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426 - Compression question

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Posted by: buck_y_lee---------------------

Howdy folks.

I need some feedback please. I can kick my 426 past TDC without the decomp when the engine is cold. Once it's warm, I cannot.

It starts well, runs well, pulls well. No smoke.

It sucked some water in a few weeks back, so I'm thinking that I may have scorched the ring/piston, but the fact that it runs well and has compression when warm is confusing. Could it be that a valve is sticking, allowing me to kick it through when cold, maybe gummed up with contaminated oil residue?

The top end was only done 6 months ago, and it doesn't have all that many hours on it. All easy hours too. hhmmff.. I have checked the decomp cable and actuator and all is in order.

Please share your thoughts.

Thanks in advance,
buck.


Posted by: Air Chunk---------------------

I would check the valve clearance first


Posted by: buck_y_lee---------------------

yes, I guess I should. Though I had them re-shimmed only 20-30hrs ago.

Thanks Air Chunk.


Posted by: Hick---------------------

Quote:
Originally Posted by buck_y_lee
Howdy folks.

I need some feedback please. I can kick my 426 past TDC without the decomp when the engine is cold. Once it's warm, I cannot.



Two observations from my experience w/ this bike:

1) Crack the throttle or turn up the idle and you will not be able to kick it through.

2) Both my YZFs ('00 and '01) exhibited this characteristic even when new. I've read many posts to the same effect at Thumpertalk.

I don't think your valves are sticking, and would characterize this as "normal" for this bike...


Posted by: buck_y_lee---------------------

hhmmm. Now I am second guessing myself as to whether this is a 'new' observation. I am almost certain that the first time I noticed it was after the bike was submerged a few weeks ago.

Could you explain to me why opening the throttle or idle screw would prevent this from occuring? Is it because more air would be sucked in to the cylinder, therefore increasing compression?

Thanks Hick.

Anyone else experienced this with their thumpers?


Posted by: Hick---------------------

Quote:
Originally Posted by buck_y_lee
Could you explain to me why opening the throttle or idle screw would prevent this from occuring? Is it because more air would be sucked in to the cylinder, therefore increasing compression?


Exactly.

You are welcome.

I also suspect a dry cylinder doesn't build compression as effectively as a well oiled (and warm) cylinder does.

I guess only you know how much water and debris could have gotten sucked into your motor when it was "submerged," but by checking the valve lash you should be able to rule out a stuck valve.

Hope this helps.


Posted by: buck_y_lee---------------------

Sorry Hick, what is the valve lash?

It took in a lot of dirty water. Probably a litre or so.


Posted by: Hick---------------------

Quote:
Originally Posted by buck_y_lee
Sorry Hick, what is the valve lash?




valve clearance = valve lash

Same thing.


Posted by: berudd---------------------

Hmmm, my 426 does not do this. If I put weight on the kick starter in a few minutes it will push through but I have never been able to kick it through TDC. Not new and not have 2 years of riding. On occassion when I accidentally do try to kick it past TDC it damn near breaks my ankle. If you can easily kick it past TDC check the valves and consider a new top end. Get a compression tester (or have it tested), I think manual outlines some specs on this but I may be wrong. If it hold compression like it should, then maybe you are just a hoss and kick like a mule!


Posted by: buck_y_lee---------------------

Thanks again Hick.

Nah berudd, I'm a lightweight.

Will get into it on the weekend. If valve clearances are in spec, top end will be pulled off and rebuilt.

Thanks for the help guys.


Posted by: jmics19067---------------------

if you have ever hit the decompresion lever while the bike was running you could have nicked the cam follower a little bit too cause a burr. MIght be just enough to stick on a dry cold aluminum start up but when it warms up the aluminum expands and gets oil..... Just a thought... if you have the valve cover off, take a close look at the right side exhaust valve lifter. There is also a chance of a slightly bent/corroded/dirty/worn valve stem where it has to ride in the valve guide doing the same thing between hot and cold


Posted by: buck_y_lee---------------------

Thanks jmics.

Geez I have a lot to learn. This will be my first time pulling an engine apart. My uncle is pretty good on the tools, so hopefully between the two of us and the manual we'll locate the problem and get it back together without further damage!

Thanks for the suggestions everyone. Keep them coming. The more the merrier.



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