99yz400f locking up

7 replies to this topic
  • bob huffman

Posted January 24, 2016 - 04:18 PM


Bought this bike with a new top end rebuild. The guy that sold it started once but died and he thought it was an electrical problem.
I got it running right away but it ceased after only about 30 seconds of running.
After i tore into it i found a broken rod that had cracked the case and ruined the jug. I replaced the case, rod, piston, crank, and jug.
After i got it back together it ran great for about for the ten minutes i had to ride it. I shut it off and put it away for then.
Next time i got it out i started it up first kick and it ran for maybe five seconds and it ceased up, im completely sick about it and havent even bothered tearing back into it.
Any idea what on earth could be causing this issue?

  • harmetp

Posted January 24, 2016 - 04:53 PM


Maybe this sounds dumb, but does it have oil and coolant?  Does it overheat or just stop running.  Could be a bad oil or water pump.  

  • Vic The Rider

Posted January 25, 2016 - 08:21 AM


Maybe it's leaking coolant into the cylinder through a crack or bad gasket and it hydro locked. Just a thought when you say it seized remember these bikes don't have a auto decompression cam you have to activate it manually.

  • bob huffman

Posted January 26, 2016 - 11:25 AM


Yes has antifreeze and oil. Thought in the beginning starter gear was getting jammed. Would free up after messing with kicker and rocking back and forth in gear. Took it to shop and the guy said everything looked ok. Got back started up. Blew up. Rebuilt. Started a couple of times ran ok for maybe a total of 10-15 min. Then seized up again. Really haven't messed with it since. Frustrated. Can anyone help !

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  • harmetp

Posted January 26, 2016 - 12:34 PM


So it's not actually overheating, just getting up to normal temp and then blows up.  I'm not sure on these engines (never had my 426 apart), but does the piston have a slight offset to the wrist pin and could it have gone in backwards?  

  • harmetp

Posted January 26, 2016 - 12:38 PM


When you had the top end rebuilt was the lower connecting rod bearing ok and nothing wrong with the crank?  Years ago pressed together cranks could have the pin rotate a little which ruined the motor.  I'm just throwing out ideas since what you are experiencing doesn't happen unless everything is worn out and maybe not even then.  

  • grayracer513

Posted January 26, 2016 - 02:40 PM


If the piston were installed backward, the mismatch between the valve reliefs in the piston and the valves themselves would prevent the engine from rotating. 


The OP also states that he replaced the crank, although he did not state what the crank was replaced with.  The 400, 426, and the '03 450 were subject to rod bearing failures on occasion that can lock the engine. 


The engine needs to be torn down, the point of failure located, and the cause determined.  Then the condition that lead to the cause of failure must be found and addressed.  For example, if the cams seized, and you determine it happened due to lack of oil, then the thing that caused the loss of lube must be identified, such as blocked oil passages or a worn or broken oil pump.

  • bob huffman

Posted January 26, 2016 - 05:12 PM


I appreciate all the information you have all given. It's my sons bike he has done most of the work. I know the crank and rod where used and came together. Seemed like they where in decent shape. No play in rod to crank connection. Piston end of rod looked nice and smooth. When it blew up first time it actually busted the rod off the crank. Piston wasn't seized in cylinder. Wrist pin not seized in rod or piston. And crank and rod connection didn't show signs of seizing up. All looked like they where getting oil. It's like the engine came to a sudden stop and the rod flew apart. Could a gear in the engine lock so hard as to cause this? I'm not sure what the problem is this time. It seemed to be running fine. To my knowledge it doesn't have an offset to the wrist pin. Guess he'll have to pull it apart again. I just did a rebuild on an 85 xt 350 and haven't had any issues. These two bikes are like comparing apples and oranges though. Thanks for all your input. We appreciate it !

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