getting compression back in 400F-Help!

[zcookie49]

A friend of mine has recently had a new piston and rings installed in his 99YZ400F. seriously, not mine , a friend....
Anyhow, he had a friend do the job.
Needless to say, once he installed motor back into the bike and put back together, when you go to kick the bike, there is no compression.... He has probably torn it down 4 - 5 times, readjusted the cams and chain, apparently having all lined up with tension, reinstall, and no compression.
Now he is pretty much finished with messing with it..probably going to take to a shop to finalize
I thought that before he did that, I could post a thread here and see if anyone has some suggestions toward this. Would appreciate any help here. Thanks....

 

[ThumperPower]

Sounds like a stuck open valve. He has torn it down 5 times and found nothing?

even with the motor out of time he will have compression. There is no decompresion release on this bike if i recall. If there were i would suspect that to be the problem. But anyway i would say his problem is in the head somewhere, all it takes is a small leak. Seeing as he put a new piston and rings in it, did he have the valves recut, or replaced?

These are just some thoughts, but i would look at the head to start.

 

[sfc crash]

doesn't top dead center put the bike in a condition where all valves should be fully seated? that is where the lil' tick mark lines up in the timing hole. put it tdc,take the head off,and i'll bet'cha dollars to hondas that the problem will be very appearent.all valves should be closed.:cool:

 

[will pattison]

you can easily check for a bent valve because you can press them by hand. if they don't move or don't spring back, then there you go. however, i've never seen it happen. you have to think about what could cause a valve stem to bend. the case for an impact on the valve is nonexistant as far as i know with this motor. if somthing happens to cause the valve to stick in the valve guide, then it might bend, but again, never seen it happen. it sounds to me like an assembly issue.

wp.

 

[lump-dog]

Had the very same problem. There was a "burr" in the head where the valve springs sit. When the bike was cold, the valve spring retainer would hang up on the burr, causing it to stick open, therefore no compression. Weird thing was, I would still be able to get it to start, and once warmed up, the compression "came back" the next time I would start it, and remain for the rest of the day! Once it cooled off overnite, though, no compression when starting. I couldn't find anything wrong with the spring, valve, retainer, but I replaced them all any way, and no more problem. FYI, it was the exhaust valve on the right side of the head, the one that the decompression arm actuated, maybe a good place to start looking.

lump-dog

 

[gasgasman]

A friend of mine had his YZF eat his lunch, with the same problem.

Turned out to be the camshaft. The sprocket slipped on the cam. Throwing the cam timing out.

 

[will pattison]

yamaha's best-kept secret!:eek:

that makes 4 of them that i know of, me included!

wp.

 

[zcookie49]

my friend took the bike to local shop, apparently he was 1 tooth off on the timing and that for some reason, 1 of the exhaust valves was clicked in, so when the stroke cycle started, the valve wouldnt close all the way... So you guys with the insight of a valve not shuting properly were correct. Luckily, it wasnt any thing broken or such....now only if it werent calling for rain this weekend....

 

[gasgasman]

Originally posted by zcookie49 ..now only if it werent calling for rain this weekend.... [/B]

A reason not to ride? Tsk..tsk..tsk:mad: :mad: :mad:

 

[SFO]

Originally posted by lump-dog
Had the very same problem. There was a "burr" in the head where the valve springs sit. When the bike was cold, the valve spring retainer would hang up on the burr, causing it to stick open, therefore no compression. I couldn't find anything wrong with the spring, valve, retainer, but I replaced them all any way, and no more problem. FYI, it was the exhaust valve on the right side of the head, the one that the decompression arm actuated, maybe a good place to start looking.

lump-dog

Do you mean there was a burr in the bucket bore?
If the retainer was hanging in the bucket bore shurley the bucket would remain depressed as well?
The top edge of the bucket bore where it intersects the cam journal?
I noticed the decomp actuator was a sharp transition from flat to edge and radiused it to prevent it from raising an edge in the bucket.
Maybe I am misunderstanding something here...

 

[zcookie49]

yeah, yeah, yeah, i know what your thinking with the rain comment, but driving 2 hours to mx park in upper Pennsylvania with snow to ice to rain mix at 31 degrees can sometimes be a deterrent to the group... I am going day to day......:whiner: