Picture of Curing 2-cycle engine problems
Many 2-cycle engines have problems and are sent to the junk yard before their time. Typical problems include hard starting, rough running, a need to adjust the carburetor during use to keep the engine from stalling, a need to rev the engine to keep it from dying, and not starting at all.

I will assume the user knows to replace the spark plug regularly, to use fresh fuel, and to replace or clean the air filter.

With time and normal vibration, the screws that seal the crankcase from air leaks loosen just a little and air gets into the engine through leaks in the crankcase gaskets.

First check the mounting screws for the carburetor and tighten them. Then go to the screws that hold the cylinder head to the crankcase body and those that cover the end where the crankshaft comes out of the engine. See the yellow circles on this photo of a weed whacker engine. If any of these screws loosen as little as a quarter of a turn air begins to leak into the engine and the fuel/air mixture either is not pushed into the engine on the piston's downstroke or it becomes too lean for the engine to run by pulling in extra air during the piston's upstroke.

After about ten years of use, no amount of tightening on these screws will make a dead engine run. Chances are the gaskets have become hardened beyond their ability to seal the engine. Dismantle the engine completely and install new gaskets. You may not be able to buy the proper gaskets, but you can buy a sheet of gasket material. Use the old gaskets or the engine castings as a pattern to cut new gaskets. Your engine will run like new again.
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smokedout1020 days ago

Hello wondering if you can help i have 49cc goped it used to be weak when accelerated a little bit but would go up to top speed. When i was using it just stopped I heard a sort of popping noise and I stopped then the engine turned off I couldnt start it afterwards because i couldnt pull the cord and it was stuck at first i thought it was flooded so I removed the spark plug and it was still hard to pull so i remove the pull start rope to see if it was stuck and i tried to hand crank the engine it only moves two full turns either way before it sounds like it hits something and cant move it after that?

Phil B (author)  smokedout1020 days ago
I do not know anything definite, but from what you describe, I would want to see what is inside the cylinder. I am wondering if the piston could be seized due to scored walls or a broken ring. You may or may not be able to see anything by looking into the spark plug hole. You may need to remove the cylinder from the engine case.

I have a Chinese clone 90cc 2 stroke engine,air box is coming apart and wonder if you can use a regular cone air filter or do you have to use the air box?,cover air intake and it starts and if keep parcial covered will run but when push throttle down speed up and down and dies when you let off.

Phil B (author)  robert.burrow.1211 month ago
I am not expert. My experience was that have modified an air cleaner and the engine ran, You may need to adjust the idle screws because the air flow through the new air cleaner may be a bit different. I think it should work.
There is no certain way if starting . Just hard to start and I don't know why! Maybe thinking the compression . But would it even crank?
Phil B (author)  coreystalvey1 month ago
I was thinking my son-in- law's Husqvarna chainsaw has six or so steps for starting, including a compression release valve. And, it is less horsepower than your engine. The steps must be followed in order for it to start. Even old John Deere tractors have a compression release to open before yanking the flywheel.

I have a 1972 mercury 20 hp 2 stroke boat motor. it is hard to crank but when it cranks it runs good. new sparkplugs , gas , carb cleaned , gas lines cleaned. any suggestions?

Phil B (author)  coreystalvey1 month ago
I am not familiar with your engine. I do know that pulling a starter cord on a 5 HP engine can really take all of the strength a person has, and you have 20 HP engine. I used my son-in-law's chain saw, and it was 5 HP or less. It came equipped ith a compression release button and very precise instructions on how to use it during starting. Are there any special starting instructions for your engine?
mashaca2 months ago
Hi! I have a Toro 2-cycle snowblower that was given to me in non-starting condition. The previous owner had mixed 10-30 oil with gas on the last winter he used it. Anyway, I replaced the spark plug and replaced the gas with the proper mixture. The snowblower started up no problem but would not run with the choke at full; engine was revving up and down but it ran for several minutes. Now I tried starting it and it will run but only if I use the primer bulb to feed gas, as if the carburetor is not feeding gas automatically. Please advise. Thanks!
Phil B (author)  mashaca2 months ago
Notice that in some of my responses to recent questions I referred to an on-line set of diagnostics charts and gave a link. Look for a symptom that most closely matches your problem. Then try the proposed solutions until you find one that solves your difficulty. You are likely correct that the flow of fuel is restricted. The cause could be due to a dirty fuel filter, but also to a poorly functioning air vent to the fuel tank. (Loosen or remove the cap on the tank and see if that makes a difference.) There could also be dirt in the carburetor. If there is a sediment bowl, dump it out and wipe it clean.

