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

[palsouth]

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]

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]

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]

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.

[chazmuffin]

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]

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.

[chazmuffin]

Hi Phil B. Very informative post. My weed eater bike motor recently stopped working after a full week of perfect performance. I thought I had mixed old 2 stroke oil (in the proper ratio BTW) with gas, giving me my problems. After new gas and synthetic oil, I have the same problem. I will check to see if my "head" bolts are tightened down. Also, i read about a reed valve further in the posts. I recently blew out the carb and the engine ran well off the bike but I don't recall seeing a reed valve between the carb and crank case. Is this where it should be? I have inspected the exhaust and it is squeaky clean. No matter what I try (checked spark, compression, clogged exhaust, turned back mixture), I get gas dripping from the intake.

[Phil B]

The reed valve seals against backflow from the crankcase to the carburetor when the engine piston is on the downstroke. It is usually on the end of the crankcase opposite from the combustion cylinder. It is one or two pieces of very springy thin steel over a smooth surface around a hole or holes.

[Phil B]

Thank you for your comment and for looking at my Instructable. If you have gasoline dripping from the intake, it sounds to me like you may have a heavy float in the carburetor. I realize you may have a very small carburetor that does not use a float, but has a diaphragm. If there is no float, I would look carefully at the carburetor. Be sure the two halves of it seal tightly. Look at the diaphragm to be sure it is not too loose or torn. (I once had a Briggs and Stratton 4-cycle engine that uses a diaphragm in its carburetor. In time that diaphragm stretched a little. Just looking at it you would never guess there was anything wrong with it. But, the engine would not start for love or money. I worked on a few of those engines for friends, too. If the engine was four or five years old and I could find nothing wrong with it, I would just replace that diaphragm. Magically, the engine started just fine after replacing that diaphragm.) Look also at the needle valves, especially the one that is supposed to limit the amount of gas coming into the carburetor. It has a rubber tip on it. Look at its seat for dirt. Maybe, if there is no filter in the gas tank or gas line, you might want to install a filter to protect against fine pieces of dirt blocking a valve open so gas pours into the carburetor. I hope this helps. It is all I can think of now.

[joseph.kincaid80]

ok, i need help. I have a 85 honda nb50 aero scooter. Motor ran good till the other day, it just started slowing down then died. Havent been able to start motor since. Tried carb cleaner/starting fluild, no go. Replaced spark plug, used the ether, no go. Cleaned carb and all fuel/vacum lines, made sure i had spark, no go. Look inside, through reed valves which looked good, at the c. shaft, arm, and piston itself...no go. Went and got another s. plug on the one in a mil. chance i wasnt getting enough spark...no go. Now, after reading your article i'm gonna try to tighten crank case bolts, and then, if that doesnt do it, find gasket sheet and replace all the gaskets

[Phil B]

At 27 years of age, it is likely the engine gaskets are leaking. Certainly, your description of an engine that ran, but then slowed and died sounds like screws loosening and allowing air to enter the crankcase could be the fault. I hope it is working for you soon. Please report back when you have solved the problem.

[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?

[kd80project]

I have a KD 80 dirt bike ran great for years however after a divorce I stoped rideing it with my son so it has sat in my garge for ten years now. My son and I are starting a project together and thas to ride it again. We drained fuel the 10yrs ago so thas ok, we made up some new fuel mix,(the auto oil thing is pluged) we repalced the old fuel lines,new spark plug, new filter, it kicks over sounds like the compresstion is fine but it not start. We put it in 5th gear started running,poped clucth it started, whoo whoo the joy ! but real weakly and turned off. After running our selfs too my near death. (I am fat and out of shape) IT WILL NOT START ????? AND NEW a problem it seems like its in nutral even if not holding the clucth on. is it possiable we striped the gears push starting it ?

[Phil B]

You said you drained the fuel tank. I expect there could still be some old gasoline in the carburetor itself. The usual practice is to run the engine until dies after draining fuel. That is a more certain way to void the carburetor of all fuel. If there is still fuel in the carburetor, it has probably formed some varnishes and you may need to dismantle the carburetor to clean it.

As I mentioned, in ten years it is possible your engine gaskets no longer seal the crankcase as they should and the engine sucks air so that the fuel mixture is too lean to run the engine.

I do not know what kind of clutch you have, but it is always possible a key sheared or something else that effectively disengages the drive train. I expect you will simply need to take it apart and see what you find.

[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.