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.

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.

Comments

[BIGHAIRYDUDE]

hey i got a weedwhacker on my bike it ran good a while ago but now it only runs on half choke, there is barley any power when you go full throttle and the engine rpms are slow and it will bog down if you gun it from the start any ideas?

[Phil B]

I am sorry, but I did not see your post earlier. The choke makes the mixture richer. If it runs at half-choke, your engine is not getting enough fuel or it is getting too much air. Is there a fuel filter that could be dirty? Are there any obstructions in the carburetor or water in the fuel system? Are there any tiny air leakages in the crankcase? It has been a long while since I have worked on small engines with any regularity.

[thekorgoroth]

I have a liberty 50cc piaggio scooter, after a serious blow the carburator base suffered really badly,the engine started to uncontrollably accelerate on its own due to the extra air that was coming in the system and also the bike wouldn't reach its maximum torque, so i replaced it.
No more acceleration but now the bike sometimes is as before or still wont give me its full power i used 100 octane gasoline (worked fine) after 3 refills with normal unleaded fuel it wont rev past 2000 rpm...

and if i keep the throttle down it stop me and my mechanic have absolutely no idea what is going on.

any thoughts?

[Phil B]

This is just a thought. My brother-in-law had a small motorcycle several decades back when I was first married, and it would not develop full power. The centrifugal fly weights on the automatic spark advance were stuck. He removed the cover over them and used some penetrating oil to help loosen them up along with working them by hand. Suddenly it was a much different, more powerful engine.

Engine problems can be confusing; but, every engine needs good compression, good spark delivered at just the right time, clean fuel mixed in the right proportion with clean air. In the case of a two-cycle engine, the crankcase must provide a good seal from outside air, the reed valve must seal properly, and the exhaust ports must allow free movement of gases without any choking or restriction. I know saying these things is a little like telling someone in financial difficulties that the problem is either taking in too little or spending too much. But, a careful examination of each (air, fuel, spark, and compression, etc.) will usually locate the problem eventually.

Thanks for looking at this and for commenting. I hope you are able to resolve the problem soon.

[dartmouthguy]

H, Phil- ( or WIMC) I rescued a small Tecumseh tiller from the dumd and and got it running nicely, but it's loud. I removed the muffler and there's nothing inside. The case is perfect. Can I 'pack' the insides of the muffler case with something non-flammable and effective, like insulation or SS steel wool? I know one absolutely must not run a 2-stroke w/o the muffler - back pressure and all that.

[wobbler]

Not too sure stainless steel wool won't catch fire. Normal steel wool burns easily, especially in an air flow and only takes a spark to start it. Then it burns really hot.

[gunguru94]

runing it with out a muffler will not hurt it. back pressure (if there is a lot of it) does nothing to the engine but rob power so it will not hurt it to run with no muffler  ( it will just be loud) like some motorcycles have straight pipes which have very little back pressure but are also loud. If you do use fiberglass be careful to keep an eye on it because if to much carben builds up on it or if the muffler gets allmost pluged it can burn up the exhaust side of the piston...
I've worked in a small engine shop since i was like 7 .

[dirtbiker809]

i beg to differ. back pressure may not be as vital in four stroke engines, but it is for two strokes in order to scavenge gases. without a properly tuned exhaust, your engine will run like crap because without back pressure more unburnt fuel is being wasted.

[LiquidLightning]

It's not back pressure you want, its negative sound waves. When the exhaust pulse hits a change in diameter in the pipe, the sonic wave changes direction. It has nothing to do with restriction.

[jsadler1]

You are correct. In fact in a two stroke engine the exhaust system acts as if it is a cam shaft. Degree and velocity of exhaust determine the volume and velocity of the intake charge. When power is the only consideration expansion chambers are used in two stroke engines. What tends to occur is that at a very narrow rpm range great power can be made but at all other rpms power is less than if a more restrictive system is used. Since almost all engine applications involve running at various throttle openings almost all uses require a muffler type system rather than expansion chambers.

[dartmouthguy]

Thanks, gunguru, & everyone who replied.
I bought a new muffler for this bugger and guess what, thw new muffler has notthing inside it either! Ya live - ya loin..... And it's still God-Awful loud!!!
On a 2-stroker, there's a reed instead of a valve on the intake side of the cylinder which can go south w/o back pressure, or so I was told more than a few years ago.
The thing won't till, though. I think the worm gear inside the wee differential has definitely gone South - a project for the Winter
Happiest of Thanksgivings to all of you, and I'm hopeful everyone will have somewhere to go for this special American day.

[wobbler]

Not all two strokes use reed valves, some just use ports with the position of the piston effectively acting as the valve as it covers them.

[Phil B]

I am hardly expert on this. I know some mufflers come packed (not too tightly) with fiberglass wool like you find in home insulation. That should be easy to find. I would try either that or go to a small engine shop and see if they could fit the engine with a new muffler for only a little money.

[dartmouthguy]

Thank you, Phil- I'll stuff some rock-wool or somesuch into it and go from there. If it doesn't work for long I'll buy a muffler. I wanted to ultimately swap it for a scooter. God Bless

[Phil B]

Thanks. And, may God bless you, also!

[wobbler]

I always use Red Hematite when I make up any seals like this. You can even use it as a joining compound where there's no gasket although, I'd want the faces to be pretty flat anyway. It's totally heat resistant and I even use it on head gaskets. It also helps to keep gaskets temporarily in place so they don't shift as you're mating the two parts together.

[satan_poop]

I have a few of the same weed whackers do you know the size of the engine on this one?I make gopeds/mopeds here is the link for my video http://www.youtube.com/watch?v=fDlUUScQuwI

[Phil B]

I am sorry, but I do not know the displacement of the engine. It was purchased in 1989 and is the only size available then.

[satan_poop]

is the high screw on yours covered with white silicon?

[Phil B]

I do not remember the high idle screw being covered with silicone, white or otherwise. It was always a pain to remove the air cleaner cover/choke housing in order to adjust the idle screws. That improved after I realized my gaskets were leaking and I replaced them with fresh gaskets.