1982 mercury 50hp

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heycookieman

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I just picked up a starcraft fm 160 with a 1982 50 hp merc. I did a compression check on it and all cyl. read at 70 lbs psi. I know this is low but why would all 4 cyls read the same? Should I be worried or do you think putting oil in the cyls would help. The boat did sit about 2 years. What I'm thinking if the the motor was no good I'd have a couple cyls high and 1 or 2 low.Thanks
 
Hopefully the motor was fogged before sitting for 2 years but it sounds like the cylinder walls are dry. You could spray some oil in it and test it again. I would double up on the oil ratio on the first tank.
 
JMichael the plugs were all in thats the way I told to do it and the throttle was not open. What would the open throttle do on the test? mpnelle I'll double the oil up and I do run my motors a little rich anyway. Thanks for the replys
 
Like JMichael says all plugs need to be removed to reduce the drag on the starter. On a 4 stroke, the throttle needs to be open so as not to restrict air getting to the cylinders during the compression test, it's not really necessary on your 2 stroke. If you retest, I have a feeling your motor is pretty healthy.
 
I open the throttle regardless of the type engine to be tested. You have to have air coming into the cylinder for an engine to develop it's max compression and the butterfly valve in the carb can prevent maximum air flow into the cylinder if it is closed. Having the spark plugs in on a multi cylinder engine will add more load to the starter because of the compression in the other cylinders. This will slow down the engines rpm while turning it over and cause your compression reading to be lower.

P.S. Since the motor has been sitting for prolonged period, I would add a little oil to each cylinder and turn it over a few times slowly to get the cylinder walls lubricated.
 
Thanks JMichael and mphelle, I did put oil in the cyls after I checked it so its been in there a couple of days. Ill try it today before I take the motor off to start the rebuild and let you know. I'll post pics asap. Thanks again
 

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