small engine - hydraulic lock?

Question

Stihl string trimmer, 2-cycle. It's been fading, and hard to start when hot. I thought I'd check the compression when hot. (It was 50-ish). Then, I thought I'd squirt some oil in the cylinder, and see what effect it would have on compression. (Not much.) ...

But, now it's really hard to turn over, when the plug is in. Oops. Just two little squirts of oil. It spins easily with the plug out. Hydraulic lock?

I dried out the crankcase, and put it upside down to drain the cylinder. Everything seems dry, but cranking is still hard. Any thoughts or suggestions would be appreciated.

Thanks, George

digitalmaster · Accepted Answer

hydraulic lock is caused by having a liquid(incompressible) in the cylinder.2 squirts of oil will not do it.besides if you spin it without the plug any appreciable volume of liquid will be ejected through the hole. perhaps the oil freed up a stuck ring and what you are feeling is the proper amount of compression???

John Lawrence · Answer

Any amount of oil above the required fuel/oil mixture will cause a dramatic increase in compression ratio. This is true whether too much oil was added to the mixture or "squirt" in. You'll end up breaking a pull cord. You have to spend more time pulling the motor with the plug out. If it was mine, I'd put a small amount of mixed gas in the plug hole and pull it over for a while with the plug out. Then wait until it's dry and try starting. Good luck.

George · Answer

That took care of it.  Thank you.George

John Lawrence · Answer

Glad to help.

small engine - hydraulic lock?

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