if one does not function- a single piston with restricted exhaust system and one compression ring is not. The carbs on these machines are junk, live with it.

The photo of the piston is not straight from an engine if you look at it with an open mind and a sense of humor. It has been beaten with a hammer. Where is the wrist pin and the connecting rod? The cylinder?

tcambrón (author)2016-04-21

gaww these 2 strokes are a pain for me.

I always had problems with my rc car - traxxis tmaxx, went through 5 carbs and 4 motors. I haven't owned an rc car for over 10 years now. But now I am having problems with my used pocket bike - mx3.

I recently installed a brand new (advertised as new) carb on it. I let it idle on choke a few minutes to warm up. Raced around a few minutes until my testing-muffler had fell off.

So, I got the muffler issues mostly fixed. I decided to do some local street runs. I did choke warm up a few minutes. I noticed that it idles pretty high rpm without choke. Tons of nice blue smoke with choke and cold-no-choke. But then there was no more blue smoke after 10 minutes of run time and the idle rpm seems to be higher. I did a couple more passes and started to notice that it would bog a little at full throttle and I would have to throttle about 75% just to get going.

I think I seen an idle screw. But um, I don't think I even seen a High/Low tune.

blc1983 (author)2016-03-31

i have a cr125 elsinor motor not sure what kind of cdi box it needs. any info would b nice thank u.

turbochargedAdam (author)2014-04-08

In my workshop I have a 49cc 2 stroke engine, it wont run at all when the choke is off, and will only run properly when the choke is full on, any ideas why this could be?

When the choke is on it means that it is "choking" the motor and giving it less air but the same amount of gas. That would make it a richer mix which is more gas/less air. To make ur engine work well without the choke is by making the main jet (the big one) much larger probably 3-5 sizes bigger which the sizes go up in 2-3 number intervals. Remember to try to tune the bike with the idle screw before trying to put new jets in it.

Jones E.B (author)hopekiller302015-10-25

??? Just rebuilding my 50cc. Put 70cc upgrade, and new carb, but still need to run air/fuel screw all the way in to make it stay running???

john.brennfleck (author)2014-09-09

I have a 39cc razor for my son I have changed the plug, bought a new carb but it still bogs out after it get hot and quits if I catch in time I can keep it going with a shot of either but only a short time when it is cold it runs great but also take either to start the carb has no screw adjustments

down jet and check for leaks. scratch that, the other way around.

Check for cracks around the carb

Jones E.B (author)Jones E.B2015-10-25

Actually how is the throttle cable. You could still be stuck wide open

benjamin.namssorc (author)2015-09-12

A great guide! However, I have come across many small two-strokes with Walbro carbs that will not tune because of problems within the carb. The fuel pump diaphragm and needle valve sometimes need to be replaced, especially the diaphragm. This link is really useful:

PanaYotisF (author)2015-02-15

i would like to ask a question about a techncal subject on my husqvarna wre125.
i have bored the cylinder to 57 mm and as i couldn find a piston to husqvarna specs i ordered a honda cr 125 this one it is the same the only difference is that the cr's piston is a bit dome.what mod do i have to do with the head now to fix the comression ratio?? how many cc more does the head need ?? the stroke is 54.5 the compress ratio 8.8/1..
awaiting your response if anyone knows
thank you...

michael.rae.3762 (author)2015-02-14

Hey guys, so went to start my 951 xp seadoo jetski the other day and found a pretty scary problem. It took a lil to start due to a flat batt but when it did i cycled the revs a few times it revved to peak revs on its own... i pulled the leads off the plugs but it kept revving, so i turned off the fuel and disconnection positive and negative off the batt, still revving, so i started taking a spark plug out.. by this time i think the carbs ran dry so it stopped..

So ive got a 2 stroke that was running fine a month ago but now its peaking on its own with no spark? Accelerator cable has free movement and carbi's seem to b opening an closing properly but is hard to tell without pulling the exhaust apart. Spark plug tips are mocha brown. Do i need a carbi rebuild?

LiquidLightning (author)2012-01-10

This guide is great, but you should add that this process will only apply on walbro carburetors. Also, after you finish, pull the spark plug and check the color of the tip.
If it is mocha brown, it's perfect, black means too rich, and white means your motor may just explode (too lean). Also, when it bogs down, typically that is over lean, and when it sputters (known as four stroking), it is too rich.

It is actually not true that u can tell how the tuning of an engine is from the spark plug, especially with two strokes even if u use a specific amount of oil per gal of gas and have used that same amount before. U will only be able to tell extremes from the exhaust giving white smoke if too rich or too much oil in the gas.

