1989 yamaha 90 hp compression question

[tampante33]

Just bought this boat with a 90 HP yamaha outboard on it. Ran fine except for a slight misfire. Did a compression check and it is 80- 70- 75. Just want to know if that is bad for this motor.

 

[JB]

Re: 1989 yamaha 90 hp compression question

Howdy.

Welcome to iboats.

Marginal. The 10psi spread is very slightly over 10% and suggests a decarbon job.

Some will call it worn out and call for 100psi+. I don't agree, but at least do the decarb, then do the compression test with a new guage and the method spelled out in the Engine FAQs Forum before you start fixing stuff that ain't broke.

Good luck.

 

[tampante33]

Re: 1989 yamaha 90 hp compression question

Thanks for that info.... what is decarbon, and how do you do that

 

[robert graham]

Re: 1989 yamaha 90 hp compression question

Several ways to do the decarbon, but I would use a quart of the Yamaha Ringfree and use it in your gas tank at the "deposit removal" dosage, go out on the lake or river and run your motor at various RPM's. Check spark plugs for fouling which can occur during decarbon process. You should consider using the Ringfree all the time at the "deposit prevention" dosage. You can purchase Ringfree online or from your Yamaha dealer. Good Luck!