Chainsaw or String Trimmer Crankcase Seals

16 years ago

The owner of a small engine repair shop showed me how to check for a crankcase leak with no test equipment as follows:

Remove the carb and the rubber boot if present. Remove the spark plug. Hold your thumb over the intake and stick one finger in the spark plug hole. Have somebody pull the starting rope. A definite vacuum can be felt if the there are no leaks in the crank case.

Its not a common condition for the seals to be bad but after checking fuel lines, tank filter, carb, exhaust, spark and compression and all are found good, suspect a leak in the crank case.

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Comments (3)

nevada_walrus
16 years ago
Sorry but that test is bogus.

Until the piston gets down to the ports you will get cylinder suction. You are not in any way testing the integrity of the crankcase.
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weed_cutter
16 years ago
Maybe the trick is to feel vacuum on your thumb covering the intake? I can't understand the spark plug removal part.

OK,OK. Thumb over the intake, index finger in the hole and plug boot on the little finger. Then pull the rope. Got it.
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nevada_walrus
16 years ago
OK, I get it now, just wasn't reading in the right meaning. I still wouldn't rely on this as a accurate test however. Its the same as thumbing the plug hole to check for compression. Just not reliable. There ain't a 2 stroke hand held that I can't hold my thumb on hard enough that compression will blow pressure past and that's the way that test is suppose to work.

As for crank seals, they have to hold both vacuum and compression. The seals on 2 strokes are designed to work both ways. After all, they do call crankcase compression the "primary compression" because it comes first. The crankcase on a 2 stroke is in reality a blower on a drag car.

Looking to hold for a minute 5-8 psi of both vacuum and pressure with proper tools is something a thumb cannot read or measure. This is something that someone even older then me may have been able to rely on when engines ran much richer on fuel and had fully adjustable carbs to compensate for minor engine wear. It just isn't a usable or reliable method on modern low emission engines.
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Chainsaw or String Trimmer Crankcase Seals

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nevada_walrus

weed_cutter

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