Curing 2-cycle engine problems

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.
Remove these adsRemove these ads by Signing Up
sean72394 months ago
Thank you for your reply. Yes when I removed the bottom of the carb I filled up the bowl with gas to see if the floats were working correctly and not being stuck nor cracked and they both floated to the top. The only other thing I can think of is that the tab on the float valve has been weakened and moved thus not allowing the floats to fully go to the top and controlling the amount of gas.
Phil B (author)  sean72394 months ago
I am assuming your floats are either cork or a solid block of lightweight semi-porous plastic. Even when they have become too heavy they are light enough to float, but not with the required buoyancy. I had this problem with a car I once owned. I got the car new. After seven years gasoline would mysteriously run out of the carburetor and onto the exhaust manifold when the engine warmed up. The float was a black block of semi-porous plastic. It seemed nice and light. But, when I pressed a fingernail into it, I saw a little moisture where my nail made an impression. The float was light enough to float, but had absorbed enough gasoline to be too heavy when the engine was warmed and the gasoline in the carburetor became less dense. The float worked well when the engine was cold and the gasoline was more dense, but not when the engine was warm. When i got a new float, I could suddenly tell the old float was a bit heavier. (We are talking about only a few grams.) And, the engine suddenly performed perfectly when I installed a new float. You would need to weigh your old floats with a scale capable of measuring a small number of grams accurately and you would need to know the factory specifications for the weight of a new float to determine if your float is not heavy. If your carburetor rebuild kit did not include new floats for your carburetor, replace the floats. I think you will be surprised.
sean72394 months ago
Hello I was wondering if you can help me? I have a 1990 Polaris Trail Boss 250. I am having the problem where it will start and run great when its cold but will die when its hot. It will not start again once hot until it cools down. The bike is oil injected and I have checked the system. I changed the head gasket as I had a compression leak. The compression is at 150psi cold and about 144psi warm. I have cleaned and replaced all the carb jets and needles in the carb. I have a shop manual that I got and set the carb specs back to factory settings. It seems that the carb is getting way to much gas. I have tried to adjust the amount of fuel going to the engine but with no success. I have checked the stator and spark. I have great spark cold or hot. I am all out of ideas on what to do next. Bolts are all tight around the engine. Could you please help me? Thank you!!!!!!!!
Phil B (author)  sean72394 months ago
If the engine is getting too much fuel and you cannot adjust by any usual means, have you checked the carb. float. Even the plastic block floats can absorb gas and become heavy.
TexasBuilder5 months ago
I'd like to add my two cents for troubleshooting the dreaded "two cycle engine starts when cold, but dies when hot"...

I used to have a '76 Yamaha Dirt Bike that would start like a champ and run peppy; However, once you took it out, just as you started to break a sweat, the peppy 125 would die, and not restart...

So, upon troubleshooting everything else in the book, it turns out that the problem was that as the engine heated up, the scalloped out cylinder wall would begin to lose compression. This was not very evident, as the visual inspection of the engine cylinder wall looked all nice and smooth. Calipers revealed that the cylinder wall was scalloped. Once the engine was given a machine job and new piston / rings, it ran like a new engine.
Also, another thing to check are the reed valves, which I think may also be prone to temperature related changes.

So, now I also have a chainsaw that exhibits this same behavior. I will troubleshoot it and report back what I find...

