help with 02 426, low compression and no start.


72 replies to this topic
  • 84rm250

Posted October 08, 2009 - 04:55 PM

#1

Motor has been rebuilt with new rod kit, piston&rings and head was rebuilt by an old man at the shop in town.
Got it all together and it seems to only have half the compression it should. I can kick it over easy with no decomp lever(and i'm not to big at 5'11 180lbs) and it wont start.

I check'd the timing over and over and seems to be fine, one sproket almost looks like its off a tooth but if you shift it that tooth it lands way off. So i'm guessing its fine.
Checked valve clearance and nothing is to tight.
:busted:
Any suggestions, tips, or help would be greatly appreciated!
Thanks in advance:worthy:

  • mkporn

Posted October 08, 2009 - 08:15 PM

#2

what was done to the head, why did you have to rebuild crank and head? what cause it to need to be rebuilt that far in the first place? Did you discover why it grenaded to begin with or did you just rebuild it to freshen it up?

  • 84rm250

Posted October 08, 2009 - 09:03 PM

#3

It broke a valve, witch hit the piston, witch bent the rod.

Head and cams were inspected for cracks, warps and any other damage. Had the valves guides replace, new SS valves with springs and hardwear, new lifters and shims.
Motor was split and cases were cleaned and bearings and tranny were all inspected. Crank was rebuild and trued.
New piston and rings, the cylinder still show'd cross hatch.
It was put back together using moose gasket set, manual and a good torque wrench.

  • wobblyknobby

Posted October 08, 2009 - 09:36 PM

#4

One or both off your cams has slipped inside the sprockets. Just replace them with good stock ones out of a running engine.

  • 84rm250

Posted October 08, 2009 - 09:43 PM

#5

Is this true?? Any way to check the cams and tell if they have?

  • wobblyknobby

Posted October 08, 2009 - 09:50 PM

#6

Easiest way to check your cams is up against a known good pair. That gear is pressed on and slips if the valve gets smashed up against the piston. It can leave a bad spot on the cam and lifter cup also.

  • wobblyknobby

Posted October 08, 2009 - 09:52 PM

#7

It sounds like you have already found out the cam is off when you were checking the marks.

  • 84rm250

Posted October 08, 2009 - 09:59 PM

#8

Marks all look fine, cam sliping in the timing sproket sounds possible.
I dont really have another set of cams laying around for it is the thing. I'd have to order a new set witch I dont want to do if its not going to fix it.
Is there any way to check them and make sure they are bad?

  • wobblyknobby

Posted October 08, 2009 - 10:13 PM

#9

You could use a degree wheel to know precisely or you could compare it to the owners manual by eye. I see cams on ebay very cheap but you must verify that they are out of a running engine and not one that ate a valve. If none of your intake valves were bent, then that cam might be ok, and vice versa. Also, after rebuiding it, my 426 did not have stiff compression until it ran for a minute.

  • 84rm250

Posted October 08, 2009 - 10:22 PM

#10

Could you kick yours through over and over with no decomp lever at first?
If I recall right it was two on the intake side. Could have been both tho. I dont have a degree that would fit/work on the bike.
But now you have me really wondering about this.
I never even thought about this possibility. Gotta love TT

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  • wobblyknobby

Posted October 09, 2009 - 12:44 PM

#11

Could you kick yours through over and over with no decomp lever at first?
If I recall right it was two on the intake side. Could have been both tho. I dont have a degree that would fit/work on the bike.
But now you have me really wondering about this.
I never even thought about this possibility. Gotta love TT


Yes I could kick it over without the decomp before the initial break in. But all the same, more likely than not, your cam gear has slipped also. It happens when you smash a valve. I would hit up e bay for a good intake cam. Heck, a new one is $110. Also, I have seen bikes run with slipped cams but they run like crap and are impossible to tune.

  • JJRace

Posted October 09, 2009 - 01:33 PM

#12

Just a simple check, but take the lever off the decompression cable to make sure it has slack in it. If the cable is routed wrong, it will stick the valve open.

Otherwise, if you have to buy new cams, look into a set with the exhaust autodecompressor on it. 03+ yz450F cams will work also. Then you can lose the decompression cable, buy a new plug for the whole in the head off Ebay or TT if they still have them and dont worry about the starting drill.

  • grayracer513

Posted October 09, 2009 - 02:07 PM

#13

03+ yz450F cams will work also.

Not exactly. The timing marks won't line up in a 426 head. You can use a 450 exhaust cam if you still have a 426 intake by setting the intake cam up on it's marks and timing the exhaust off of that, but if you had two 450 cams, you'd need to use a degree wheel.

  • 84rm250

Posted October 09, 2009 - 02:15 PM

#14

Thanks for the help guys, TT rules.

I checked decomp lever and its got around 1/4 inch of play before it touches the lifter cap. So that should be ok.
Maybe rings just need to seat and compression will be fine? But i still cant get it to start.
No easy way to check for slipage?
I just dont want to end up spending 250 on new cams and end up not fixing my problem. I'm sure you can all relate to that.
Found some stockers on ebay that said they came out of a runs bike but who can trust that these days.

  • yzhaulin

Posted October 09, 2009 - 03:10 PM

#15

Here are some pics of the stock cams from my 01 426. I posted these before for a guy with a similiar problem. Hope they help.

Posted Image

Posted Image

Posted Image

  • 84rm250

Posted October 09, 2009 - 03:21 PM

#16

Thanks for the pics, this is from what i can see how mine look. They both sit in the head facing away from each other.
Thanks again!

  • 84rm250

Posted October 09, 2009 - 09:56 PM

#17

Put it all back together again after checking timing for the 3rd time. I kicked it over about 10 times and it back fired and shot flame out the pipe once and thats about it. Witch leads me to think the timing is off.
Is it bad to keep kicking it over like this trying to get it to start? Never being ran before i'm worryed about not have enought oil to everthing.
Any help would be awsome, thanks!

  • Sider_steve

Posted October 09, 2009 - 10:20 PM

#18

Just out of curiosity,did you have the correct timing mark located on the flywheel?
Not sure if you have a manual for the bike,but if you don't you can get one here.
http://www.yamahaown...ook.com.au/?r=0
Sounds like you are close.

  • grayracer513

Posted October 10, 2009 - 07:37 AM

#19

Pull the spark plug and use a probe to verify that true TDC is where the flywheel says it is. The key may be sheared or out of position.

  • 84rm250

Posted October 10, 2009 - 10:53 AM

#20

Ya I have a manual a useing all thnd i'me right marks. Using the I on the flywheel andlining the dots up on the cams with the head.
I have use'd and pice of wire to make sure it was on TDC. Everything lands and lines up like i should be dead nuts on timing. But it dont act like it..





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