BrianJ42 months ago

Hi my friend baught a suzuki ts100 bike, it will start fine and idel fine, but when he puts it into first gear you have to have the revs basically full before it will pull away anything less it seems like its going to die yet in any other gear it is fine any ideas?

thanks in advance

Phil B (author)  BrianJ42 months ago

Take a look at the link in my response to bike22u7 immediately below. It is a link to general diagnostic charts for either 2-cycle or 4-cycle engines. I would guess the problem you describe would be considered "low power." This sub-link discusses multiple causes for that problem.

You will need to check out the probable causes one by one.

bike22u72 months ago
I have subaru 6hp motor on my power washer, it starts on the first or second pull but if i stop it when its warm and try to restart it wont start. Any suggestions. Thank you!
Phil B (author)  bike22u72 months ago

Is that a 2-cycle or a 4-cycle engine? I have not experienced the problem you describe and am not a lot of help. Here is a link to a diagnostic tool for getting a start on solving your problem:

When I was younger, I got my hands on a couple of auto repair manuals. They had diagnostic helps in a special section similar to the link above. It was always a matter of looking for the best description of the problem, then looking at possible causes, and trying them until I found and fixed the problem.

submarine243 months ago
Hello PHIL I have a scooter with 2 cycle engine that I bought used the guy said that it needed a new throttle and he also said in needed a primer bulb. He started it by spray the carb with starter fluid and it ran. So I only fixed the throttle cable but i didnt fixed the primer bulb and sprayed it with brake cleaner because I didn't have any starter fluid and it ran good for about a couple miles. But then I was about to run out of gas so I refilled it with gas but the scooter already had gas when I bought it. It still ran fine and it ran for like 5 miles after I started and sprayed. But then the next day I spray it and
pull it to start and it wont start it just sounds like its going to turn on but it doesn't take off it just turns off after a second. Is it because it doesn't have a primer bulb or could it be the gasoline I put in there I mixed it with oil according to ratio on the gas cap which says 25:1 ??
Phil B (author)  submarine243 months ago
I would begin by removing the spark plug and looking at it to see if it gives any clues. Often one can find photos showing color and types of deposits on a spark plug for various engine problems, and for a properly running engine. I do not know if such a chart exists for 2-cycle engines specifically. Also, when you have pulled the starter several times and then remove the plug, do you see evidence of too little fuel or too much fuel?
Hey Phil I have a echo ht225 hedge trimmer and when I accelerate I get sum leakage from fuel tank,what kind of sealant do you recommend?
Phil B (author)  ruben.i.sanchez3 months ago
I assume your fuel tank is plastic rather than from sheet metal. Can you tell where the leak originates? (You could wipe the tank dry and clean with a paper towel. Then rub flour over it. Watch to see where signs of wetness appear first.) The leak could come from a crack at a seam in the plastic, from the fitting where the fuel line attaches to the tank, or from the gas cap. The gas cap could be missing a gasket. The gas cap also needs to breath so air can get into the tank as the engine burns gasoline flowing from the tank. A sealant should not be necessary if everything is as it should be. Often you can buy a new gas tank cap. I hope this helps.

I have a husqvarna backpack blower 130 that starts ok, but when throttled it will die out. Gas and plug are new and good. When I take the airbox plastic cover off it runs great and accelerates perfect and runs great. I put the plastic cover back on and it will die out. There are 4 ports on the cover and they aren't blocked. The foam filter is clean and blower runs great with it in place. Any reason why

Phil B (author)  john.theisen.943 months ago

Did this problem develop suddenly? Is there anything you can recall that would have caused a change? It appears (as you describe) the engine is not getting enough air for a normal amount of fuel, unless you remove the air filter cover. Is the choke fully open, or could something allow it to close partially without noticing that?
jrkbrg3 months ago

Phil, I

have a 25cc Troy Bilt Back-Pac leaf blower that will not start. New gas, new plug (getting spark). I took it to a Troy Bilt service dealer and they said the piston and cylinder were scored and it would not draw gas and was not worth repairing. It cost $44 for the diagnosis. I can pay the $44 and pick up my machine or they will credit the $44 towards a new blower. Does the "scored piston & cylinder make sense?

