Yz426f not starting and just backfiring

8 replies to this topic
  • brandi696

Posted March 20, 2011 - 02:43 PM


:banghead:Hey guys I am new here, and have few questions for someone.
I have a 2001 yz426f and it has been completely gone through, changed to the decomp cam to hot cams,(no more decomp lever) so here we go, after we shimmed it and put it back togther it started and ran great, so we shut it off, then we were able to restart it 6 times after that, then this morning I went to start it so I could break it in today and all it is doing is kicking over and backfiring, so we tore it back down to check the clerence on the vavles there just fine nothnig worng with them. What in the heck is wrong with this stupid a$$ bike? Runs great lastnight, then nothing today. As of right now I am ready to throw the "B" out and run it over with my truck.
Thanks guys....:thumbsup::bonk:

  • wanawakeboard

Posted March 20, 2011 - 02:49 PM


mine does that sometimes, a couple of slow rolls on the throttle before I kick and it will usually go, maybe confirm the plug gap if it still wont start. mines weird sometimes one kick other times pop pop out the exhaust..

  • grayracer513

Posted March 20, 2011 - 08:41 PM


Fouled plug is my first guess.

462's are notorious for that when you start them up and then don't actually take them out and run them long enough for the deposits to burn off of the spark plug. The 400/426 has fairly weak spark at cranking speeds, and it's easy to slop a plug up to the point where it effectively won't work. Try a fresh new plug in it. Then be sure you adjust your pilot screw for the new cam.

Another point that you may be overlooking with the new cam. When you had the manual lever the routine was to:
  • bring up against compression,
  • pull the lever,
  • advance the kick starter about one more inch,
  • then draw back and kick.

What the auto decomp does is to take care of steps 2 and 3 when you do steps 1 and 4. In other words, what you should be doing with the AD setup is to push the engine up against compression and then draw the lever back up and kick it right from there. Don't over think it and try to put the engine somewhere else in the cycle.

  • brandi696

Posted March 21, 2011 - 04:06 PM


Hello again,
So today we changed the plug while in the shop and also put the stock cam back in and it wont even fire.:thumbsup: Have no clue what the heck is wrong with this dang bike. Yesterday it would kick over and try but back fire and today nothing, and yes we checked the spark and it is awesome. What could possible bve wrong with it? Please help if you can anything at this point is worth trying.
What we have treid,
put oem valve cam back in,
changed the plug 8 times,
shimmed and checked timing
cleaned carb (very well)
put fresh gad in it.
checked the spark again still good strong spark.
What is the heck could it be?

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  • wanawakeboard

Posted March 21, 2011 - 05:10 PM


sometimes its hard to figure out when our bikes act up, when I am frustrated I try to go back to basics, what does that engine need to run?
fuel (air mix at proper ratio)
Spark (at the right time)
and compression. (plenty of it)

If you have verified each of these then it is likely it is a combination of problems or even a random issue.
I wish I could think of specifics to try but sometimes the advise to just go back and think about the basics can be helpful. Frustration clouds judgment.

  • brandi696

Posted March 21, 2011 - 05:35 PM


I have to say at this point I am giving the bike a week to run. We went back over the basics started at point 1 and still nothing, we have the stuff for it to run but it just refeuses to actually run, saturday we were able to start it and let it run for 10 mintues, and after that it started just fine. But then sunday nothing but a backfire, today nothing. This even has the mechanic in aww. He can not figure it.... Just tired of it all ready.

  • brandi696

Posted March 21, 2011 - 05:37 PM


I do have a question though do you have to do anything special to the nuetral switch after you put the comp cam in? Cause we took it off and put a block off plate on it, but it did not say if there was anything special we had to do after we took it off. Does anyone know about this? Hopeing maybe this is the problem?!?!??!

  • brandi696

Posted March 21, 2011 - 07:15 PM


Well we went back at again and started from point A. And still nothing, just can not figure it out. The beast has a week the ole man says and its gone. I just can nt understand why all of sudden this happened. Compression is awesome, spark is great, fule air miture is right on and still nothing. What I am wondering is; is it going to matter that we put a neutral switch block off plate on? Everyone says that that wont cause a starting issue. We have even took it a shop and it has the mechanics stumped also. THey said there was no logical reason for it to be doing this. Any new idea of what this could be?

  • grayracer513

Posted March 21, 2011 - 07:54 PM


If you suspect the neutral switch, you can duplicate the condition that is created in neutral by grounding the light blue lead that went to the switch. That makes a slight difference in ignition timing and resets the rev limit down to 7000.

One thing I wonder is whether you are strictly paying attention to the closed throttle rule when starting. If you are, that might be the problem. Try cracking the throttle open just above idle to get it going.

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