Compression Problems

[Lostfisher]

I had just bought a late 80's model 40hp Yamaha oil injected. The motor looked mint, but after I had gotten it I did a compression test and got 120 out of the top two cylinders and only 70 PSI out of the bottom one. Is there different ways that i can test the power head to further identify what it could be, E.g. the rings, the piston itself, even a bad gasket, etc. Also I have a feeling that maybe the oil injector wasn't working and that is why the powerhead lost compression. Other than that it looks mint.

 

[rodbolt]

Re: Compression Problems

time to pull the head and inspect the cyl wall.
oil injection problems are very rare and usually show up as a damaged crankshaft and a rod hanging out of the block.
most times piston damage is lean air fuel ratio or overheat.

 

[Lostfisher]

Re: Compression Problems

I took off the head and looked at the wall of the motor it seems to be fine. I also noticed that some one else was into the power head the screws were finger tight on the motor. I did notice some pitting on the piston head itself. I put a bunch of grease on the head gasket just incase it was the loose bolts thet caused it to leak but it did not make any diffrence. Also I read that if you put oil into the spark plug hole you can identify whether or not it is the gasket. If the PSI stays the same than it is the gasket if the PSI goes up than it is either the piston or the rings. because the oil creates a seal. After squirting oil into the hole my PSI jumped up from 70 to 90. I'll be taking the head apart tomorrow, hopefuly I'll be able to put it back together. Any Info I should know about taking the head apart so that I dont break or ruin anything?

 

[rodbolt]

Re: Compression Problems

if your talking about the power head simply remove all the bolts,dissconect all controls,battery cables shift brackets fuel lines.
I doubt you will be reassembling tomorrow.
and before you start get a service manual,preferably on from yamaha.
and for future reference, that oil trick is simply useless and if to much oil gets added it bends connecting rods and breaks starters.
on a two stroke if it has low compression it can only be a piston issue or a head gasket issue and both require head removal for inspection.
any scuffing on the cyl walls deep enough to leave copper after rubbing a penny acrossed them, deem the cyl unfit for further service until its bored or sleeved.

 

[Lostfisher]

Re: Compression Problems

Are there any ways that I can identify whether or not it is the head gasket or the piston. the gasket looked fine to me. but if it saves me a whole day of terring this thing apart that would be great.

 

[mphelle8vld]

Re: Compression Problems

Even if your cylinders look great, if you have a stuck ring from overheating it will show itself as low compression. Sounds like the previous owner was aware of the compression problem since you found the loose head bolts. The head gasket on that motor was not meant to be reused, once the metal seal is compressed on installation, it will not be able to seal properly again. I would replace with a new gasket and check again, if it's still low on that cylinder, might be time to pull it apart and check the rings.

 

[Bluefish]

Re: Compression Problems

Also, check the head to see if it is warped I think 4 to5 thousands from end to end is the maximum tolerance