Phil B (author)  jrkbrg3 months ago
Rings and cylinders do suffer damage for various reasons. I had a 2-cycle lawnmower that failed with a scored cylinder after about 15 years. Old age was a factor. Using the wrong oil in the fuel mixture causes damage, too. Can you look into the spark plug hole to see the cylinder walls? Or, can you remove the cylinder to look?
jrkbrg Phil B3 months ago

Phil, thanks for your reply. I take it the reason given is a good possibility and will look into your suggestion after I get it back. Look's like there is a new Stihl in my future.

Thanks again.

Phil B (author)  jrkbrg3 months ago

There is a possibility you might be able to find another engine on Craigslist or eBay. But, you would need a way to be sure it does not have the problems you have now.

bill.harbilas3 months ago

Why would my 2-cycle, Suzuki Toro leak a little gas when I shut it off? I rebuilt the carburetor with 2 new gaskets, bowl seal, and float pin. It runs fine. This carburetor has a peculiar feature, a small 1-1.5 inch black line the curvers downward and is not connected. Is this to expel unused gas? I notice that some small 2-cycle motors have a line connect back into the gas tank.

Phil B (author)  bill.harbilas3 months ago

Oct 10, 2014. 1:16 PMPhil B (author) says:
You mentioned a float pin, so I assume there is a cork or plastic float, maybe even a brass float. I always assumed nothing could go wrong with a float, especially a solid plastic float. I had a new car and seven years later white smoke came out of the hood when the car idled after warming up. It had a plastic float in the carburetor. Plastic floats do actually absorb gasoline in time and become heavy. You may not notice that the float allows extra gasoline to flow because the engine manages to burn it while it is running. But, when you shut it off some gas runs out. If you have a plastic float, try pressing your fingernail into it and see if moist gasoline appears around end of your nail. There may be some other cause, but check for a heavy float. Heavy is a relative term. A float needs to be only a few tiny grams heavier than normal to be heavy. (On my car the white smoke was caused by overflow gasoline dripping onto the hot manifold.)
chainsawphil3 months ago

Hi there ... I'm struggling with my Komatsu 2 stroke chain saw. It worked fine last year and has been stored dry in the garage over winter. Went to try and start it yesterday and it is almost impossible to turn over using the pull start, it will pop over top dead centre but there is no chance of spinning it to start it. If I take the spark plug out it spins fine, I removed the exhaust and it all looks clean in the cylinder. I checked the fuel line from the tank to the carburettor and that appears to be clean, and there's a spark at the spark plug.

It is clearly not seized... it just feels like there is too much compression ? Really running out of ideas.

Phil B (author)  chainsawphil3 months ago
Is the plug wet with gasoline? Does any gasoline run out of the plug hole?

But I can see where you are coming from... the symptoms are as though the cylinder had filled with liquid .. just enough that it could compress it and allow me to turn it over but not so much that it completely stopped the piston in its tracks. I checked to make sure that the cylinder was not full of gasoline so it's not that. I was reading another thread on here where it talked about something restricting the pull cord but only when there was pressure on it... haven't taken the pull cord mechanism yet but I suspect that that may be a next steps ... very frustrating ... I've been tinkering with engines for 50+ years and 2 cycles are so basic I can't believe it's beating me !!