I was thinking that I will run the engine until it heats up and fails, then quickly perform a compression test, and see if that differs from a cold compression test... One theory of mine is that running a two stroke with insufficient lubrication in the fuel could cause this phenomena of scalloping out or "honing the bore" with the piston rings such that as the engine heats up to operating temperature, the engine loses compression, but starts fine when cold.
Phil B (author)  TexasBuilder5 months ago
Thank you. That should be helpful to many. I had never considered what you described.
botrys6 months ago
As I said I rebuilt the carb. with new gaskets. Could it be something to do with the exhaust since it starts to die after about 30 seconds as the motor starts to warm up?
Phil B (author)  botrys6 months ago
When I mentioned gaskets I did not mean only the gaskets in the carburetor, but also those between the engine castings that are held together with screws as well as the gasket between the carburetor and the engine. It still sounds like the engine is suddenly running too lean after warming up. When an engine warms up, tolerances change. Perhaps you heard the SR-71 Blackbird leaked fuel like a sieve when rolled out of the hangar, but when it reached altitude and the friction of the air at those speeds caused enough heat for seals to snug up. In your case it may be loosening up. Gaskets lose their resiliency with age and do not seal. Most small 2-cycle engines also have one mixture circuit for low r.p.m.'s, but another for high r.p.m.'s. When the gaskets on my weed trimmer were getting old, it would stumble and die when I revved it up. The problem was solved after I replaced gaskets between the cylinder and the crankcase and between the carburetor and the crankcase.
botrys6 months ago
I have a Tanaka strimmer, the motor starts, no problem, and will will rev at the beginning, however after about 30 seconds the motor tries to die, if I release the throttle it ticks over no problem. After that will rev a little but if I try to rev full it dies again. Have rebuilt the carb (new gaskets, etc), replaced the fuel line and filter, but no improvement - ANY IDEAS????
Phil B (author)  botrys6 months ago
I am guessing it is fuel starvation. That could be caused by an air leak in a gasket. If the machine is ten or so years old, get or make new gaskets. The carburetor adjustment screws for rich/lean could also be set too lean, especially on the "high" side.
Redneck03657 months ago
I have a 1975 Suzuki GT 185 that a purchased as a project and it will only start and run at high rpms (around 4 thousand) with the exhaust pipes taken off. I rebuild the carbs and used fresh gas, new air cleaner new spark plugs, good compression, good spark. I can not figure out why it will only run at high rpms. I have had the help of a former mechanic and we are both stuck does anybody have any ideas about why it is not working.
Phil B (author)  Redneck03657 months ago
When you rebuilt the carburetors is there any possibility the same gasket in each could have been installed wrong side up? (Usually the gaskets go in only one way, and it is more probable such a mistake might be made in one carburetor, but not two.) Is there a low idle adjustment and a high or run adjustment screw, or only one idle mixture screw? If there are two, it may be that the low idle adjustment screw are turned in too far. Are the parts of the throttle and choke linkages installed correctly? Is there a governor that might be stuck? Is the spark advance stuck? This has a couple of centrifugal weights that can be stuck. (These are under a round removable plate on the side of the crankcase.) Is it possible for the ignition timing to be set incorrectly, or is it impossible to adjust? I do not know what the problem is, but am suggesting some things to check.
I have a compression problem, well my neighbor does but I am the fix-it guy. So his Ryobi string trimmer is working fine one day but will not start the next. When the choke is set to full or when the throttle is not held open (with the choke in any position) it feels as though there is no compression when pulling the starter string. However except for on full choke, if you hold the throttle open there appears to be the same amount of compression as it normally would have. Although I have not visibly checked the plug but it appears/sounds like it is getting fire when pulling the starter string when the throttle is held open. I have checked the carb mounting bolts, etc for obvious spots where air may be leaking out of the cylinder but did not see anything alarming. However I do not think that there is any type of continuous air leak air as the compression appears to change depending on the amount of air being allowed in from the carb. Does anyone have any ideas on what could be causing this? I am stumped!
Phil B (author)  charlestonump77 months ago
While it seems logical that the spark plug is firing, you really cannot know for certain without removing it and checking. As you know, assumptions are often very misleading. Something you would not expect often hides under an assumption. It is odd that the compression comes and goes with changes in the throttle setting. When you find the reason why, it will all make sense.

Some engines, like Stihl chainsaw engines have a compression release valve that makes the engine easier to turn over for starting. I have been unable to find any mention of such a system on a Ryobi string trimmer. But, if there were such a system, it could be linked to the choke.

