In search of 4-stroke knowledge yz426 question (compression) thanks

Me and a friend just rebuilt his 00 yzf426. This is the third 4 stroke bike I have rebuilt (my 2 250f also)

The problem we ran into was after we had it all together. It seemed as if it did not have good compression. you could turn the motor over by hand with force. We checked the decompression release for proper adjust ment and it was fine. The motor had compression but it did not seem right that you could turn it over by hand with out useing the decompression release. We ended up starting the motor and it seemed to sound good and run good (on the stand) It idled for about 30 secs and then we shut it off. When I tried to kick it over again it seemed to have it lost compression back.

So my main question is after a rebuild is it normal for the bike to not have high compression untill it has ran a few seconds.. My 250fs i rebuilt did not seem that way.



Im not sure if this would cause a large loss of compression like you are describing, but if I leave my bike sitting for a while (i.e. 4 weeks) the oil runs of the cylinder and I can kick it over fairly easy without using the decompression release. Kick it over a few times to get the oil circulating again and its as good as new.

The first thing that comes to mind is the cam chain and camshafts. The alignment is critical, and if it's even one tooth off it will greatly effect the operation. Do a quick visual inspection with the cap off and see how they line up. When I swapped motors in my bike the "new" motor was off, and it had less compression and wouldn't produce power. After swapping cams (the gear slipped on the shaft) and lining them up correctly, the thing rips! 5th gear wheelies in the Florida sand. :thumbsup:

same thing happened to me when I rebuilt the gearbox on my 426. I put the engine back together and it had good compression. Started it, it ran for 1 min then stopped. I could then kick it through like you described. I started it up again with a lean mix of 2 stoke fuel in the carby, ran it for a rew minutes then changed back to normal fuel again. All returned to normal. This has happened to me twice. Both times was directly after I had the head off. Both times I didnt diassasemble the head, and therefore didnt clean the carbon off the chamber and valves. My theory is that a piece of carbon became dislodged and got stuck under a valve, holding it partially open. The idea of the 2 stroke fuel was to lube the valve guides and stems in case the problem was caused by a sticky valve. These days I run Motorex Valveguard in every tank of fuel.

My theory is that a piece of carbon became dislodged and got stuck under a valve, holding it partially open.

I have had that happen before too. It's most likely what happened to yours. :thumbsup:

Once, while replacing a head gasket, I used tape over the lifters to keep the lifters and shims from falling out of the head. There was still enough space for the lifters to move around, and one of the valve shims came out of its seat. When I put it all back together that valve was extremely tight, causing the motor to not have compression.

Are you sure all your valve shims are seated? Did you measure your clearances? Any valves overly tight?

Until there is a even coat of oil of the cylinder compression will be down, that is why it came back after starting and running it. New rings? Probably havent had time to seat either. Ride it! :thumbsup:

Well, we broke it in at my house and it ran fine. so we changed the oil and headed to the track, Rode for about an hour with no promblem came back and drained the oil. It looked clean and the bike done great.

thanks for the help


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