70 HP Compression Question

[motoman]

I have a 1975 70 Evinrude . I have been getting the boat ready for the up coming fishing season and put the motor on the muffs to run the fogging oil out and get the fresh gas started. I run the motor for about 15 to 20 minutes to make sure it was pumping water OK . After shutting the engine off I did a compression test on it . the numbers I had were, Top 70 psi , Middle 70 psi and bottom 72 psi . I know these number are low, but they seem to be even . To confirm the numbers I did again and the numbers were the same. Are these numbers low because I have a problem getting ready to happen or is this normal for the age of the motor? Or is it where it has set all winter and will get better as it is run more ?

 

[boobie]

Re: 70 HP Compression Question

I'd check it with a different compression gauge first and then do a good de-carb on it.

 

[motoman]

Re: 70 HP Compression Question

What all is involved in doing a decarb . Do I need to put something in the fuel tank? What will the decarbing do?

 

[boobie]

Re: 70 HP Compression Question

It will free up the rings if they are stuck from carbon build up. Check the top secret files at the beginning of this forum. Everybody always says to use sea foam, but I've always used BRP engine tuner.

 

[JB]

Re: 70 HP Compression Question

There are too many variables to say 70psi is too low. I agree that because all three are very close your engine is probably healthy mechanically.

A different gauge after a decarb might well produce higher numbers.

Good luck.