1990 Force 120 Compression on Cylinder 3

[anwalker]

Hey guys,
I just picked up a cobra 1804 with a 1990 Force 120 outboard on it. The compression tested is 145,145,120,145. The motor idles well and runs well on the top end. There is a vibration coming on plane at about 3000 rpm when the motor is under load. The motor hasn't been put in the water in about 8 years according to the previous owner. I ran about 4 gallons through the motor this weekend with sea foam in the tank but didnt see a change in the compression readings. Just wondering it if warrants pulling the cylinder head or not??

 

[Frank Acampora]

Re: 1990 Force 120 Compression on Cylinder 3

Don't fix it if it aint broke.

Yeah, one cylinder is a bit low but the engine is running well. Most Force engines will have a vibration period when accelerating so unless it is so bad that it shakes the whole boat, don't worry about it.

Since you just bought the boat, consider the engine an unknown. Change the lower unit oil, water pump impeller, fuel pump diaphragm, and put a fuel filter between the pump and carbs. Then, just double check that there is spark on all four cylinders.

 

[JB]

Re: 1990 Force 120 Compression on Cylinder 3

A decarb might clear up whatever is inhibiting #3. My guess is some carbon. Just putting SeaFoam in the tank might prevent carbon, but it won't clear it out.

But I agree with Frank. There is not enough of a problem to justify pulling the head.

 

[jerryjerry05]

Re: 1990 Force 120 Compression on Cylinder 3

Plugs burning OK?
Do a decarb with Mercury's Power Tune. Then run it.
Do another test.If your still worried?Then remove the head(gaskets only $35/40)
I bought a running motor,Comp 145 all 4 cyl.Had it sold and was checking it out(won't sell junk)removed the head and found scoring in one cyl.Lost a sale

 

[anwalker]

Re: 1990 Force 120 Compression on Cylinder 3

The plugs all look fine no signs of overly lean or rich conditions. Its funny, but when I bought the boat I pulled the gas cap and didnt smell any varnish in the tank, however; when I filled it up I can now smell varnish around the carbs. I should probably at least pull the bowls and have a look.

I agree on the compression, Its not that far off but Ill keep a close eye on it just to be sure. Wouldnt be the first time I had to rebuild. A few years back I bought a yamaha ls2000 jet boat and the motors checked out great, then the second time I had the boat out, a top corner of the piston came off down to the first ring land. The good thing is parts for this motor seem to be cheap and plentiful!

 

[jerryjerry05]

Re: 1990 Force 120 Compression on Cylinder 3

Easy to rebuild.J