chain saw trouble

1948jr · Aug 18, 2019

I put in a new piston and rings and new crank seals and now saw runs fast ,wont idle. Can you tell me what I did wrong? It is a stihl ms 391

Bob Bancroft · Aug 18, 2019

Air leak someplace. And it doesn't sound like a little seal leak either, it sounds like a big one. I'm not familiar with that saw, so can't be specific. Carb to cylinder connector? Pulse line off?

JD Seller · Aug 18, 2019

Spray a lubricant like WD40 around the crankshaft seals if you can. Also the case gaskets. If the saw changes speed you then know where your leak is.

The best way is to do a leak test on the saw. You need to block the intake and exhaust ports and then pressurize the cylinder/block through the spark plug hole. Then If it shows a leak spray soapy water around things until you see bubbles.

To seal the ports I just cut a solid gasket out if plastic oil bottles. You only need a few PSI to do the leak test.

Bret4207 · Aug 19, 2019

Yup, air leak. Check the jug to crankcase seal, the carb to jug seal and then the crank seals. All it takes is a little ripple in a seal, or an incorrect seal, to do this.

chain saw trouble

1948jr

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Bob Bancroft

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JD Seller

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Bret4207

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