Here is a link to a brief article on compression problems related to Ryobi string trimmer engines. I would take most of the engine apart, one system at a time, and make notes on what I find because it is easy to become confused later about what you saw or did not see. As the article mentions, check every inlet and outlet for air. Also, it might be good to do an actual compression check on the engine with a gauge for that purpose. And, it is always possible that a piston ring suddenly broke, which would explain the sudden onset of the problems.

I wish you well. Please let us know what the problem was when you figure it out.
southcoastdj7 months ago
Hi, I was hoping you could shed some light on my 2stroke/cycle scooter, basically I am having trouble starting the bike if left overnight or for 5hrs, in order to get it started I have to cover the air filter intake with my finger in order to get it to start, once started all is fine, ticks over as it should and drives as it should with no loss of power or performance, I have changed the autochoke for a new one, installed a new spark plug but still it will not start if left for a period of time, I am at a loss trying to figure out why its doing this as it was fine up until last sunday, any ideas?
Phil B (author)  southcoastdj7 months ago
If you must block the air intake to get the engine to start, it means air is entering the engine elsewhere, too, and the engine is getting a lean mixture. But, it is not so lean that the engine will not run when it is warmed up. Since it happened suddenly, I would suspect a screw loosened. It may be that the low circuit idle screw has been bumped and is not adjusted properly. These are usually 1 1/2 turns loosened from bottoming out. It is more likely a carburetor mounting screw or two have/has loosened. If a carburetor mounting screw loosens a quarter of a turn, that lets in too much air and you will have starting problems. If the engine is ten or more years old, consider that the gaskets may have hardened and do not seal properly. That lets in too much air, too. While you are at it, make sure the fuel flows as it should from the gas tank into the carburetor. If the engine has an old fuel filter, you could replace it with a new one.
keegan1018 months ago
Hello hello, can anyone figure this out? I have a 1985 suzuki 185 dirt bike, starts first go, idles ok, but when the throttle is applied it conks out. and after 3-4 attempts the engine revs uncontrollably to the point where it sounds like it wants to detonate and the kill switch wont turn it off nor will the key. carby looks clean on the inside, the float is ok and the throttle valve doesnt seem to stick at all. please help if you can, thanks, Kf : )
Phil B (author)  keegan1018 months ago
Find and check the spark advance weights. It sounds like they are sticking. They are under a round plate and work by centrifugal force.
trance7288 months ago
Hi Phil,
Maybe you can help me out. I have a 2 stroke powered skateboard, probably around a 32cc. It has worked fine in the past but it sat for a while. I recently began trying to tune it and have replaced fuel lines, cleaned the muffler and carb and added a new air filter. Everything runs fine, except when trying to go to its top end it becomes jerky, almost a sticky feeling. At mid range it runs fine, and if I full throttle when I am not riding on it seems to be fine with no jerkiness. I'm not sure where to concentrate. I have played around with the air/fuel screw with no effect on fixing it. This is the carb that has the idle screw on top and the air/fuel on the side. Could this be related to the gasket from the carb to the engine not being in the best condition? Any help would be greatly appreciated.

Phil B (author)  trance7288 months ago
If you have a gasket that is not in the best condition, I would replace that quickly and then see what the engine does. It may not be sealing as it should and be leaking air, which would lean out the fuel mixture. When the idle screw no longer makes any changes in the way the engine runs, it is likely too much air is getting in someplace.
MurphyDIY8 months ago
Rest of the Story... After disassembling and cleaning the carb, it started right up. I'm guessing it was a slight bit of debris (almost a smudge) on the flapper valve that is paper thin and resisted the vacuum action. Thanks!
Phil B (author)  MurphyDIY8 months ago
Thanks for the report. It makes sense. I am glad it was that easy.
Paulygriff8 months ago
I was wondering if anyone could help me with my 2 stroke engine problem I have with my bike?
Recently bought a motor and fitted it on my bike, broke the engine in properly for the first 50 miles. Running it's first gallon on a mix of two wheel two stroke mineral (Comma) oil at 25 to 1 ratio. This went really well for 220 miles. Then I put a 2 stroke motorcycle oil XL (Carlube) in at 20 to 1 ratio for the next gallon and that's when I started having problems.
The problem Is now that it starts fine and goes for a little way and that conks out. After pedalling for a while it will start again, go for a while and then conk out again.
I think maybe that I have put the wrong type of oil in on the second gallon mix?
Can anyone help me please,

