HELP please! 99 yz400f

6 replies to this topic
  • jimbob69

Posted May 03, 2007 - 08:33 AM


Alright, so last August I took a nasty fall that spelled a torn ligament in my knee, That crash, the throttle got pinned wide open when the bike was on its side, and the big end crank bearing failed.

I've just now been getting it back together and running. This is the problem.

I finished putting it together last night, with a new crank, main bearings, piston, rings and timing chain, (and gaskets of course). Started right up, ran perfect .

So this morning I go out to take it for its break in ride, and it starts right up, no problems, I take off down my driveway, at the end of my driveway i let off the throttle, putting some decell load on the engine, then I pulled the clutch in and it just died.

I go to start it up again and theres no compression, or very little for that matter. Not enough to run with for sure.

Sooo, I pulled the valve cover off, and the cams were still in perfect time, cam chain was tight, all valves have proper clearance.

What could it be! im stumped and i am sooo sick of wrenching on this thing please someone help me!!!


  • tom nolan

Posted May 03, 2007 - 08:50 AM


With no compression, check your compression release. It might be rigged wrong and holding the exhaust valve open.

  • 426_kid

Posted May 03, 2007 - 10:22 AM


decompression relase fo sure.

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  • jimbob69

Posted May 03, 2007 - 12:54 PM


No , it isn't the decompression release. I have a hotcam autodecomp, and i checked and it is working fine.

  • grayracer513

Posted May 03, 2007 - 01:16 PM


Double check that the TDC mark on your flywheel is actually at TDC, and that your cam timing is OK, and that your valve clearances are good. Then have a leak down test done. This will tell you what's leaking away your compression.

  • jimbob69

Posted May 03, 2007 - 09:06 PM


The top dead center is true TDC, i never trust the mark on the flywheel, I use a runout (I think thats what its called) guage Thing to get exact top dead center. The valve timing is right on, i know this for a fact. and I quadruple checked all the valves and tolerances are spot on.

Remember, comp was perfect for what it should be for the autodecomp cam, and it started right up, then died with no compression.

when kicking the bike over slowly by hand, when it comes up on the comression stroke i can hear air hissing from the airbox, so that makes me think i got a intake valve thats stuck open slightly from a piece of carbon or something. Does this happen?, I did a solvent test on all the valves while i had the engine apart and out of the frame, and all sealed 100%.

What im thinking is maybe the solvent loosened up some carbon in the intake port, and a small chunk or flake got just right in there.

Idk, grey could you chime in?

Another question, anyone know of the best bang for buck when buyin head gaskets, I've had nothin but bad luck with cometic (blew three of them last summer) then i switched to OEM with no problems at all. but they charge fricken 35 bucks around here for one. and thumpertalk store aint too much less.

thank you


  • grayracer513

Posted May 04, 2007 - 05:20 AM


If it is an intake hanging open (and yes, that does happen), a check of clearance will find one with more than it should have. In that case, you can try tapping downward on the lifter to bounce it back shut. In the case of the center intake, you may have to remove at least the cam cap, if not the cam, to accomplish that.

As far as cam timing, I'm sure it was perfect when it started, but is it still, that's the question. Check again, if you haven't since it quit.

If it turns out to be the valve sticking from carbon, you ave two choices after you free it up. One is to pull the head and clean it out. Te other is to get a good "top engine cleaner" from an auto parts store. On a car, you would simply pour slowly through the carb as the engine runs at a fast idle, but that's obviously not possible with a side draft carburetor, so you'll need a squeeze bottle or a syringe of some kind and a hose on the end. Don't run the fluid in too fast, or you'll hydro lock the engine. Pour it through, shut it down and leave it sit 10-15 minutes, then run it fairly hard. Most TEC's can also be added to the gas.

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