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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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.75 days 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.
Avtrill9 days 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.malone15 days 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.
wheelz69619 days 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.

Phil B (author)  wheelz69619 days 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)  wheelz69618 days 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.

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)  wheelz69617 days 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.

avaella20081 month 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.

avaella20081 month 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)  avaella20081 month 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.
tezwarby772 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 .

Phil B (author) 4 months ago
Tezwarby77, adding extra fuel to the mixture and choking the carb. air flow means you have extra air leaking into the engine someplace. Look for loose screws and hardened gaskets that no longer seal.
tezwarby77 Phil B4 months ago

Thanks Phil it looks like I have a crankcase seal gone on the clutch side of the engine so I will replace both sides.

Thanks Terry.

Phil B (author)  tezwarby774 months ago

There was a time when people would bring an engine to me and ask me to get it running. It is a new experience to troubleshoot engines I have not even seen. Thank you for reporting back and letting me know what the problem was. It makes sense. I think you will be pleased with the engine's response when you get the new seals in place and the air leaks are gone.
who8dapple4 months ago

I followed your advice and tried some starting fluid (2-3 second blast) about a half dozen pulls to no avail...took out the Spark Plug for a look see. It was pretty wet, so certainly getting fuel. At this point, I decided to dry it out with a little butane torch and gave the cylinder case the same treatment for approximately 30 seconds. Reassembled and managed to start it with about 5 pulls...(no priming, just full throttle) I think heating it up was just a fluke...i think it was getting flooded!

I ran it for about 10-12 minutes at full throttle and noticed how much fuel it was consuming...quite a bit more than normal. I did a few cool down restarts 15, 30 and eventually 60 minutes apart. It started up within 3-4 pulls with no priming and just full throttle. My guess, 3-4 push of the bulb is enough to flood it. Ryobi manual does suggest 10x for a cold start, and i've never done more than 4-5x in the past...

Now to figure out how to adjust the carb without the special tool.

Big thanks for all your helpful suggestions Phil!

who8dapple4 months ago

Need Help...Ryobi CS26 String Trimmer, about 2-3 yrs old. Briefly started 2 weeks ago, before i did the following; Put in new Carburetor and Carb Gasket, Fuel Filter, Fuel and Air Line, New Spark Plug (Sparking) fresh mixed gas. I've attempted to start it more than a half dozen times, and can't get it to start...any suggestions would be appreciated!

Phil B (author)  who8dapple4 months ago
With problems of all sorts it is good to ask, "What was the last thing that happened before it quit working?" Although every engine needs spark, compression, and clean air in proper mixture with gasoline; chances are your problems are related to something you did to the fuel system. Are the mixture screws set properly on the carburetor? The "L" (low) screw should be about a turn and a half backed out from fully seated. The "H" (high) screw should be about one full turn out from fully seated. Make certain the air filter is clean, itself, and not restricted in any way that impedes the flow of air. Remove the fuel line from the carburetor. Gasoline should flow fairly freely from the fuel line. Remove the spark plug. Is it clean, or caked with carbon deposits? Wrap a bare copper wire tightly around the body of the spark plug and connect it firmly to the metal frame of the engine. Pull the cord rapidly a couple of times with the engine kill switch open so it does not short the spark voltage to ground. There should be.a nice blue spark. If the plug is more than a year old, replace it. An old plug can look very good, but it fails to spark when under normal engine compression. Was the plug wet with gasoline when you removed it? If so, that is evidence the engine is flooding. If the plug was dry when it was removed, try putting the plug back into the engine and torque it properly for a good seal. Then spray ether starting fluid into the air cleaner opening and pull the rope. Did the engine fire or run? A very dry plug likely indicates fuel starvation, which could be caused by an air leak or by improper carburetor adjustment, or by a blockage in the carburetor. The compression on a small engine is not always easy to measure with a compression testing gauge. Often you try to spin the engine by hand, and the compression causes the crankshaft to stop and bounce in reverse direction, if it meets minimum standards.

I am curious why you decided to change so many parts of the fuel system on a machine that is only about three years old.

I hope something her helps.
who8dapple Phil B4 months ago

Hi Phil, thank you for your response! The unit was running rough at the end of last season, and I did manage to start it 2 weeks ago while cleaning it with carb cleaner. This particular model does not have a carb rebuild kit available, hence purchasing a new one for $22 didn't seem too outrageous. I cleaned the (basic) Air Filter, Checked the Exhaust for Clogs/Carbon build up, replaced the Priming Bulb, Fuel and Air Line, and also changed the Spark Plug.

I did check the plug for spark, and it produced a nice blue spark. I mixed fresh fuel with a 50:1 mix, but the only thing that was new to the equation was the addition of Ethanol Stabilizer as per recomendation from the guys at HD (possible culprit?).

I will get a can of Starting Fluid to see if that may help!

Thanks Kindly

RY28020.MOV(402x714) 23 KB
Phil B (author)  who8dapple4 months ago

My son-in-law has a Husqvarna chain saw. He always gets gasoline made without ethanol to use in it. The starting instructions say to apply full choke and squeeze the primer bulb five times. Then pull the cord five times. Open the choke and continue to pull the rope as needed. But, in my experience, the engine fires and dies after about three pulls of the cord. Then it starts and continues to run on the next pull. That is from a full cold start.

who8dapple4 months ago

Oh, i forgot to add that I installed a new primer bulb as well!

Phil B (author)  who8dapple4 months ago

You should not need 50 pulls of the starter cord to make the engine run. I would try some ether starter fluid into the air intake before the first pull and see if the engine runs or at least fires. I would also want to remove the plug after 5 to 10 pulls and see if it shows evidence of flooding or fuel starvation.

who8dapple4 months ago finally started after approximately 50 consecutive pulls. It ran great and idled well. Did a few hot starts in 2-3 pulls, did a cool down for 45 mins and started right up. But, after about 1 1/2 hrs cooling, it will not start up again. Can I upgrade to a hotter spark plug, and would that help?

tezwarby774 months ago

Hi I have a Yamaha G1 golf buggy with a 215cc 2-cycle engine, it will not run without the choke on, I have richen it up on the carb screw as much as I can, it has a new plug in , the filters are good . Can you give me any idears please

sean72399 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)  sean72399 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.
sean72399 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)  sean72399 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.
TexasBuilder10 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)  TexasBuilder10 months ago
Thank you. That should be helpful to many. I had never considered what you described.
botrys11 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)  botrys11 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.
botrys11 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)  botrys11 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.
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)  Redneck03651 year 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.
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