Many thanks Paul
Phil B (author)  Paulygriff8 months ago
I would take the spark plug out and examine it for any deposits on it. What is their texture, thickness, color, etc.? Compare the appearance of the spark plug and any deposits on it to a spark plug chart to get clues about what may be happening with your engine. For certain, I would go back to the oil mixture that worked, even if I threw out what did not work.
Phil B (author)  Phil B8 months ago
There is the possibility the oil in the second gallon of mix does not provide enough lubrication and the engine runs extra hot, which causes it to bind up with extra resistance and quit. Pedaling for a while allows the engine to cool so clearances can go back to normal. This is a very brief article, but it has a couple of possible clues.
MurphyDIY8 months ago
Ok, thanks. I'll see if I can pull it apart to inspect and clean. It's labled "ZAMA", without bulb on top and quite small. I think it has diaphram which I saw after taking off two screws on side (I believe) - was intact. Definitely flooding. Thanks again for your help and contributions to instructables! I'll tinker a bit more, but may have to make a sacrafice to the landfill gods (hate doing that!). Any suggestions as to brand I should buy instead for lead blower?
Phil B (author)  MurphyDIY8 months ago
I remember small carburetors like that with a name like "wilco" or something similar. I think they are all very similar. Carburetors have a needle valve that regulates the flow of gas into the carburetor. Anything that causes that valve not to seal causes flooding. That could be a little piece of dirt, like a shred of grass. A broken or missing spring can also be a problem leading to flooding. I think you can buy those carburetors new or used quite reasonably. I have now doubt your leaf blower will work well again.
Phil B (author)  Phil B8 months ago
It appears the Zama carburetors are a Chinese copy of the Walbro carburetors I remember. The Zama models are used on a variety of brands, including Stihl.
Phil B (author)  Phil B8 months ago
I looked on YouTube. The carburetors I remember were by Walbro. Here is a video on rebuilding a Walbro carburetor, but the principles are the same. There are a couple on Zama carburetors, but the quality makes them very difficult to use profitably.
MurphyDIY8 months ago
Yes, I believe it is. Certainly after I pull the plug a LOT of gas comes out the first few pulls - more than a mist. It's almost like too much gas is getting in there. It's possible my gas is leaking somewhere out of my tank - it looks lower after it's set - and is was seized again - difficult to pull starter.. Is that possible?
on further inspection, I took off the top looking into the piston under the carb (from a side view) and the lower piston area was filled with gas - doesn't seem like that would be normal.
Phil B (author)  MurphyDIY8 months ago
I suspect something is wrong in the carburetor. If it has a float, the float is heavy and needs to be replaced. If it has no float, there should be a rubberized fabric diaphragm that pulls the right amount of gas into the crankcase. The diaphragm may be torn or loose. At any rate, your engine is flooding.
MurphyDIY8 months ago
Need help please. I have a small consumer based RYOBI leaf blower (not professional style). It was working fine, then quickly became hard to start then seized up (difficult and jerky to pull cord). I pulled the plug and cycled the engine several times and it pulled very easy. Still would not start. Good spark, but changed plug anyway. I emptied gas tank and again pulled plug and cycled it to remove gas from cylinder. What was strange was after doing this and putting the plug back in without filling up gas, it started! When I filled the tank back up (new gas and new oil mixed to spec) it would not start. I repeated the last sequence and it once again started empty but then would NOT start after I filled up the tank or basically put any gas in tank. This must be characteristic of something. I'm not an engine guy, but I felt someone would know what was going on with this description? Any help would be appreciated.
Phil B (author)  MurphyDIY8 months ago
After you put fresh gas in the tank and are unsuccessful at starting the engine, is the plug wet with gas if you remove it right then?
palsouth11 months ago
Hi, Phil, I'm having a problem with a Tecumseh 2 cycle engine on a small tiller. After it runs for a little bit and gets warm, it starts to surge. I can see the speed adjusting lever, by the air intake, moving back and forth . . . on its own. I have the throttle wide open, but the lever is going back and forth, and the engine is surging then slowing down. After doing this several times, it just dies. If I let the engine sit for a couple of minutes, I can restart it and it runs okay again, but only for a few minutes. Any thoughts?
Phil B (author)  palsouth11 months ago
Another thing to try is running the engine with the cap on the gas tank loose. There is a vent on the cap and what you describe could happen if the gas cap cannot breathe.
Phil B (author)  Phil B11 months ago
If the gas cap cannot breathe either find a way to open it or get a new cap. You do not want to run the machine a long time with a loose cap.
Phil B (author)  palsouth11 months ago
My immediate thought is a restricted fuel supply, perhaps from water in the sediment bowl or a dirty fuel filter. I would remove and clean the sediment bowl. I would also remove the float and see if gas flows freely from the bottom of the carburetor.

