Introduction: 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.

Step 1: Clean the Exhaust Ports, Too.

Picture of 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.


fred2strk (author)2017-10-12

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.

Phil B (author)fred2strk2017-10-13

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.


5949 (author)2017-09-04

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

Phil B (author)59492017-09-05

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.

Jkshartley (author)2017-08-09

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

Phil B (author)Jkshartley2017-08-09

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.)

Nathan da dude (author)2017-06-20

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?

Phil B (author)Nathan da dude2017-06-20

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.

ThomasG221 (author)2017-05-15

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?

Phil B (author)ThomasG2212017-05-18

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.)

ThisGuyLovesToBuild (author)2017-04-27

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)

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.

chrisrat (author)2017-03-27

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

Phil B (author)chrisrat 2017-03-27

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

chrisrat (author)Phil B2017-03-27

thanks phil will try it

spunx (author)2017-03-26

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 ?

Phil B (author)spunx2017-03-27

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?

spunx (author)Phil B2017-03-27

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!

Phil B (author)spunx2017-03-27


I have never been employed as a mechanic. I simply worked on my own machines. Once someone brought a chainsaw to me that one or two others had not been able to start. I discovered carburetor mounting screws were loose a quarter of a turn from vibration, and it started after tightening them. Our string trimmer began to stall and need continual carburetor adjustments at ten years old. I theorized the crankcase gaskets were no longer supple enough to seal and the problems went away when I installed all new gaskets. A mower lost lots of power when I discovered exhaust ports were partially blocked with carbon deposits. Troubleshooting guides I had did not discuss these things, yet they seem more than likely to become problems. So, I posted a simple Instructable about these three things. Now all sorts of people want me to diagnose their engine problems without seeing the engines. Sometimes I can make a suggestion and it works. But, I am much better on problems I have had than one I may never have experienced. And, I no longer own anything with a small engine.

zak1982 (author)2016-12-08

hey all my 71 cc scooter starts idles but mont move when i accelarate any ideas what it may be ? if i lift back tyre of the ground it goes when i put it down an accelarate it just revs upbut wont go forward ???

Phil B (author)zak19822016-12-08

It sounds as if your engine is running as it should. You simply are not getting power through the drive train to the wheel. Is the centrifugal clutch engaging? Is there any kind of locking pin that has sheared? You will probably need to remove shields and follow the drive train to see what is not engaging and what you can do about it.

zak1982 (author)Phil B2016-12-09

i dont no if its engaging iam only new to this stuff anything i should lookout for ? if i lift the back tyre up of the ground it spins like mad if i sit it down and accelerate it doesnt move

Phil B (author)zak19822016-12-09

Thank you for the photos. It appears you have the commonly used drive system, which is a centrifugal clutch on the engine shaft, a sprocket on the drive wheel, and roller chain to connect the two. A centrifugal or speed clutch contains weighted sections held inward toward the shaft by springs. When the speed of the engine reaches a pre-determined threshold, the weights overcome the force of the springs and move outward to a bell-shaped housing that has a small chain sprocket on it. The centrifugal clutch is probably about the only thing that can go wrong. The clutch weights might be stuck with rust or corrosion so they do not engage the bell housing, or the bell housing might have an oily or greasy coating that causes it to slip on the weights, or the weights could be excessively worn and not engaging the bell housing. A new centrifugal clutch is not terribly expensive. Make certain a replacement fits the same shaft size and locks onto the shaft in the same manner as the original, if you decide to replace the clutch,

mikealmand2 (author)2016-11-29

I put the wrong ratio of oil to gas in my Ryobi backpack blower. I was using it and it started to bog down and then quit altogether. I drained all the gas/oil mixture, added the correct ratio. I've been unable to crank it. Any suggestions?

