1982 50 hp mercury compression

[heycookieman]

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

 

[emckelvy]

Re: 1982 50 hp mercury compression

Either an issue with the compression gauge or just need to crank over the engine a bit longer. Note that if you pull it with a rope that it'll be quite difficult to get up to "normal" compression readings.

Normal practice is to let the engine crank until the needle on the gauge stops moving. This may take 3-4 seconds of cranking or so. I'd recommend putting the motor on a flusher and having just a bit of water flow, to lubricate the rubber impeller while you're doing the compression test.

The fact that the readings are all even is encouraging; it's extremely unlilkely that you'd get even readings of any kind in a powerhead with bad rings/pistons.

HTH..........ed

 

[heycookieman]

Re: 1982 50 hp mercury compression

Thank you emckelvy for you response. I did crank it for 4-5 sec. if not longer but not with a flusher. I'll do that as soon as it warms up. Thanks again.

 

[quicktach]

Re: 1982 50 hp mercury compression

Four equal readings is pretty good. Yes 70 psig is low, but I wouldn't worry too much for now. Curious ... is there much carbon build up on the tops of the pistons?

 

[GA_Boater]

Re: 1982 50 hp mercury compression

When I got mine after sitting for untold years, compression was only 50 across the board. That was cold and no oil. I got her running and never bothered checking again because she runs so good. In your case, I think 70 is good for starters, once you get her running see what it comes up to.

If it is running, try testing when warm and use a freshly charged battery, too. Also use some Seafoam in the gas, it helps free up sticky rings

 

[heycookieman]

Re: 1982 50 hp mercury compression

Thanks quick and GA nope the plugs were pretty clean and the battery was fully charged.I'll gas some gas and run the seafoam. Thanks again