Instructables
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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submarine2417 hours 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)  submarine244 hours 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 days 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.945 days ago
John,

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?
jrkbrg15 days 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)  jrkbrg14 days 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 B14 days 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)  jrkbrg14 days 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.

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.harbilas14 days 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.)

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)  chainsawphil18 days 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

mike8man22 days 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?

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)  mike8man22 days 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. 
husq125b26 days 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)  husq125b25 days 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. http://www.ehow.com/info_7944826_two-trimmer-not-s...

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 Spatula1 month 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.71 month 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.
Avtrill1 month ago

Hello I have been having problems with a goped I was given by a friend. The engine is a 2 cycle Zenoah G23LH (22.5cc). Since I have owned it I had to replace the top end ( head,piston,rings,wrist pin,needle bearing,etc.) I also rebuilt the carburator and changed the fuel lines and throttle cables. The problem I am having is last time I had it running I couldn't seem to keep it from running lean. I tried adjusting the H and L carb screws with no luck. After running for about a minute the engine didn't want to turn over like it was seized. I removed the head re oiled everything and have it back together but am afraid of it seizing. I did notice very slight marking on one side of the cylinder that makes me think it might of been misaligned. When I turn the flywheel now it seems smooth and like it has good compression. Any ideas at all? I'm getting to be an expert at tearing it down and not enjoying it very much. One other thing I noticed is the original head it came with had some porting done to it by my friend.

TY TY I looked into this and yes found a leak in my fuel line behind the fuel bulb will be replacing the line soon and will tell you how i went. The leak is only visible when flexed otherwise it cant be seen.

I assume that would be the reason my Talon Whipper snipper would run for a bit (5 minutes) then die and not start again (would run a few seconds and die) as air would be getting sucked into an open hole in the fuel line easier when it heats up.

Phil B (author)  shithappeens.malone1 month ago
It is so easy to make an assumption that later proves to have been false. I had a 1991 Chevrolet Lumina sedan that often stumbled and died at stoplights. Spraying carburetor cleaner into the air intake tube helped some. The real problem was a slit inside one of the pleats on the underside of the air intake tube. With engine heat and with vibration the slit opened or closed, creating a big mystery. I found it after phoning a mechanic who had just begun a Saturday radio program. He had very few callers and was glad to keep me on the line talking with him rather than filling dead air.

I am glad you rare making progress. After the cause of the problem is found, it all makes so much sense.
wheelz6961 month ago

Hi phil , i was wondering if you could help me figure out a problem im having with a 49cc 2 cycle scooter motor , bought it at the pawn shop, for some reason it only idles while choked ,give it gas and it bogs down , when the choke is off the wheel just keeps spinning with a medium rev , hitting the throttle makes it go faster without any bogging, tried setting the air/fuel screw to 2 and a half out from closed to make sure that wasnt the problem but had the same result. Any ideas? Thanks for your time.

scoot3.jpgengine1.jpg
Phil B (author)  wheelz6961 month ago
If you need to choke the engine to get it to idle I suspect an air leak into the crankcase. (Choking reduces the amount of air entering the combustion chamber.) That may mean you need to tighten carburetor mount screws or screws holding the halves of the crankcase together (or where the cylinder head mounts to the crankcase. If that does not solve the problem, it may be that the gaskets are old and do not really seal anymore. I described making your own from gasket material. One other commenter had similar problems and found an air leak in one of the crankshaft bearing seals.

You were right!! original owner must have just thrown the thing back together , 3 loose bolts holding the crank case, Started up with a little throttle , ran it with the choke off for a second but i think the throttle screw wasnt in enough , it idled fine with a little throttle held down but died when i let off , i turned the throttle in a little more but i think its having trouble starting when its hot , couldnt get it started again before rippin skin off my fingers pull starting it, gotta take a break for the day , any reason for it not starting while hot?

Phil B (author)  wheelz6961 month ago

When things warm up they expand and move. Did all of the screws remain tight, or is something still loose? How new is the spark plug? An old plug may not work under compression, although you did say it started and ran. Automobile carburetors sometimes experienced vapor lock. It happened because too much heat made its way to the carburetor. The solution was often to design an insulating spacer between the engine block and the carburetor. It could even be that something moved and is shorting out the ignition. A wire would likely be pinched, in that case. It is difficult to know. You probably just need to check things while looking for anything that is not quite right.

Update , got to ride it and got it to idle great , still getting a little hot but not as bad. Spark plug is the one that i bought it with and will be replaced with a new one soon.

https://www.dropbox.com/s/mx3fuhcyo02mog3/Video%20...

Bolts were still tight after the first tightening , The extra rattling is the cover for the gas tank , its cracked pretty bad and rattles like that , After running it i have to let it sit for about 10 minutes to get it started again , Starts with full throttle with the choke on, flip the choke back and forth for about a minute to keep it going and bam! it sits there idling with no wheels spin, same thing every time to start it, this thing is way quieter and far faster then i expected , WoW! Thanks a ton , will keep ya updated on the overheating problem.

Phil B (author)  wheelz6961 month ago

I looked on the Internet for information on hard starting with a hot engine.. What I found suggested such problems are related to fuel delivery, specifically vapor lock. I would still look for anything that is just not quite right. As I have mentioned before, an engine needs an adequate supply of clean fuel properly mixed with air in the correct ratio, proper compression in the cylinder, and a good spark delivered at the proper moment. A 2-cycle engine also needs to be able to evacuate the exhaust gasses efficiently.

avaella20082 months ago

It is about 5yrs old. I will run through your list and see what happens. I am letting my pride get the best of me on this one but I want the satisfaction of fixing it. I will run through it again next weekend and let you know. Thanks for the advice.

avaella20082 months ago

Phil, I looked through the thred and didn't see anything that sounded like the issue I am having. I have a Weedeater Edger that I have replaced the carb, fuel lines, and fuel filter, and air filter. It won't start. I shoot some carb cleaner in and it starts briefly and then dies. This is the same issue that was happening before all the replacements. Thanks for any suggestions you might have.

Phil B (author)  avaella20082 months ago
How old is the edger? If it is more than ten years old, the gaskets probably are not sealing well enough to draw fuel into the engine and need to be replaced. Is there good spark?

In order to run an engine needs a free flow of air, fresh fuel and air in the proper rich/lean mixture, compression, and good spark delivered at the proper moment. 2-cycle engines also need unrestricted expulsion of the exhaust. Sometimes what an engine does or does not do gives a strong clue about which of these is not as it should be. Otherwise, the user simply needs to work through these one by one, putting aside assumptions that can mask the problem.
tezwarby773 months ago

Hi I would appreciate some advice on how to get the generator belt pulley off the crankshaft end, on a G1 Yamaha Golf buggie, and the magneto plate which is fixt with a woodruff key .

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