Phil B (author)mikealmand22016-11-29

I had a similar accident with a small garden tiller. I went searching on the Internet. What I found suggested removing the spark plug and putting about a teaspoon of oil into the combustion chamber. Then put a socket wrench with a long bar on the flywheel nut. Turn the engine slowly in one direction only until it is free. Put the spark plug back in, put back any parts you removed, and try starting it. In my case, the engine started and ran fine. I am sure I did not do it any favors and caused some extra wear. We sold the house where we lived at the time and left that tiller with the new homeowner. I cannot say if it is still working, but expect it is.

JerryG64 (author)2016-11-15

My son's two stroke 4 wheeler will run fine for a while and when he shuts it off, it won't restart for a while

Phil B (author)JerryG642016-11-16

Here is a link to the problem you describe, but with 4-cycle engines. Still, some things would still be applicable. As it mentions, heat causes things to move. In the case of your 2-cycle engine, a mating surface may be opening and letting in too much air. Check all screws and crankcase bolts for tightness, assuming the gaskets are still good. Is the a good spark you can see when you turn the engine over during the time it will not start? Remove the plug wire and set up a small gap to a grounded wire, maybe with another spark plug. I seem to remember hot start problems can be caused by fuel percolation problems. Is there any reason why the fuel system would have overheated and caused a vapor lock? As the article notes, there could be several different causes.

cable860 (author)2016-11-12

Hi, I have a Craftsman 5 HP. Tecumseh engine snow blower. It wouldn't start this year so I cleaned out the carb, changed the fuel, filter and all brand new gas. After priming it the engine starts for a few seconds then dies. I keep pumping the primer as it starts to die and that doesn't do anything to help it keep running. I eventually noticed the muffler was missing a bolt and was not secured and it was damaged pretty good. I'm getting one off Ebay. Would the muffler have anything to do with my issue of getting it to run for more than a few seconds?

Phil B (author)cable8602016-11-12

Is that a 2-cycle or a 4-cycle engine? If it runs a short time and then dies, that sounds like fuel starvation. You would need to look for anything that could restrict fuel flow or cause the mixture entering the combustion chamber to be too, too lean.

cable860 (author)Phil B2016-11-13

Thank you for responding. It is a 2 cycle. The blower sat for 2 summers and 1 winter without being used. I changed everything from the tank to the carb. I pulled the bowl and cleaned it and the float and needle and sprayed what I could get at without pulling the carb. The carb is probably gummed up. I was trying not to have to pull the carb. I probably need to pull it and clean the whole carb. There's a hole where the bolt that holds the bowl in place that has an opening. Is there a part in there that needs to be cleaned that could be causing this?

Phil B (author)cable8602016-11-13

I do not work on engines regularly, and do not now own anything with a small engine on it. There was a time when I had to keep a couple of small engines running,

If the carburetor sat for a couple of years with old fuel in it, chances are real good something is gummed up. Then there is also the total age of the machine to consider. A couple of people here had older machines. Ten years seems to be a tipping point. At that age, gaskets, and possibly also crankshaft seals no longer seal. Air leaks into the crankcase and makes the mixture far too lean for the engine to run. But, those usually do not start and run for a short period. They just do not start, except with starting fluid. Then they stumble and die.

A lot depends on what type of carburetor your engine has. I know Briggs & Stratton used a pulse carburetor with a diaphragm, but that is usually on 3.5 HP engines and 4-cycle. In time the diaphragm is no longer taut enough to function and needs to be replaced.

Some carburetors have a float. Those can be come "heavy" by a few grams and they do not shut the gas supply off when they should. But, you see moist gas on the spark plug or running out of the engine and the exhaust smells like fresh gas, if it runs at all.

I mentioned fuel starvation. That can happen when the vent cap no longer lets air back into the gas tank because the little vent is plugged. But, the engine usually runs for a few minutes before stumbling and dying. You can test the engine by loosening the cap just a bit to see if it no longer stumbles. The air cleaner can be restricted. The fuel filter (if it has one, usually in-line between the tank and the carburetor) could be restricted. It is good maintenance to replace fuel filters periodically, anyway.

