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25hp EFI merc, No compression?

john01

New member
I have a 2006 merc 25hp 4/stoke EFI ( OR107138 )

This engine will not run or even pop. i checked compression with throttle wide open and can only get about 10psi on all 3 cylinders. what could be causing this? i know this engine has decompression valves built on the cam but i should get more compression than 10psi right?

I did put new rings thinking maybe it was that. (this engine does have a lot of time on it) but the compression did not change. im stumped, and dont know what to try next.. any help would be greatly appreciated.
 
Sorry i should have said that i did have the head and valves check by a machine shop, and everything checked out good. I double and triple checked the timing marks on the crank and timing gear. i thought timing must be off too. but the marks where right. the flywheel key and cam gear key was good. thats why this makes no sense..

I did a leak down test and each cylinder holds pressure, but at about 40-45 degrees BTDC the exhaust valve opens and releases it. I know it seams the valves are opening way to early (like the timing is off) but i timed this as the book says. aligned the mark on the crank to the mark on the block, #1 at TDC and then the #1 mark on the cam gear to the the mark on the block.

Am i doing something wrong?
Could the camshaft be twisted?
When and how does the compression relief system work?
 
There is no way that the engine ran till it " wore out " to having 10 psi compression !-----So something happened to it to change timing I would say.------Sounds like you bought someone else's problem.
 
Okay. Here's what I'm seeing: The timing marks (or how you're looking at them) is off.

Here's what to do; You need to find what's called the 'split overlap' position of the valves. Put # 1 piston at TDC and look at the valves. If you're on the right stroke, BOTH intake and exhaust valves should be just a tiny bit open. (If one of them is way open, rotate one revolution and try again.)

When you're on the right revolution, move the flywheel back an forth while watching the valves until you get them both slightly open. Then look where the TDC mark on the flywheel and pointer line up: you should be VERY close to TDC. If you're off a bunch, then that's the problem.

Jeff.
 
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