Small Engine Runs Rough

[frank1492]

This 40-year old Tecumseh has just had a new carburetor, so the carburetor is pretty much ruled out. The plug is brand new, and I just replaced the air filter. When *not* under load, and run at medium to high speed, the engine runs unevenly. The misfires are somewhat random. The engine rarely dies. I would say there is rarely more than one firing failure in an "episode," and when the engine next fires, it emits a puff of smoke, signifying a rich condition. Then it will run fine for a few more seconds before another "episode." I have not checked the points, but have bought a new set and a new condenser. I have not checked the head bolts for tightness, nor the condition of the shut-off grounding wire. Have not lubricated or checked the governor arm. The muffler leaks slightly. Could someone please indicate where they would go next, then where, then where, etc.? Of course I would like to avoid pulling the head. Help much appreciated! Frank

[Bob]

I was wondering about the compression. I don't mean with a compression checker, how does it feel when pulling the starter rope? If this is a vertical cylinder, compression can be improved by removing the plug, setting piston at max down stroke and pouring about 1/3 cup of paint thinner into the cylinder over night. Change oil next day. Works like a miracle.

Bob-tx

[frank1492]

A good thought but I don't believe that's the case.There is a good solid feel when the starter cord is pulled- plenty of resistance. Thanks anyway though!

[Jack]

You may have a weak mag on a 40 yr old engine. Tecumseh are pron for such. Check out how strong it is.

[Straw Man]

Replace points and condenser then see how it runs. Might want to snag a compression gauge and check that. Depending on frequency of usage, 40 years is a long time to go without some kind of valve and or cylinder/ring/piston work in a splash lubricated engine. Unless you notice some weeping where the head meets the cylinder, don't bother with the head bolts. Make sure that the carb and manifold are properly seated to the intake. Did you get all the old gasket material off the intake?

[Oren]

The magneto contacts were one guess for me. The contacts become pitted over time an need cleaning... replacement after 40 years :-) A weak/failing magneto might act like this.

First check the mixture screw for adjustment...imo.

-- Oren

"If things get any worse, I'll have to ask you to stop helping me."

[frank1492]

Thanks to all for your excellent advice. The points were badly burned. Replaced these and the condenser. Engine starts and runs fine now. Idles down to practically nothing! (All of which is very remarkable since I guessed the points gap!) Again thank you! Frank P.S. The new carb I installed did not have a fitting for the crankcase breather tube, which currently is left dangling. Can anyone suggest what to do with the breather? (I assume plugging it is not an option.) I could rig a port just inside the air filter if that's a good place.

[frank1492]

The engine appears to have good compression, based on the pattern of resistance which I encounter when trying to start it. No weeping at the head. No gasket material was stuck to any surface.