Most small engine carburetors are fairly simple, but there can always be small passageways that are blocked with gums and varnishes from old gasoline.

And, a main point in this Instructable is that (as above) gaskets are assumed to be tight and sealing, but a mounting screw even one-quarter of a turn loose lets in air that makes the mixture lean. Usually those keep the engine from starting at all, though.

You could get some carburetor cleaner and spray into the engine. Let it soak. Then blow it out or chase it with starting fluid to clean it out. You will probably need to do this a few times to open up whatever is gummed up.

As you likely know now, it is a good idea to run the last bit of gasoline out of the machine at the end of the season. I tried to do that with our mower, leaf blower, string trimmer, and garden tiller. When I did that I never had any trouble at the beginning of the next season. (We no longer have any of those machines.)

If you cannot find the proper gaskets, you can often trace and cut your own from a gasket material you would get at an auto parts store. A mower repair shop can probably get the right gaskets for your if you need some.

cable860 (author)Phil B2016-11-13

I totally thank you for all your help and advice. I found a muffler locally through Craigslist extremely cheap compared to Ebay. I originally changed the fuel, fuel line, filter, spark plug and sprayed carb cleaner where I could and still no results. This time I found more ports and openings that needed cleaning and I double dosed cleaned them. Everything runs great now!! Never again will I leave old gas in there. Thank you again!!


Phil B (author)cable8602016-11-13

Thank you for the good report. I love success stories. It is a bit of a pain to run the engine at the end of the season until the gas is gone, even if you make your best effort to drain all you can from the tank first. It seems so wasteful, but it is worth it. I am glad you got it without too much difficulty. Now, for your sake, I hope you do not have a lot of snow and really do not need to use the blower!

cable860 (author)Phil B2016-11-13

I'm sure i'll regret this but when it ran so good and the satisfaction I felt when it was complete I actually was hoping to get to use it soon. I'm sure after one time i'll be praying for spring.

Phil B (author)cable8602016-11-13

I am not aware of your location. I spent eight years west of Cleveland, Ohio. We did not get the heavy lake effect snows they got on the east side, but it was still more than enough snow. I was the pastor of a church. The church owned a Gravely 4-cycle tractor with mower and a snow blower attachments. It was parked in a lean-to next to our garage. I did the responsible thing and made certain the engine oil and filter were fresh and the 2 volt battery was charged. I figured I needed to keep the "L"-shaped driveway around our house clear after a storm so I could get to the hospital or someone's house if there were ever an emergency and people wanted the pastor right now. We also had a preschool with a church parking lot that ran between two streets a block apart. The preschool needed to have two wheel tracks across the parking lot cleared so parents could enter from one street, drop or pick up kids, and exit on the other street. After a snow, I was the one on the scene who, by default, made those things happen. I was glad to have such good equipment and did not mind the maintenance work to keep it ready too much. The clearing of the snow, especially ice crystals blowing back into my face did not thrill me.

Mills-boy12 (author)2016-11-12

I'm a yz owner and at the track I've accidentally mixed too much two stroke with petrol and the bike was having problems starting please get back with advice please!

Phil B (author)Mills-boy122016-11-12

I can only guess. Have you checked the spark plug for oil fouling? The exhaust ports from the cylinder head to the exhaust pipe and muffler could be partially occluded from carbon deposits, depending on how long it ran with too much 2-cycle oil in the fuel mixture.

cable860 (author)2016-11-12

Also I haven't changed the plug yet. I bought the wrong one. I need to take it back tomorrow and exchange it.

JboyA (author)2016-10-05

Good day mr. Phil! I have this honda dio 2 stroke 50cc engine, it stocked in a very long time, i would like to restore it, so i cleaned the carburetor change the cdi,ignition coil, sparkplug and statorcoil, i change the oil in the oil tank and a fresh fuel, so it has a fresh fuel,full compression, good spark, and propper lubrication, but it wouldnt start i change the cylinder gasket but it wouldnt start the engine, i am so confused right now what should i do? Thanks!