Nothing runs out of the plug hole when I turn it over with the plug out and the spark plug gets wet with gas when it's in. Thanks. Phil

mike8man3 months ago

Phil I'd appreciate your comments on a two-stroke multi-tool (hedge trimmer. chain saw etc). The engine was working fine until a piston ring broke and gouged a groove in the cylinder. As parts for this 10 month old engine are no longer available (the engine was a replacement for an earlier one which developed a crack in the crank case), I replaced the damaged cylinder/piston/ring with the same from the earlier engine. BUT I did manage to get it to run for 20 min or so, but only at one almost max speed. Since then it will not start. I've tried a new plug - there is a spark - there is some compression (no idea if it is enough), I've changed the carb and checked the bolts holding everything together, Any ideas please?

mike8man mike8man3 months ago

Thanks Phil. However I may have found the unexplained fault. The old undamaged cylinder, piston and rings are now also damaged in about the same place, therefore poor compression, but no reason why as there were no broken piston ring bits in either the engine or muffler before I reassembled them.

The retailer is going to strip down an old m/c and send me another cylinder/piston/rings, so fingers crossed.

Thanks for your help, I'll let you know what happens.

Phil B (author)  mike8man3 months ago
I know only those problems I have experienced, and I have not experienced this problem. But, here is a free diagnostic help for small engine problems, both 2-cycle and 4-cycle. I would look at the list of possible causes, make some guesses about which to try first, and eliminate them one by one. 
husq125b3 months ago

Hey Phil, I have a Husqvarna 125b gas blower. It starts, idles, runs and performs great....except for restarting the unit after it's hot. If it sits for 10-20 minutes, I cannot get it started. I've tried choking and not choking. If it sits for a good hour to 1.5 hours, and cools all the way off, it'll fire up and run great. It doesn't die on me when running hot. I just get it started again if it happens to be hot. Could it be a coil going out? Your thoughts and suggestions are much appreciated

Phil B (author)  husq125b3 months ago

I have not dealt with the problem you describe, but from what I can find on the Internet, hard starting problems when an engine like yours is hot are usually related to carburetor problems. Here is a link to an article I found.

I wish I could pinpoint the problem for you.

Hi Phil. I have a '73 Yamaha DT 250 project bike. It was experiencing runaway/preignition due to a leak at the head gasket. I rebuilt the top end. Now it starts with a little help from starter fluid and will idle correctly for a few minutes. Once it begins to warm up the rpm's start creeping towards redline. Are there more leaks I've missed?
Phil B (author)  Battle Spatula4 months ago
I am sorry for the delay in getting back to you. I have not had this problem. It is always possible there is another leak. If it runs fine until it heats up, I wonder if something is opening up due to heat, even though it sealed when cool. I looked on the Internet and could not find much. Something in the back of my head makes me wonder if the spark timing is correct and the spark advance is working correctly. I also wonder if it is possible the throttle is opening from vibration when it should not. I wish I could give you a definite answer. Perhaps someone else has experience or knowledge with this.
I'll try replacing the reed valves, re-timing the magnetos, and installing new points.

Hi Phil. I have a 50cc PGO Rodoshow 2 stroke scooter. The bike runs fairly easily. No problem with starting, just give it a bit of fuel and it starts right up.The problem is when you accelerate it just clogs up. The bike only goes about 10mph.

I thought it was the exhaust because it didn't have the original on. I changed it with a original one and the problem still exists.

The air and fuel screws were turned in fully when I got it. Ive adjusted them to where I feel I am getting the best response.

When I open the throttle fully, the engine struggles to build revs up.

The spark plug obviously works. Do I need to change it though? The engine looks like it had a recent top end rebuild, it has a new head which has non matching bolts.

I took the carb off today. Just cleaned the jets. Problem still persists.

Phil B (author)  sarel.vandenberg.74 months ago
You looked at the exhaust. I trust you checked the exhaust ports from the combustion cylinder to the muffler chamber to be sure they are not partially closed. Does your carburetor have a Low (L) and a High (H) setting? Starting uses the Low circuit in the carburetor, but faster revolutions and more power uses the High circuit. As I mentioned, the High adjustment screw should be turned out about one turn from fully seating at the bottom of its adjustment.

Also, does you bike have a centrifugal weight spark advance? As I mentioned to one other person, my brother-in-law had a motorbike that would not generate power above a certain rather low amount. The weights on the centrifugal spark advance had seized from corrosion. He removed a round side plate to access them and applied penetrating oil. Then he worked them back and forth until they were free.

A new spark plug never hurt anyone, unless you know for certain the present plug is like new. If you have not, check the air cleaner to be certain is is working like new.
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