compression check

4

41merc

Member
I recently searched for a "compression check how to" and found a good description. It said to remove all plugs attach the gauge then turn the engine with the throttle wide open. My question is shouldn't the fuel line be disconnected? It seems doing this with the line on will allow a lot of unburnt gas into the engine.:confused:
Thank you,
 
Disconnecting the fuel line wouldn't keep the fuel sitting in the carburetor bowl from going into the engine. You'd have to drain the bowl totally to prevent that.

No harm in unburnt gas going into the engine - especially wide open throttle - won't even be enough to flood it.

The method you described is how everyone I know tests compression (including me) and no one worries about the unburnt gas. Just be sure to put out your cig before the test.:rolleyes:
 
Rather than remove all the spark plugs before doing the compression check, is it ok to just remove one plug at a time, do the compression check, reinstall the plug and then move on to the next plug?
 
My understanding is to take all the plugs out, encase there is a leak between adjacent cylinders. Also to do the test with the engine warm.

I have never had any problem with fuel and have never unhooked the gas from the carb. Also turn it over a few times per cylinder, so the gauge gets compression from the tested cylinder twice.

2cents...
 
if all the plugs are removed whitch they schould be. you will have very little or no vacuum at the carb. witch means gas will not be pulled into the eng . do the test as the manual says. the manual will tell you if comp. is low put oil in cyl it will explain why .
 
richard crow said:
if all the plugs are removed whitch they schould be. you will have very little or no vacuum at the carb. witch means gas will not be pulled into the eng . do the test as the manual says. the manual will tell you if comp. is low put oil in cyl it will explain why .
Click to expand...

Agreed. Also by pulling the plugs the engine will spin faster, saving the battery and giving you a better reading.
 
Hi All, Isn't there another part of the test when you locate a low cylinder?

I think this is right on OHV engines, but if you find a low one, put a tablespoon of engine oil in the plug hole and test again. If it stays low then the leak is in the top of the engine, if it comes up then the leak is in the rings. The theory is the oil helps seal the rings. Is this valid in valve-in-block applications? Seems like it might not be. :confused:

What is a good compression number to get in a 21 stud engine?

-VeryTangled
 
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