NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Discussion of 2 stroke engine rebuilding
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Discussion of 2 stroke engine rebuilding

"I am in the process of refurbishing an old Eska 7.5 H.P. outboard boat motor. The engine (a 2-stroke Tecumseh) runs fine when under load at full throttle.

I can only get the engine to idle at a high RPM. If I lower the idle speed, the engine will eventually die.

Once the engine is started and idling, any attempt to put it into gear will cause the engine to die. Sometimes it will stay running and I can get the throttle revved up at which point the motor will push the boat and run just fine. When I lower the engine speed (approaching the dock) the motor will eventually die.

I am totally stumped. I've completely rebuilt the carburetor (new seals, etc.), replaced the condenser, points, and spark plug. I've set all adjustments to factory specs yet it still won't idle or go into gear."

(From: Al Savage (asavage@iname.com).)

Although I haven't worked in the field in 15 years, two stroke theory doesn't change much .

Is your high idle (out of gear, unloaded) smooth? It shouldn't be. Two strokes without electronic mixture controls are almost always calibrated to be slightly rich when unloaded, as the nature of all three induction systems (piston port, reed, rotary valve) is such that adding a load leans the effective mixture. Something to do with flow dynamics, I didn't need to know, as I don't design them.

You've covered the common problem area -- ignition. I suspect you have an air leak somewhere. Upper or lower crankshaft seal, reed plate gasket, intake gasket, and upper housing gasket are common areas, in that order.

Mind, this is generic two cycle troubleshooting advice; I don't pretend to know the model you're working with.

To track these things down in the shop, we'd build custom block off plates and pressurize the crankcase to a few inches, then watch the gauge to see the leakage rate. Not terribly practical unless you're rebuilding, but you sometimes found porous castings and cracks that way.

When I'm feeling adventurous I'd spray starting fluid (with a tube nozzle attached) around all the seams with the engine running to see if the run behavior changed. Not recommended; too dangerous.

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