What am I missing? HELP!

10 replies to this topic
  • DRH

Posted January 26, 2006 - 01:24 PM


1999 YZ400F Major backfire on decel. Have completely checked and sealed the exhaust system to where I am satisfied that there is no exhaust leaks. I have installed a #48 pilot, (richer than stock) at 1 1/2 turns out. The valves are adjusted to specs, the timing is dead on and the carb is clean inside and out, and the slide is in the correct postion. I have interchanged three different exhaust systems including the original stock unit. Nothing but nothing has made one single change to the backfire condition. Interesting is the fact with the choke out no backfire symptoms exsists. The bike fires up on the first kick every time.....It has to be something simple. But what. Oh by the way, all other jetting specs are stock....Thanks

  • 2012 450 xc-w

Posted January 26, 2006 - 02:58 PM


Adjust your fuel screw

  • kawirider

Posted January 26, 2006 - 03:17 PM


Adjust your fuel screw

what he said

  • WR450F_RDR

Posted January 26, 2006 - 05:22 PM


Your bike is a little lean.

  • jrcgaf364

Posted January 27, 2006 - 03:07 AM


on my 99 400f i have the fuel screw out 2 full turns. but thats neither here nor there due to the fact that my jetting isn't the same

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  • 2012 450 xc-w

Posted January 27, 2006 - 05:59 AM


reccomend purchasing an aftermarket fuel screw that can be adjusted with your fingers, makes doing this much easier than with the stock screw.

  • DRH

Posted January 27, 2006 - 04:12 PM


The adjustment I made was from stock to the #48. The backfire was there with the #42 and it is still there with the 48 Not even a hint of letting up.

  • DRH

Posted January 27, 2006 - 04:16 PM


It use to have the Zip Tye one, but the thing kept backing out and had a sloppy feel to it. I wasn't happy with that particular product. To your knowledge does any of the after market screws come with the spring, o-ring and washer?

  • grayracer513

Posted January 27, 2006 - 04:28 PM


Before you pull any more hair out over this, check your exhaust system for leaks, such as a loose midpipe clamp, or the joint at the head. Air leaking in is a major cause of this problem, and if your exhaust leaks air, you'll never get the bike rich enough to make it stop.

Do be aware that decel popping at a reasonable level is a normal condition.

  • kizzle426

Posted January 27, 2006 - 04:28 PM


You're supposed to use the ones that came on your stock one. It says that in the instructions. Maybe that's why you had problems with it?

  • DRH

Posted February 04, 2006 - 06:01 PM


Dispite by best efforts to insure that the exhaust was sealed, as it urned out, after many many hours of tinkering with the jets and pilot screw, it was in fact the header pipe where it mounts into the muffler. I tightened the clamp to a snug fit, and on a hunch, I removed the mounting bolts to the muffler from the frame, and without so much as an effort, I was able to slide the muffler off the header pipe. The stock gasget was no longer of any use. Not having a new one to install, I reinstalled the muffler, and cranked down big time on the muffler clamp and the popping all but disappeared. I am sure a new gasget will eliminate what popping is left. Now I bet I wil be slightly on the rich side, having installed the 48 pilot jet........

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