compression test ?

[rweboard67]

i am going to have a mechanic service a 1970 merc thunderbolt 650 65hp.. he wants to see at least 125 psi in each cylinder...i have seen posts here and read else where ,a little lower is o.k.. at least 110psi..is this correct... even though the motor has been taken care of, but has not run for a few years ( been garaged ) i feel it might pass the big test.. i am a little anxious that anything less then 125psi he will want to breakdown the motor.... at what range can you live with ?? thanks alot !!

 

[boat1010]

Re: compression test ?

I am by no means a mechanic but have always understood that it isn't really how high the compression is but more if they are even, and not a big difference between cylinders. Of course there is a limit to that but usually if they are within a few pounds of each other I am thinking it is a pretty good motor. And I have to agree that higher is better but don't think that 110 is a bad number if they are all like that.

 

[robert graham]

Re: compression test ?

If the motor runs, then maybe run a de-carbon dose of product like Ringfree through a couple of tanks of gas to remove carbon, free up rings, then run compression test. I wouldn't be too quick to let someone talk me into rebuilding a 40 year old motor that still runs pretty darn well. The decarbon would be good for the motor anyway. Good Luck!

 

[rweboard67]

Re: compression test ?

thansk alot boat1010.. this is the impression i have also.. i just need to confirm it with someone else... if the compression comes in , lets say.. at 110psi on each 4 cylinders .. which is on the slightly lower end of good compression.. then basic tune up is want i really intend to have done and not talked into a more exstensive project.. i just have to figure out a 110psi compression reading, then a 125psi, is the difference on how hard i can push the outboard.. just want to get it running for the short boating season we have in new england..then the next off season look into recondition of the motor..

 

[rweboard67]

Re: compression test ?

thanks for the reply robert !!! i need some ground to stand on : ) the mechanic restores and repairs old vintage outboards.. i feel fortunate to have connected to him... he has a passion for what he does... which can drive up the cost beause a certain psi reading is only acceptable for a basic tuneup by our phone conversation.. he sound like a great guy..