no compression and will not start



30 replies to this topic
  • no_thump

Posted July 14, 2001 - 02:40 PM

#1

DETAILS: 99YZ 400 RACED LAST 2.5 YEARS.
DROPPED A VALVE, VALVE WENT INTO PISTON AND THEN INTO HEAD. SO I REBUILT THE MOTOR, NEW PISTON, HAD CYLNDER RESLEEVED. PURCHASED NEW HEAD AND NEW VALVES TO GO WITH THE HEAD. VALVES LIFTERS GO UP AND DOWN WITH THE CAMS, THEY APPEAR TO OPEN AND CLOSE. TIMING IS MATCHED UP WITH THE I ON THE CRANK AND EIEI ON THE CAMS JUST LIKE THE OLD MANUAL SAYS. PROBLEM: THIS DAM BIKE WILL NOT START, BACK FIRES OH ABOUT EVERY 10 KICKS. AND I DO NOT NEED THE DECOMPRESSION LEVER, MOTOR TURN OVER EASY. WHAT THE HELL IS WRONG? WHERE IS THE COMPRESSION?

PLEASE HELP!!

  • Hick

Posted July 14, 2001 - 02:54 PM

#2

You may have a valve hanging open somehow. Did you check the clearances?

Maybe you inadvertently put a kink in your decompression cable when you reinstalled it?

BTW, you left your Caps Lock key on.

  • rob1980

Posted July 14, 2001 - 04:28 PM

#3

Did you get the new valves matched to the new head? This ensures a gas tight seal when they are closed.
If the first posts suggestion doesn't work, make sure there is no leaking compression out the exhaust or intake when the piston is going up on the compression stroke.
Try removing the exhaust and listen for leaking compression on the compression stroke.
Once I rebuiltmy XR 250 and got the valve seats matched to the valves. The guy did a poor job and when I put it back together, it had no compression. It turned out that the exhaust valves were leaking on the compression stroke due to poor machining of the seats. Let us know what you find.
Rob

  • no_thump

Posted July 14, 2001 - 05:27 PM

#4

Sorry about the caps. I always leave them on for simplicity. kink in the decompression cable is not the problem. the decompression mechanism visually works fine. vavle hanging open and check clearance is probably where i need to ge next. i don't have a clue on this at all. although it seems the cam lobes make contact all 360 degrees.

Originally posted by Hick:
You may have a valve hanging open somehow. Did you check the clearances?

Maybe you inadvertently put a kink in your decompression cable when you reinstalled it?

BTW, you left your Caps Lock key on.



  • no_thump

Posted July 14, 2001 - 05:37 PM

#5

the new valve were stuck in the new head by a 25 year veteran at 45 buckoroo's an hour. or 65 dollars for the 1.5 hours he charged me. i should give him a call and ask him what he did.
i can't believe the valves or sticking. if the valves did stick. would the lifter heads still go up and down with the cam lobes?

what do you think the local yamaha shop would require to make this bike run

Originally posted by rob1980:
Did you get the new valves matched to the new head? This ensures a gas tight seal when they are closed.
If the first posts suggestion doesn't work, make sure there is no leaking compression out the exhaust or intake when the piston is going up on the compression stroke.
Try removing the exhaust and listen for leaking compression on the compression stroke.
Once I rebuiltmy XR 250 and got the valve seats matched to the valves. The guy did a poor job and when I put it back together, it had no compression. It turned out that the exhaust valves were leaking on the compression stroke due to poor machining of the seats. Let us know what you find.
Rob



  • Hick

Posted July 14, 2001 - 05:39 PM

#6

Originally posted by no thump:
...check clearance is probably where i need to ge next. i don't have a clue on this at all. although it seems the cam lobes make contact all 360 degrees.


If they really are in contact over 360 deg.s then you would have zero clearance and I would be surprised if it DID start in that situation. At TDC there should be clearance between both cams and the shim buckets.

.10 - .15 mm on intake
.20 - .25 mm on exhaust

You should be able to get a smaller sized ordinary automotive feeler gauge between the lobe and bucket. To adjust the clearance you swap the shim located underneath each bucket, they have their thickness’ inscribed on them, to create more clearance get a correspondingly smaller sized shim.

Look in the manual under the Inspection/Adjustment section.

Hope this helps.

  • no_thump

Posted July 15, 2001 - 03:47 AM

#7

Thanks, if i can find the time and the energy i'm going to tear the head off and properely shim the valves as well as inspect the valve seat with cams installed.

I must have several or one valve remaining open.

i'll let every one know what i found. sure appreciate the good help!!

