Engine compression questions

Code54 · Dec 27, 2014

I recently picked up a 1996 yamaha Kodiak - it seems to run decent and the transmission checked out -high, low, all the gears. While it needs a lot of beauty work, and some brake work overall for the cost (few hundred bucks) it does not seem too bad.
Today when I started messing with it I figured I would test the cylinder pressure to see how bad the rings were - figured they would be shot. Now I did the pressure test cold, wide open throttle using the starter to crank the motor over. The gauge locks on the highest reading and I had a strange one - about 150psi, three checks, all within a few PSI. I looked up the pressure in the service manual and it last 120-131 on the high side. I seem to be 20PSI over what I should have. My only thought it carbon buildup, maybe? Maybe I am wrong?
As I said it sounds decent when running and it is not making any weird sounds. It has an aftermarket pipe on it and that concerned me thinking it may not have been jetted for the pipe but notice the spark plug was not showing it was running hot (did have carbon filing, possibly running rich)
What is the best way to deal with this? I could pull the jug, OHC, etc and try cleaning it up? Leave it alone and run some decent cleaner in it (seafoam maybe?)

This is just a cheap old ATV to mess around with and let the kids ride, not some major show quad by any stretch but I would like it too be reliable more than anything. I already bought a battery, all the brakes and some other minor parts but do realize this is a $1000 machine in good shape - I just want to make it safe and reliable and am not sure what all could be causing this and the way to handle it.

Thanks in advance

tcartwri · Dec 27, 2014

Check the valve lash.

Winmac · Dec 27, 2014

Gauge could be off, perhaps you cranked it longer than Yamaha recommends for the test, I wouldn't worry on bit about the reading.

Tough old bikes, ride rough and slow got a big bear with 20k km on it, engine hasn't been touched.

brain55 · Dec 28, 2014

tcartwri said:
Check the valve lash.

Yep, excess valve lash causes late valve timing and in turn increased cylinder pressures.

Brian

SpudHauler · Dec 28, 2014

Always use the same gauge to check compression and watch how quick it comes to the highest amount.

The faster it rises usually indicates better ring seal.

And comparing amounts to other engines on one gauge eliminates errors.

It will be the readings you take in the future each year after doing a tuneup that will help indicate if your engine is still healthy.

Code54 · Dec 28, 2014

I checked the valves today and the intake valve was a little on the tight side so I got it adjusted and started to change the oil but ran into a little problem. Seems someone stripped the bolts that hold the oil filter in so that took a bit to get it apart. Has some fine metal filling in there and some sludge. Seeing the filter also filters the tranny I am guessing this maybe normal? (is that correct) I also have NO idea how long it has been since it was last changed but judging by the bolts it may have been a while.
Anyway, I ordered the bolts so once they get it I can get it all together and recheck my readings. I am now interested in seeing if it changes with the valve adjustment.
Thanks for the information - I love learning about this stuff. Wish our local schools offered some kind of night course on small engines/ATVs.....

Code54 · Dec 28, 2014

Winmac said:
Gauge could be off, perhaps you cranked it longer than Yamaha recommends for the test, I wouldn't worry on bit about the reading.

Tough old bikes, ride rough and slow got a big bear with 20k km on it, engine hasn't been touched.

WOW - Off the top of my head, thats what, over 12000 miles. That is awesome! I have no guess how many miles this one has on it by I doubt it is close to that!

I did notice that in LOW 1 it is slow but the thing pulls like a tractor. Never had one with high and low range but must say I really like it. Maybe kind of run crawling in the creek during the summer!

tcartwri · Dec 28, 2014

Code54 said:
I checked the valves today and the intake valve was a little on the tight side so I got it adjusted and started to change the oil but ran into a little problem. Seems someone stripped the bolts that hold the oil filter in so that took a bit to get it apart. Has some fine metal filling in there and some sludge. Seeing the filter also filters the tranny I am guessing this maybe normal? (is that correct) I also have NO idea how long it has been since it was last changed but judging by the bolts it may have been a while.
Anyway, I ordered the bolts so once they get it I can get it all together and recheck my readings. I am now interested in seeing if it changes with the valve adjustment.
Thanks for the information - I love learning about this stuff. Wish our local schools offered some kind of night course on small engines/ATVs.....

I am not immediately familiar with this machine, but if it has a wet clutch then finding sludge in the case is normal.