2 stroke (author)2011-11-12


ilpug (author)2011-06-15

I have a Husvarna weed whacker. recently it started to become impossible to start. i would pull the cord about 5 times before it would even turn over a couple times. i took it all apart, cleaned it, greased it, and reassembled. it has a new spark plug and a new fuel filter, although the air filter is a bit dirty, but i have run it before much dirtier. i have been looking for the tuning pins, but i cant find them! i found two little pin things that are right on the carburetor, but they have no markings or any slots for a screwdriver. any ideas?

bleachworthy (author)ilpug2011-06-15

Firstly, the air filter being dirty blocks air coming in, and will make the engine run rich, and your spark plug will eventually foul out. Second, its hard to tell which needle on the Carb is which. Best way I've found is to tighten both all the way closed, and gradually open one a quarter turn at a time until it sputters. If you open one more than 3 whole turns with no results, close it back up and try the other. In your case this may be difficult, since you have no way to grip the heads of the needles well. If you can, take a photo of your Carb so we can get a good look at it. Its completely possible that yours is not adjustable (thanks to the EPA), in which case I advise an aftermarket (different) carbourator. Most have a simmilar bolt pattern, and will interchange nicely. just be sure your donor motor is about the same size. My mccoulloch has a homelite Carb, because replacements and rebuild kits don't exist anymore for my make. Hope this helps!

bcampbell61 (author)bleachworthy2011-08-23

i have a 2006 kawasaki bayou 250 it runns good if u can start it and once u get it running u must drive it but as soon as it gets hot it runns fine but if its cold an u go to start it ull have to pull start it like 300 ft b fore it even tries to start plz help irf i cannot get it fixed ill scrap it!

ilpug (author)bleachworthy2011-06-16

thanks for the quick reply. today i cleaned the air filter, and checked the sparkplug, which was fine. then i took the whole machine apart, cleaned what was dirty, and put it back together. same problem. i will look into aftermarket carbs, because it really looks like this one was made to not be adjustable. on closer inspection of the pins, I found that they are perfectly round, and impossible to turn even with needle nose pliers. The instruction manual does say that you must turn those pins to tune it, but i think it was for a different model.

bcampbell61 (author)ilpug2011-08-23

check the gap on the magneto!!! and the gap on the spark plug!

bleachworthy (author)ilpug2011-06-16

Well, you can always try to file or grind a flat surface on the sides of the needle heads for grip. Your best bet is to use a dremel, or other small rotary tool and make a slot for a screwdriver on the heads. If you can get them to turn, take them out and clean their seats. Make sure when tightening that you do not overtighten, this can permanently damage them! These things are super-fragile when it comes to tightness

bcampbell61 (author)bleachworthy2011-08-23

yea my montgomery ward chain saw did the same thing except it was the gap on the magneto!!

sergeantday (author)2009-05-30

Got a Stihl BG 85 that will not run at high speed. New air filter, new sparkplug, even cleaned the spark arrester. Factory book limited in problem solving. Attempted to adjust carburator settings but this failed. Gas filter is clean. Anyone got an idea how to correct problem Thanks for your help and stay safe

big trav (author)sergeantday2011-01-19

Could help if you stated what the problem is!!!!!

ilpug (author)big trav2011-06-15

@seargentday: have you tested the distance that the throttle lever moves hte actual throttle?

@big trav: he did state it: it wont run at high speed.

TSC (author)2011-05-25

When your done tunning it will it idle just right ?

MrTrick (author)2007-11-08

I'm sorry, but 2-stroke motors are more efficient (speaking in power-weight ratio) than 4-stroke motors because they have one power stroke out of 2, whereas a 4-stroke motor only has one power stroke out of 4. The number of moving parts is a minor aspect of efficiency that is dwarfed by other things like compression ratio, and displacement is a measure of engine size. Also, if you are going to take a picture from the wikipedia article discussing 2-stroke engines, it would be a good idea to: a) quote the article directly when talking about theory b) attribute the source of the image.

big trav (author)MrTrick2011-01-19

In theory you are right but in practice you are WAAAAAAAAAYYYYYYYYYYYY wrong, four strokes are more efficient and will run for easy 4 times as long on the same amount of fuel. Yes per cc 2 strokes USED TO produce more horse power, however some of the modern 4 strokes have matched or exceeded what equivalent 2 strokes produce with much higher torque. 2 strokes are ineficient in the way they use fuel due to a number of reasons one is the fact that they blow half of their fuel charge up the exhaust to achieve nothing, another is the fact that the engines lubricants are being carried in the fuel instead of in a sump completely separate to the combustion cycle. I think you should look at how a 2 stroke really works and all of the data which shows just how inefficient and polluting 2 stroke engines really are.

THEN you will see why I love

MrTrick (author)big trav2011-01-19

I didn't say fuel-efficient.
I said weight-efficient. As in, for a given power output, 2-stroke motors can be lighter.
(Even if, as you've said, they use more fuel)


And yes, 2-strokes make a wonderfully awful noise. :-D

bleachworthy (author)MrTrick2007-11-08

ah, good point, and I did not get that image from wikipedia, I got it from and the displacement comment is a "duh" thing. I mentioned that the displacement difference, because it really does matter in this world of bady needed efficiency.

TheBlunderbuss (author)2009-05-23

What choke setting should the engine be at, at any time? That is, if the engine has a choke (both my weed-eaters do).

jbeidle (author)TheBlunderbuss2009-05-29

The choke is only for cool weather starting. It provides a richer fuel-air mix for the initial start. Most 2-cycles instruct you to choke, pull until the machine tries to start (usually 2-3 pulls for a well tuned engine). Then you move to the half-choke position and pull to start. Once you got the engine running, leave it at half-choke for a short time (anywhere from 30 - 60 seconds, depending on the manufacturer) until warm. Once the engine is warm, you won't need the choke.

