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Lm 318 dropping cylinder

C

chris_holt

Contributing Member
Looking for some input. I have twin lm318's and my port engine has been dropping and picking up a cylinder at idle. Plugs, wires and cap are new. I have done multiple compression tests and some times it comes up good, other times, low. At higher rpm this engine lags behind the starboard engine and it is obvious it is not generating the hp that the starboard is. Possible lifter issue? What else can come and go like that?
 
Well an ignition coil could cause a intermittent miss but if the compression tests are not consistently good I may suspect a sticking or burnt/corroded valve. I would follow up with a leak down test.
 
Before you spend all the time checking everything, Disconnect your tach and see if the problem disappears. Also see if you have sideways play in the distributor shaft. If you can move it even a couple of thousandths, it's a problem.
 
I thought about the tach and coil but that can't affect compression readings. At idle you can hear the cylinder picking up and dropping off. The rpm will go from 750 to 700 and back to 750. Sometimes the compression on that cylinder is 120 and sometimes it's 50. There is nothing consistent. I will try a leak down test this week. I assume I should try it both dry and wet?
 
chris_holt said:
I thought about the tach and coil but that can't affect compression readings. At idle you can hear the cylinder picking up and dropping off. The rpm will go from 750 to 700 and back to 750. Sometimes the compression on that cylinder is 120 and sometimes it's 50. There is nothing consistent. I will try a leak down test this week. I assume I should try it both dry and wet?
Click to expand...

I Missed the whole part about the inconsistent compression. I must get some glasses for myself, it's the second time in as many weeks.


A Leak down test is what you need to do. Sounds like you may be pulling the heads.
 
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the only thing that can cause such a difference in compression, is a valve that is not seating properly. possibly the vale seat itself is loose, or you have a hydraulic lifter that has internal problems that is causing it to become hard, then normal. i would remove the heads. there will be obvious discoloration of the valves in the cylinder affected. it may be as simple as a valve job. worn valve components can give the same symptoms. a bad distributor or coil can cause a cylinder to drop, then come back, but it will in no way affect the compression.
 
Agreed. Time for a valve job.

Jeff

PS: A sticking valve at high rpms can cause a LOT of damage! I wouldn't push your luck here.
 
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