Curing 2-cycle Engine Problems




Posted in WorkshopCars

Introduction: Curing 2-cycle Engine Problems

About: I miss the days when magazines like Popular Mechanics had all sorts of DIY projects for making and repairing just about everything. I am enjoying posting things I have learned and done since I got my first ...

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.

Step 1: Clean the Exhaust Ports, Too.

Remove the muffler. The exhaust ports are under it. In time carbon residue from the burned oil in the fuel mixture begins to close off the exhaust ports. An engine is an air pump. What cannot get out blocks what needs to get in. Move the piston to the downstroke position (most distant position away from the spark plug) and knock off carbon deposits with a screwdriver. Turn the engine over by hand a few times to blow the carbon granules out of the engine.

Once I used our small 2-cycle garden tiller. After ten minutes the engine siezed and stopped. I had mixed 2-cycle oil with the gasoline, but it was old by the time I was using it. I found advice on the Internet that said to let the engine cool. Then use a wrench with a long handle to slowly turn the engine over. Turn in the same direction, not back and forth. That engine still works very well, although I am sure it suffered a little. Lesson learned: always use freshly mixed fuel.

When a gasoline engine of any kind will sit unused for any length of time, drain the fuel from the tank and run the engine until all remaining fuel in the system has been burned through the engine. This prevents gums and varnishes from closing off small openings.



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    5 Questions

    Hello there!
    I recently purchased a 86 Suzuki lt250r 2 stroke atv now when I bought the atv it ran good but you had keep giving it gas so I research it first thing I see is start it warm it up n adjust fuel/air screw to were it idles just right so that's what I did I noticed after I did that it did start to smoke more white smoke out of the exaust also there is oil coming out of exaust but I heard that was normal for a 2 stroke so anyway went to start it one day after riding it 2 days in a row wouldn't start pulled plug it was bad got new plug started right up yesterday went to start it today won't start pulled the plug black oily / maybe burnt gas smells like fuel more than oil on the end of spark plug. Any advice on what could of caused this and white smoke from exausy would be greatly appreciated all I am completely new to 2 strokes my first one.


    I am not a small engine mechanic. In this Instructsble I simoly offered some thngs I found that are not always discussed in troubleshooting charts and their cure.

    Here is a link to a troubleshooting chart for 2-cycle engines.

    It is not always a fast prccess, but explore possible solutions to problem descriptions that most closely describe your symptoms. Be sure you are using new clean fuel properly mixed for your engine.

    Hello so my inline trimmer,starts up idles perfect.but when the throttle is pulled it fires an gos even a couple times of pulling the throttle,youll think it's good to go,you go to start using it an then it starts to bogg down I think they call it.
    But only sometimes dies mostly when throttle is fully pulled it pulse's at a low idle,let go an it gos back to the regular idle setting....plz plz help


    I would not be surprised if you have an air leak in one of the gaskets due to a loose screw. Have you worked through the things in this Instructable?

    how to install uncaged needle bearings on 2 stage snow blower crank


    i do not inow. Check YouTube for a video on it.

    I have a champion generator. The plug is good. It fires fine and runs until it gets hot then bogs down. Any ideas?


    Does your carburetor have a float, or is it pulse carburetor using a diaphragm? If it uses a float, the float could have absorbed enough gasoline over the years that it has become hesvy. When the engine warms up the fuel becomes hotter and less buoyant. The float does not shut off the flow of gasoline when it should snd too much enters the engine, causing the engine to stumble. Also check to see if debris is blocking the air flow through the cowling and cooling fins. If these things are not your problem, do an Internet search for hot run problems in a 2-cycle engine.

    I have a 2 cycle back pack leaf blower. It starts up ok when the choke is closed and runs half choke, but when I open it up all the way the engine bogs down and shuts off.


    The answer to your problem is in the Introduction above. You have an air leak, probably from a poirly seated gasket. Choking the engine reduces the amount of air entering through the carburetor so the mixture is more nearly what it should be due to extra air entering through leaks. Tighten carburetor mounting screws and crankcase screws. Replace gaskets if tightening screws does not solve the problem.


    Chainsaw engine - I pull the starter and only get one pop, like the egine won't turn over freely. Starter rope OK, took off air filter, took off muffler - same. Took out spark plug, turns over fine.

    Any suggestions? Thanks, Mark

    1 reply

    Did you check through the engine for problems I mentioned, like air leaks? It sounds like the engine is not getting the right fuel at the right mixture in the right amount. Several things could cause that. Those include air not getting back into the gas tank through the vent in the cap, an obstructed fuel filter, a sticking float (although you likely have a pulsed carburetor with no float), proper mixture screw settings, and no air leaks at the carburetor and engine gaskets.

    you posess some excellent knowledge. I am new to this site and I had a question to

    ask of you but I am not sure how that is done on this. If you could indulge me and tell me how to do this I'd appreciate it.

    1 reply

    Thank you. In my lifetime I have had many problems to solve and little money to pay others to solve them for me. The resolution was to learn what I could and solve them myself. The great thing about Instructables is that we can all share things we have learned or developed ourselves.

    Many people ask questions in the comment section just as you did. Others click on the member name and send a private message from the member profile page.

    If your question is related to a problem with a motor, check the previous comments to see if it has already been discussed. Otherwise, I am not a professional mechanic. I posted something fairly simple about a couple of problems I had that were almost never discussed. In the old days we bought manuals with troubleshooting charts and worked our way through the charts until we solved the problem. That is still a very good approach, except those charts are now on the Internet and accessible free of charge. I linked one in several comments below.



    My leaf blower works fine when sitting on the ground when I pick it up and move around swinging side to side or up-and-down it bogs down and died any ideas

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    1 reply

    When it is on the ground the engine is probably at idle, but when you pick it up and swing it around, it is probably with more throttle. Is that correct? If so, swinging it around is not the only thing that is happening. The high side of the carburetor may not be properly adjusted, but the low side is. Look for the letters L and H by two mixture adjustment screws. L is usually open 1 1/2 turns from fully seated and H is usually open 1 turn from fully seated. Those are starting points. Fine tune it after that. It may also be that you have an air leak in the fuel system and the engine is experiencing fuel starvation when you apply throttle. Check screws for looseness. Replace the gaskets if more than ten years old.

    I have a style 2stroke strimmer it in runs great but only when air filter taken out any reason for this ?

    1 reply

    The engine is not getting enough air when the air filter is in place. Removing the filte increases the air flow. I am assuming you have a filter that uses a pleated paper element. (Some engines in the past used an oil bath filter in which the air had to pass through a reservoir of oil, which trapped the dirt.) Try replacing the filter with a new one. Conceivably you might have too much fuel getting into the engine because of a rich mixture or a heavy float in the carburetor (unless your carburetor uses a pulsed diaphragm rather than a float). In that case removing the air filter also manages to restore the proper air to fuel ratio. If it has been quit a while since you replaced the filter, try that. (Some air filters are a piece of foam soaked in oil. Those also need to be cleaned in soapy water or a solvent and oiled. Drip some oil onto the foam and squeeze it in your hand a couple of times to spread the oil out through the filter.)

    i have a 2 stroke engine mounted on my bike and its supposed to run on 16:1 fuel ratio, but i put 50:1 in it, and now my back wheel its connected to wont spin, (btw its a bump start) what should i do to fix it?

    1 reply

    I am not trained and have not been employed as a mechanic. I did once run a 2-cycle engine with properly mixed fuel that had become too old. In your case, there was not enough oil in the fuel mixture for the engine's requirements. The two situations are roughly the same. My engine seized up after a few minutes. I did some searching on the Internet. The solution was to squirt a teaspoon of thin oil into the spark plug hole and turn the engine slowly in one direction only with a long wrench on the flywheel nut until it broke free. It is not a guaranteed cure. In my case, it did work. It depends on how much damage was done. My engine seemed to work normally after it broke free.

    I have a 2 cycle engine mounted on my bicycle. Its made to be bump
    started. With stock parts it ran fine. I've driven it over 20 miles.
    Now I've installed upgrades and it starts up just fine, but it
    immediately dies if I'm not going downhill, regardless of whether I'm
    using the throttle or not. After trying to start it multiple times, I
    took the spark plug out and noticed that it had liquid fuel on it. The
    exhaust ports are clean. Do you have any tips for fixing this?

    1 reply

    First, I have never been trained or employed as a mechanic. A wet plug indicates too much fuel is reaching the combustion chamber. Could your carburetor have a sticking or a heavy float? ("Heavy" is relative. I had a carburetor with a plastic block float. After 7 years it had absorbed enough gasoline to be "heavy," even though it still felt very light in my hand.)

    I have an old two stroke Echo engine that won't fire at all, it has no build up, has a new spark plug, and I just cleaned the carb and replaced the fuel line, what could be my issue,(the carb works, it was tested on a similar engine)

    1 reply

    I am not a mechanic, nor have I ever been. I tried to list some things in this Instructable that are not usually mentioned in the available troubleshooting guides, but I encountered them by experience. Those may be the solution to your problem, or not. For an engine to run it needs the correct mixture of air with clean fuel, compression, and spark. All sorts of things can interfere, like a blocked vent in the gas tank cap, a dirty fuel filter or air filter, gasket leaks that allow too much air into the mixture. In response to some of the comments below I did list a link to a troubleshooting guide. If your engine is old, there is a strong chance the crankcase gaskets are not sealing and are allowing too much air into the fuel mixture.

    hi phil

    i have an old chain saw motor that i have just had given me changed the plug and it will run with no problems for about 10min then it won't start. could this be because the motor was made to run on leaded petrol and we in the uk are now on unleaded petrol or is it something eles. i only want to use it to smoke rats out for the terriers

    2 replies

    You said it is old. My string trimmer shown in the photos behaved similarly. New gaskets all around solved it for me.

    thanks phil will try it

    hi phil

    I have a fs55 strimmer which won't start. It fires once through the carboretor. It all looks fine inside - no damage. New coil + new plug and not blockages as would be expected in the exhaust. Any ideas ?

    2 replies

    Is the spark plug wet or dry after it has fired and quit? How many years old is the machine? Have you tightened all carburetor and crankcase screws? Are the mixture screws properly set? Is the air filter clean?

    Hi Phil

    Thank you for responding so fast.

    I read all your advice ( great advice that is nowhere else on the web ) and have the machine in pieces as i really expected to find an obvious fault but NO ! lol

    I bought a couple of a guy that tried and failed to fix it!
    but this is the newest of the the two. You know the internet is very thin on this paticular problem. Now its guts are hanging out i will order a gasket kit and some silicon / clean and puit it back together but i can understand how its misfiring into the carb like that.
    You see my initial thought was the flywheel setting had been snapped off as i had this on a chiansaw recently which was my cocky mistake and i'm suprised you never suggested that as the primary suspect. I will return when i have done the above and let you know what came of it ..
    Cheers phil!