I'm thinking bent valves and badly bent at that. Could have been caused by either a bad cam chain adjustment or bad/broken valve springs. A leak down test will quickly point out the culprit.
25 PSI is really bad. I've had pistons kiss the valves on a race bike due to a loose cam chain adjuster and the motor still made about 90 PSI of compression. Suzuki 1150 motor but more or less the same in re cam drive and cam chain tension adjustment.
Twere me, I would check/adjust valves then recheck compression. If it was still low, and I bet it will be, I would then go the leak down route. On this engine, I wouldn't be checking for the rate. I would be listening to see which end was leaking. Good possibility it could be both.
We need a bit more history and background of the bike before the test.
How many miles on the engine
How do you run it? Loaf around or keep the RPM's up?
When was the last time it was run. Did it run Ok then? Any untoward banging, booming or clattering?
What caused you to want to have the compressions tested?
Has it been sitting long?
IT sounds like it will spin Ok? any noises from the upper end while it's spinning?
As OnD said a pressure differential or leak down test is the only way to tell where the air is going.
Since you were spinning it over and if you would have heard something going on in the upper end and would have stopped spinning it over I'm going to stick my neck out and GUESS you have a couple of stuck rings. This would or would not be confirmed by a PD or leakdown test.
Give us some answers and maybe we can matrix this out.
had the head off and lapped all the valves they wernt bad no visable damage to any part it of the head the compression was tested because it would not tick over when it was cold fine once warmed up. Took it for a run once the guy managed to start the bike and it went up to a ton no worries no smoke or rattles or bangs it only has 17k on the clock after lapping the valves still got low compression think maybe the rings next
???? in re how you are running the compression test.
Proper way is:
All plugs out.
Throttle wide open
Gauge installed and properly sealed in one cylinder. (This is really important).
Crank till gauge stops increasing in pressure (should take about 4 or 5 chuffs).
If pressure is still low, squirt a little motor oil in through the spark plug holes and crank the engine a few times. Then try again. If it improves it was the rings, if not, look to the valves.
By the way, take a careful look at the gauge while cranking, it could be a bad check valve in the gauge giving a low reading. Could also be the seal between the gauge and the spark plug hole. This really needs to be tight which means a clean surface to seal against and a good O-ring type seal at the adapter.
Not being picky, just trying to garner information to help.
By air intakes off do you mean you have removed the throttle bodies?
If the bike is down that far, it is a simple matter to pull the valve cover. Once done, check the valve clearances. If the valves are bent, clearance will have increased. Good time to check/verify cam timing as well as cam chain tension.
Again, watch that seal where the adapter goes into the spark plug hole. While cranking the engine, watch the gauge, if it goes up to say 40 and drops back to 25, you have a leak.
Cam timing? If it was off when you re-assembled the head to the motor, you may have now bent the valves.
My advise would be to pull the head again and pour something like lacquer thinner into the combustion chambers. Look for it getting up into the ports. Or fill the ports with the stuff and see if it winds up in the chambers.
I just don't see stuck rings unless the motor was really a bunch of carbon on the pistons and you managed to scrape some into gap between the piston top and the cylinder walls. Even there, the second compression ring would still allow more than 25 PSI.
You sure there isn't an old spark plug washer stuck to the head?
I don't see how the bike would even run if it had 25psi in 2 cylinders. Also if the rings are shot that badly that the compression is that low wouldn't you have major blue smoke coming out the tailpipe?
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