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Low compression on one cylinder

Yamaharider74

Yamaharider74

Pro
Joined
Dec 23, 2013
Messages
140
Age
49
Location
theodore,saskatchewan
Country
Canada
Snowmobile
07 Yamaha apex
hello I have a boosted apex 06 with an 09 engine took a compression test and 3 holes are 120 the 4 hole is 60 it's supercharged with head shim ,bigger injectors , aftermarket fuel pump running 12 pounds of boost rings look fine but that cylinder looked carboned up or sooted up at top edge of cylinder and also valves look black on that
Cylinder going to get piston measured for out of round and install new rings of measures good maybe rings are done from being boosted all the time
Any thoughts suggestions?
 

Yamaharider74 said:
hello I have a boosted apex 06 with an 09 engine took a compression test and 3 holes are 120 the 4 hole is 60 it's supercharged with head shim ,bigger injectors , aftermarket fuel pump running 12 pounds of boost rings look fine but that cylinder looked carboned up or sooted up at top edge of cylinder and also valves look black on that
Cylinder going to get piston measured for out of round and install new rings of measures good maybe rings are done from being boosted all the time
Any thoughts suggestions?
Click to expand...

Bad valve or maybe head gasket would be my gas.
 
When did you do a compression test? After it has been sitting since last season? Was it running fine last time you rode it?
Or was it running poor and you did his compression test to start troubleshooting?
 
mbarryracing said:
When did you do a compression test? After it has been sitting since last season? Was it running fine last time you rode it?
Or was it running poor and you did his compression test to start troubleshooting?
Click to expand...
Hi ever since I started this project I have always had trouble getting the afrs at an idle to show any numbers like it was pouring to much fuel in but when driving it was good tons of power just at an idle it smelled like to much fuel no matter what I did with the programmer so I decided to do a compression test and the one cylinder was at 60 other three at 120 so I figured that was the problem compression down on the one cylinder and with it being boosted it made up for the low compression with the extra air being forced in , I never took a compression test before this
 
The reason I was asking is after my Apex sits a season, the cylinder leak down is horrible due to the valves corroded at the seats until it is started and run for a bit. But this is obviously now not that case, since you have an issue running too.

I'm with MrSled, I think the problem lies with the valves in that cylinder, leaking and not sealing. If you had piston damage from boosting that is causing that low of compression you'd have piston crown damage from detonation, eroded away to the first ring land. It's not the piston or cylinder.

How many miles on the valves or since last clearance adjustment?

Start with bolting the cam's back in the head and checking the clearances... likely have a valve that is seated in enough to not have any clearance and is staying slightly open, hopefully it's just as simple as changing to smaller lash pill under one of the caps.
 
Last edited:
hi and thanks ,yes i was doubtful on the piston as soon as i pulled the head off the cylinders looked like new yet and all pistons looked good from the top side but i decided to pull that piston out that was down on compression and rings and piston all looked perfect also
as far as the miles on the last valve adjustment i have never done it and not sure if previous owner has ever done it either but i am definatly going to check that out also
will bolt the cams in and do some checking and measuring
 
Make sure you don't have a broken exhaust valve spring, too.
 
For future reference you should have performed a cylinder leak down test before you tore in to the engine. If you have a low cylinder during a compression test a leak down test will guide as to weather it's leaking past the rings or intake or exhaust valves. Always a good idea to check valve clearance before tear down.
 
Yamajon1 said:
For future reference you should have performed a cylinder leak down test before you tore in to the engine. If you have a low cylinder during a compression test a leak down test will guide as to weather it's leaking past the rings or intake or exhaust valves. Always a good idea to check valve clearance before tear down.
Click to expand...
Okay thanks good to know I will know better for next time will purchase a leak down tester also
 
mryamaha said:
So what was the verdict on this one??
Click to expand...
Well I checked the cylinder that was low and the cylinder looked good piston ,rings all look good ,I then checked the cam shaft clearance and the valves on that cylinder are needing a valve adjustment they are to tight so will change the pills on those that was the intake valves, I then turned the head upside down turned the cams till valves were closed on that cylinder took isopropyl and dumped in where the valves seat and it sat in there all afternoon never went down so valves must be seating well
So I will put it back together reset valves and hopefully all goes well
 
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