For this tuning procedure, you need to be able to start and warm the engine. Tuning a cold motor causes more problems.

big trav (author)jbeidle2011-01-19

If the engine is cold and has not been started for a couple of hours or more then use full choke, after a couple of pulls the engine should fire, then you flick to half choke and it should fire and run within the first 1 to 3 pulls, then you should immediately flick on to the run position. you may find it a little "doughy" for a few seconds while it warms up, but then it is right to go. This said I have a ryobi brushcutter which is a right pig if the fuel is more than a week old, and find with stale fuel it will not run unless it is half choked, Piece of crap that it is!!!

BUY STIHL, they are the best

BIGHAIRYDUDE (author)2010-12-06

hey i need to ask you a question i got a new motor and the primer bulb will suck in and out pretty fast and there are no visible cracks. but i can see any gas going through the lines, on the side with the big screw where gas is sucked in, when i unscrew it it releases pressure and gas just spews out like crazy from the lines. i cleaned the carb and i just can fix it i dont know what is wrong if you could help it would be well appreciated

blah, I need to read things right the first time... I'm assuming you have a diaphragm style carb, it sounds like a valve is not opening when it should. It's probably going to be underneath the diaphragm, you may need to remove one or two SMALL screws to release a little lever that the diaphragm plate presses down. Under this lever should be a spring, and under the spring either a ball or a pin/needle like thing. Be sure everything is clean, and be as gentle as possible in every step. In testing, to prevent fuel spray, open your tank cap first. Let me know if this helps!

i did exactly that before and it didn't make a difference! i have no idea what to do, and i have worked to small engines for a while like rebuilding golf cart engines and weedeaters of all sorts but this stumped me.


wow-amazing (author)2009-04-22

Heh, more like 15,000 rpm (that's what my goped does, but it's a little farther from my vital organs). Is there a way to tune the high-end without full throttle?

bleachworthy (author)wow-amazing2010-12-06

sorry it took so long, the only good way to tune it is to use WOT, I'm sure you can fine tune it in any range, it's just very tedious, and can require specialty tools like airflow meters, and air/fuel ratio meters (Spendy).

BIGHAIRYDUDE (author)2010-09-05

my weedeater only has one screw what do i do

Look for an H or a L on the carb near the needle. I'll bet the needle is for high speed mixture.

okay well i have messed with the needle a billion times and the mix just is'nt right it doesn't seem to be doing anything it will idle but if you give it gas it bogs down but doen't quit and it won't accelerated its like theres no torque or something i made a motor bike and it has lasted about 140 miles so far and i have cleaned the carb a couple of times, and the motor is really old to, its a 25cc craftsman from i dont know when because i found it in some guys trash and fixed it up.

It's possible that your carb is plugged up a bit, mine does all the time, all you need to do is unscrew that screw all the way (making sure not to lose the spring wrapped around it), press your primer button several times until gas comes from the screw hole, or pull the spark plug and pull like crazy until gas comes out. Check the needle, make sure it isn't bent, or otherwise broken, then clean it up and put it back in, unscrew it about 2 turns for starters. if this doesn't work, you may need to pull the whole carb apart for cleaning, it's not difficult, you just need to be very gentle, and remember where things go. Most carbs I have taken apart have re-usable gaskets and seals, this makes the fix FREE!!! hope this helps, and good luck to you!

o.k thanks

dirtbiker809 (author)2010-02-09

some of this information is potentially misleading. what i was told was a 2 stroke engine has the same horsepower as a 4 stroke engine about twice its size in displacement. however, i thought that 2 strokes consumed more gallons per hour than a 4 stroke of equal horsepower and displacement. now granted, 2 strokes have tons of nuts, i just thought they were always less fuel friendly than 4 strokes

Rectifier (author)dirtbiker8092010-03-06

Nope, you're right. 2 strokes are much more weight/size efficient, but in no way are more fuel efficient.

When you see that green fuel-air mix rebound back in the animated pic - not all of it makes it back into the cylinder... Especially when not running at the resonant frequency of the pipe.
With a CVT (modern moped belt transmission) the engine is maintained at the optimum RPM through a wide range of road speeds - with a geared transmission, much gas is wasted outside of the powerband as well as the bike acting like a pig at low RPM.

On the other hand the latest direct fuel injection 2-strokes have completely insane power and efficiency - they avoid the fuel loss problem (and burning all that oil, too) by injecting fuel after the ports close, like a diesel. I personally test rode a 50cc bike with DFI up to 90kph, TOTALLY STOCK.

Source: I own a 2-stroke moped that has tons of nuts for its 50cc displacement. It also eats gas compared to similar, gutless 4 strokes on the road (almost twice as much, in fact - 3L/100km)

sawdan (author)dirtbiker8092010-02-13

This is a subject i am interested in. can you quote figures that show what "you where told"?
Just wondering. (author)2010-02-10

 Hi on the first picture you have 49cc pocket bike engine, that engine have only one idle screw how would you tune up that engine?

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