  • kmcbride

Posted July 15, 2001 - 05:24 AM

#8

Did you or the machinist put the cams back in after the valve replacement? Did you just put the shim that was there back in? I don't mean to insult you of the machinist, but as these bikes wear the valves tend to tighten from seat wear, valve wear etc. I have a '99 and have changed my shims twice. (Thanks again to Hick for sending me shims that were on back order!) I would doubt that if I were to do a valve job the same shim would apply, especially if the tightening over time is from the valve. Thus, new valve, worn seat or newly grinded seat, same shim, valves does not close all the way, no compression. I have done valve jobs on my old XR's and spent lots of time seating new valves by hand with valve grinding compound. It is critical! I have some spare sizes of shims that I can send you if you post your needs, if this is the problem.
Double check compression release with valve cover off for play over exhaust valve bucket.
Good Luck, Keith.

  • holeshot

Posted July 15, 2001 - 07:28 AM

#9

no thump:

You can't really see the small clearances by eye, since they should only be .25mm max (exhausts), so it may appear that they're touching 360 deg. around. If you've used the same shims and buckets on the new head, chances are slim to none that you have the proper clearances. I would start there before going through the hassle of pulling the cylinder head.

------------------

  • no_thump

Posted July 15, 2001 - 07:41 AM

#10

used the same shims on new valves and a new head.

Originally posted by kmcbride:
Did you or the machinist put the cams back in after the valve replacement? Did you just put the shim that was there back in? I don't mean to insult you of the machinist, but as these bikes wear the valves tend to tighten from seat wear, valve wear etc. I have a '99 and have changed my shims twice. (Thanks again to Hick for sending me shims that were on back order!) I would doubt that if I were to do a valve job the same shim would apply, especially if the tightening over time is from the valve. Thus, new valve, worn seat or newly grinded seat, same shim, valves does not close all the way, no compression. I have done valve jobs on my old XR's and spent lots of time seating new valves by hand with valve grinding compound. It is critical! I have some spare sizes of shims that I can send you if you post your needs, if this is the problem.
Double check compression release with valve cover off for play over exhaust valve bucket.
Good Luck, Keith.



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  • no_thump

Posted July 15, 2001 - 09:10 PM

#11

I have .254 mm on both intake buckets / lobes

Exhaust going from cam sproket i have
1st .504mm 2nd 20.8 will not fit (smallest i have, pretty tight) 3rd .203mm

i order or purchase some shims tommorrow. what about the bucket / lobe thast is tight. order 120 thru 150?

do we still think this is the problem?


Originally posted by Hick:
If they really are in contact over 360 deg.s then you would have zero clearance and I would be surprised if it DID start in that situation. At TDC there should be clearance between both cams and the shim buckets.

.10 - .15 mm on intake
.20 - .25 mm on exhaust

You should be able to get a smaller sized ordinary automotive feeler gauge between the lobe and bucket. To adjust the clearance you swap the shim located underneath each bucket, they have their thickness’ inscribed on them, to create more clearance get a correspondingly smaller sized shim.

Look in the manual under the Inspection/Adjustment section.

Hope this helps.



  • kmcbride

Posted July 15, 2001 - 11:59 AM

#12

No thump,
Definetly a problem! You will not know what to order until you check the shim values you have in place. Sounds like the middle exhaust valve is the culprit for compression loss, but all of them are out of spec. Double check clearances and write them down with corresponding position and add or subtract from the current shim to get you into spec. You will likely need to order these from the factory because they will be odd-sized. I tend to set clearances towards the larger end of the gap (.15 intake, .25 exhaust) as the clearance tends to tighten over time.
Did you put the shims back in their original location?
The middle exhaust valve is at least .20mm out of spec, meaning you will need a shim >.20mm smaller than the shim currently under this bucket. If you can't get any gap guage under the cam, it is likely opened. One suggestion would be to remove cams and place
a smaller sized shim from another bucket in it's place, replace cam, to allow the valve to close at TDC and then get your gap and subsequent change needed. There is a table in the manual to get the proper shim size from your 2 numbers: 1) Gap 2) Existing shim size BTW, get some assembly oil to coat surfaces; cam lobes, under buckets etc. after you have the final shims.
Hope this helps, good luck, Keith.

