Assistance appreciated on compression issue 50 hp Yammy

[jakozlow]

I am looking at a 1996 Yamaha 50 hp motor listed in the local classifieds. Motor is fairly clean. I performed compression test and found top cylinder at 120#, middle cylinder at 120# and lower cylinder at 150#. Any advice? I was told all 3 cylinders should be relatively the same. I have heard of a motor having a dead cylinder, but never heard of one having elevated compression. I did notice the lower cyl. had a little residual oil in it. Could this cause the elevated compression? Any thoughts or assistance would be greatly appreciated.

I understand 10% variance is the fine, and 15% is acceptable. Why would top to cylinders be fine and lower be so high?

Jeff

 

[rodbolt]

Re: Assistance appreciated on compression issue 50 hp Yammy

while a compression difference on a lot of inline 3cyl motors is normal and designed into the engine,yamaha wont post the actual numbers, your 30 PSI spread is a bit much.
could be carbon, could be its been rebuilt with only the bottom hole bored.
have a leakdown test run on it before buying it.
any decent yamaha dealer or most better automotive shops can do it.
test will take maybe 15 minutes and will positivly identify any pistion/cylinder sealing issues.

 

[jakozlow]

Re: Assistance appreciated on compression issue 50 hp Yammy

Much thanks Rodbolt for the insight.

Jeff

 

[JB]

Re: Assistance appreciated on compression issue 50 hp Yammy

I think the residual oil could well account for the higher reading in the bottom hole.

That is at least one good reason for running an engine to operating temp before testing.

I know of no fault that could cause elevated readings other than being tested cold, which is not an engine fault but a technique fault.

 

[jakozlow]

Re: Assistance appreciated on compression issue 50 hp Yammy

Many thanks JB.

Jeff