# Crankcase vacuum



## RKDOC

I was reading an article that stated "always perform a crankcase vacuum test when testing for poor performance on a small engine" I have a digital vacuum gauge but have never used it. I understand that too much crankcase pressure will push oil past rings and seals. What should vacuum in the crankcase be? and what will cause too low, or too high of pressure? Does it have to do with the breather assembly? How does too much or too little vacuum effect engine performance?

I know this is a lot of questions so I really appreciate your helping me understand this concept. Thanks


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## billsmowers

RKDOC said:


> I was reading an article that stated "always perform a crankcase vacuum test when testing for poor performance on a small engine" I have a digital vacuum gauge but have never used it. I understand that too much crankcase pressure will push oil past rings and seals. What should vacuum in the crankcase be? and what will cause too low, or too high of pressure? Does it have to do with the breather assembly? How does too much or too little vacuum effect engine performance?
> 
> I know this is a lot of questions so I really appreciate your helping me understand this concept. Thanks


you do a crankcase vacuum / pressure test on 2 cycle engine to check the crank seals and case for leaks you test -0.5 bar vacuum & +0.5 bar pressure
blank off the exhaust with a piece of rubber between the exhaust and cylinder
then do the same with the carb now get a compression tester fitting and connect this to your vacuum / pressure pump and perform the test should hold for 2 min

hope this helps

bill


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## RKDOC

Thanks Bill. I do vacuum leak testing on 2 cycle engines. what I am talking about is on 4 cycle engines.


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## billsmowers

RKDOC said:


> Thanks Bill. I do vacuum leak testing on 2 cycle engines. what I am talking about is on 4 cycle engines.


ok then you need neg vacuum in the crankcase to keep the oil in if you have positive pressure in the crankcase you will get oil leaks etc. normally if your rings are shot then you can get positive pressure or if your crankcase breather is not working the same as in a car.

there is a list of crankcase vacuums on one of the b/s updates when they introduced the digital manometer i just checked my manometer and i have lost the list i am shore there is someone on here who can publish a copy as i could do with one as well

bill


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## RKDOC

Thanks Bill. Is any amount of vacuum alright? or should there be a certain amount? I can't find any specs that show crankcase vacuum readings. Thanks


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## usmcgrunt

If the engine is a Briggs,somewhere on their web site,they list the vacuum specs for different engines.I have also read that 3 to 5 inches of vacuum is considered normal for most small engines.Hope this helps.


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## billsmowers

usmcgrunt said:


> If the engine is a Briggs,somewhere on their web site,they list the vacuum specs for different engines.I have also read that 3 to 5 inches of vacuum is considered normal for most small engines.Hope this helps.


cant find the list any where on the b/s site?
if anyone has a copy can they please post it

thanks
bill


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## RKDOC

Thanks for the help!!!!!!!


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