Tuning Two-Stroke Engines

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Introduction: 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

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

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!!

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!

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    68 Discussions

    The tuned pipe is not required, it is not a case of you "must" use it. Two strokes run fine open exhaust or with simple expansion mufflers. All depends how the porting/timing is set up. The tuned pipe can increase volumetric efficiency quite aggressively at resonance if set up properly - it is simply acoustic supercharging, meaning that it increases the pressure and thus density of the incoming fuel/air mixture..

    When run too lean they don't overpower themselves.. they just get too hot, start vaporizing oil and generally self-destructing via heat and friction. They make the best power tuned correctly, not over-lean. Some engines may come apart if you sieze them up, but many engines just bind up and stop or they break the con rod.

    Two-strokes generally run lower compression ratios than four strokes and also tend to waste a bit of fuel in the induction/exhaust cycle - so they tend to be less efficient than four strokes.

    Other than that, the rule of thumb starting a Husky chainsaw engine is very helpful.

    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.

    "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?

    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.

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

    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?

    2 replies

    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.

    ??? 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???

    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

    3 replies

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

    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:

    http://www.footflyer.com/fix/motor/carburetor/walbro.htm

    hi
    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 https://www.wiseco.com/ProductDetail.aspx?AppID=&ItemID=PK1449&ModelYear=&ModelID= 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...

    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?

    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.

    1 reply

    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.

    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?

    1 reply

    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!