I have a problem with a Tecumseh 3.5 HP lawn mower motor. I would like to know the reasoning for replacing the flywheel key. My assumption is that the key shears and the flywheel and crankshaft no longer are "connected" (ie: they each spin freely of each other). I had an incident where I hit a large rock that killed the engine but was able to restart it.

Now, a week or so later, the problem is that my engine will no longer stay running. If I press the gas primer bulb the engine starts but runs for just a few seconds. This tells me that at the very least I have good spark (ignition). I went online to verify my carburetor linkage since I had taken it off. Besides that I made adjustments to the governor but I have not had any luck keeping this thing running.

Maybe the two are not related?

The engine has run like a top for the last 23 years (yes 23) with no real problems until just now.

Thank you.

19bruman62, May 2011

I agree - the 2 events are not connected. Sounds like a fuel blockage - remove the carb bowl nut and drain the carb. Remove the bowl and check that it's not full of crud/water. The bowl nut may also be the jet - depending on exact model. Check it's clear. If the jet is in the actual carb. you will probably need to remove it from the engine and clear it with a piece of copper wire stripped out of electrical cable.

The flywheel key is SUPPOSED to shear in the event of a sudden stop - to prevent a re-start until you have investigated the problem. On a points engine, the engine will not fire with a sheared key. On an electronic ignition engine, you will probably get a kick-back sharp enough to discourage you from trying again without fixing it! An object solid enough to shear the key is also capable of bending the crankshaft.

phil_saunders(a)bigfoot.com

Phil Saunders, May 2011

the flywheel key is needed for the timining of the engine if your engine starts and runs for a few secs the chances are the key is fine. your problem is a fuel based one as it runs on primed fuel only take the carb to bits clean through if unsure you could take it to a local repair centre and have it sonic cleaned for a few quid. beware Tecumseh parts are becoming very rare as they went bust quite some time ago.

daz, May 2011