  • MotoStyle

Posted July 16, 2001 - 08:15 PM

#13

Thump,

Sounds like you have the exact problem that I just had. I did a top end and when it all went back together, I had little to no compression. I went back, checked my timing (spot on), checked valve lash, and found that on of my intakes was stuck open. I released tension on the cam chain, removed the cam holder, pulled the cam out of the way and took out the shim. I then reinserted the shim and found that it went back in more flush than when I pulled it out. Put it all back together and she had full comression. --- I would bet that you have the exact same problem. You don't need new shims. You have one that is not seating correctly. Do as I just explained and I bet you fix it. ---- I was furious when I got it all back together and found no compression, but felt like an ass when I fixed it with only 15 min worth of work. Be patient. These motors are pretty simple. If you don't have compression, it can only be from a number of very testable sources. Hope this helps. Keep us posted. :)

  • no_thump

Posted July 28, 2001 - 06:08 PM

#14

Originally posted by MotoStyle:
Thump,

Sounds like you have the exact problem that I just had. I did a top end and when it all went back together, I had little to no compression. I went back, checked my timing (spot on), checked valve lash, and found that on of my intakes was stuck open. I released tension on the cam chain, removed the cam holder, pulled the cam out of the way and took out the shim. I then reinserted the shim and found that it went back in more flush than when I pulled it out. Put it all back together and she had full comression. --- I would bet that you have the exact same problem. You don't need new shims. You have one that is not seating correctly. Do as I just explained and I bet you fix it. ---- I was furious when I got it all back together and found no compression, but felt like an ass when I fixed it with only 15 min worth of work. Be patient. These motors are pretty simple. If you don't have compression, it can only be from a number of very testable sources. Hope this helps. Keep us posted. :)



  • no_thump

Posted July 29, 2001 - 12:55 PM

#15

Originally posted by no thump:


WELL THE OLD 4 STROKE HAS RESUMED ITS THUMP AGAIN. VALVE WAS BEING HELD OPEN. THANKS TO ALL FOR THE HELP.

THE VALVE THAT WAS BEING HELD OPEN, I SHIM IT WITH A 120, THE SMALLEST SHIM AVAILABLE I BELIEVE. AND THIS 120 SHIM STILL HELD THE VALVE OPEN. SO I REMOVED THE SHIM AND THE VAVLE OPERATES FINE AND IS ACTUALLY SPEC.

WHAT DAMAGE IF ANY WILL ARISE FROM RUNNING NO SHIM UNDER THE BUCKET?

  • sirthumpalot

Posted July 29, 2001 - 04:09 PM

#16

I'm not a pro mechanic, but I would guess that no shim at all would mean that your valve seat or valve itself are worn pretty bad OR something in the head was bent when the piston hit the valve. Did you have the head checked out by a machine shop? If so you may want to find out if they did any grinding on that seat for some reason, perhaps they ground it a bit to far? I don't know if you would get some funny wear without a shim, but I wouldn't ride it until you find out for sure. Plus after some time as the seat wears it will likely tighten back up again and you'll have to deal with it at some point in the future. Good luck!

  • no_thump

Posted August 01, 2001 - 04:56 PM

#17

ACTUALLY I BROUGHT THE HEAD NEW FOR $475.00 BUCKS. SHOULD BE OK THATS THE CHEAPEST I COULD FIND IT FOR. MY LOCAL YAMAHA DEALERS TRIED TO SELL ME THE HEAD FOR 530 TO 570 BUCKS!! I ALSO PUT 5 NEW VALVES IN. SOMETHING IS OUT OF WHACK FOR SURE.

THANKS

Originally posted by sirthumpalot:
I'm not a pro mechanic, but I would guess that no shim at all would mean that your valve seat or valve itself are worn pretty bad OR something in the head was bent when the piston hit the valve. Did you have the head checked out by a machine shop? If so you may want to find out if they did any grinding on that seat for some reason, perhaps they ground it a bit to far? I don't know if you would get some funny wear without a shim, but I wouldn't ride it until you find out for sure. Plus after some time as the seat wears it will likely tighten back up again and you'll have to deal with it at some point in the future. Good luck!



  • MotoStyle

Posted August 01, 2001 - 05:52 PM

#18

Did you seat the valves properly in the new head? This is essential.

  • sirthumpalot

Posted August 02, 2001 - 03:48 AM

#19

Are you positive that the head which you received is brand new? It is very easy to clean up an old head and make it "look" new. I would assume that it would come wrapped in some sort of Yamaha wrapping plastic if it was a factory new head (maybe someone can verify this).

  • no_thump

Posted August 03, 2001 - 05:23 PM

#20

A MOTOCROSS SHOP WITH 20 EXP THEY ADVERTISE CHARGED ME 65 BUCKS OR 1.5 HR'S TO INSTALL THE VALVES.

Originally posted by MotoStyle:
Did you seat the valves properly in the new head? This is essential.







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