Phil B (author)JboyA2016-10-05

My engine would no longer start ten years after I bought it. Gaskets between the halves of the crankcase and between the carburetor and the crankcase hardened from age and no longer sealed. When I changed all of the gaskets the engine ran again.

JboyA (author)Phil B2016-10-05

So i must overhaul the engine and replace all the gasket

Phil B (author)JboyA2016-10-07

If the engine is more than ten years old, I would definitely replace all gaskets. I think it is very likely the cure to your problem. All you need for the engine not to start is just a little too much air getting into the crankcase and making the mixture too lean. Gaskets hardened from age make that very possible. I bought some gasket material and made my own gaskets.

JboyA (author)Phil B2016-10-07

Thank you mr. Phil. My engine is running very smooth after i replaced all the gasket. Thank you very much sir and god bless!

Phil B (author)JboyA2016-10-07

I am very happy for you. Thank you for the report. Repair manuals do not mention old gaskets that no longer seal properly. I suppose you are not expected to keep your engine, but to buy a new one.

katozd (author)2016-10-07

hi, i have a craftsman leaf blower (358797310) that i just changed the spark arrestor gaskets on. now when i turn it on, it blows blue smoke and has a high pitch noise.

Phil B (author)katozd2016-10-07

I do not have any experience with what you are asking. Can you adjust the carburetor to reduce or eliminate the blue smoke? Is the high pitch noise mechanical, like a dry bearing, or a whistling, like air rushing through a confined space?

katozd (author)Phil B2016-10-07

Its a whistling noise. I will go ahead and try to adjust the carburetor for the blue smoke.

Phil B (author)katozd2016-10-07

Is there any possibility the gasket can be installed upside down? I know that is impossible in many applications, but I have heard of it happening in other applications. A gasket upside down could plug some passageways and open others, which might make a whistling noise.

0waldogg0. (author)2016-09-28

Hi there. I have a 2 Stroke Tecumseh 3.5 mower. I loaned it to somebody who then put straight gasoline into it with no oil and then called to say its not running properly. I drained it out when I got it back and refuelled with proper mixture. Now when I use full throttle it starts sounding like a Helicopter and starts billowing out white smoke. Any ideas where I might start looking for the problem? I am a newbie when it comes to 2 stroke engines. Many Thanks

Phil B (author)0waldogg0.2016-09-29

I am sorry to hear of your engine problem. I would not lend my things to that friend again, unless the friend offers to pay for any damage to your engine caused by his error.

4-cycle engines have oil in the crankcase to lubricate moving parts, especially the contact between the piston rings and the cylinder walls. 2-cycle engines get their lubrication from the special oil mixed with the gasoline. When that oil is not present in the fuel in the proper mixture, the danger is that destructive wear will occur in the engine quickly. A compression check would give a strong clue about the extent of damage. Once damage happens, there is no economical way to restore the engine. You could check on the cost of honing the cylinder walls and installing new piston rings, but it may be cheaper to buy a new engine. If you choose to buy a new engine, look into a short block. It is the bulk on the engine and contains all moving parts, but does not include still useful things like the engine head, the ignition parts, and the carburetor. You would need to install those on the short block with new gaskets where parts need to seal.

One choice you have would be to use the engine as is, but not use wide open speeds at which you get the visible smoke. If there is scoring on the cylinder walls, the looseness it causes with the piston could cause more wear faster than normal.

You could remove the head for a visual inspection of the cylinder walls, but will need to install a new head gasket and torque the bolts to the proper amount. You or a repair shop may have a fiber optics device that allows you to peek inside through the hole for the spark plug.

I wish I had better news for you.

About This Instructable




Bio: 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 ... More »
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