You may want to make a photo of the carburetor linkage before taking any of it apart to assist in getting it back together correctly.

Also test run the engine without the air cleaner to see if that makes a difference. If so, replace the filter element.
chazmuffin1 year ago
Thanks for the ideas Phil. I checked all that you asked and tried to ride home expecting the motor to start but didn't. You know what it was? I could kick myself. I got the motor(XR-20T Weed Eater) after finding it in a crawl space and in pieces and it appeared that a dog had chewed off some of the plastic and plug wire. I repaired the plug wire and discarded the plastic. After being assured it would work I went on to make the repairs more permanent. I used a piece or rubber heater hose(no metal reinforcement) to cover the repaired high voltage wire. The hose must be partially conductive! I checked the plug and could visually see a spark but on a test ride, while I was reaching for the choke,i felt then saw a spark at my finger tips. Now it is covered with 100% silicone tubing instead. It's not as torquey as it was but a carb adjustment will take care of that. Amazing that a 16cc motor can push a man 150lbs, full back pack, and a 20lb bike. Before the troubles, the motor would get up to high idle and I didn't have to pedal for 2 blocks. Thanks Phil, all those tips were great. I was starting to think the rings were shot. After trying a few of your suggestions I saw the "short" and all is well in the world.

Thank you, thank you, thank you.
Phil B (author)  chazmuffin1 year ago
When you said gas ran out of the intake, I assumed the problem had to be in the carburetor. Ignition problems and carburetor problems sometimes fool you into thinking it is one when it is actually the other. I once worked on a 4-cycle mower engine for a friend. It would not start. A primary side wire near the ignition coil was shorting to the ground, but I could not immediately see where the insulation was worn away. An engine needs clean air, good spark, properly mixed fuel, and adequate compression to run. I usually check the air filter. If I cannot tell if the filter is dirty or not, I just remove the filter for testing. Looking at the spark plug will often tell if the engine is flooding. A little ether into the carburetor throat will also tell you if it was not getting gas. If it starts with the ether when it would not star before, chances are the engine was not getting gas. While I have the spark plug out, I usually ground the metal base to the engine with a wire and aligator clips. Clip the ignition secondary to the plug terminal. Turn on the ignition if there is an on/off switch and pull the starter cord. There should be a bright blue spark. A thin yellow spark is not good enough. Also, some plugs will show a spark with a test like this, but if the plug is old, it may not fire under compression.

I am glad you solved your problem. I do not feel like I was much help. You did a good job of observation leading to discovery.

Get More Out of Instructables

Already have an Account?


PDF Downloads
As a Pro member, you will gain access to download any Instructable in the PDF format. You also have the ability to customize your PDF download.

Upgrade to Pro today!