03 450 new motor, back firing

10 replies to this topic
  • bigwhitediesel

Posted April 01, 2004 - 04:44 PM


I searched backfiring, but didn't find much help.
My top end blew up, and the dealer replaced it under warranty. Before this happened, she ran perfect. Now, when at decel, it back fires nonstop, until I hit the gas. And it pops when trying to start it.

Where do I begin?
The dealer had no business messing with the carb, right?
BTW, they had no idea what caused the piston rod to bend backwards.

  • felony

Posted April 02, 2004 - 03:06 AM


turn your fuel screw(on bottom of carb,front of float bowl)out a half turn to see if that helps.you have a lean condition which is causing the backfire on deceleration.backing the fuel screw out richens the pilot circuit.if you go more than 2 and a half turns out or so,you may want to go with the next size up pilot.look at a manual for instructions on performing this task,good luck

  • Fastest1

Posted April 02, 2004 - 04:40 AM


BWD, When doing a search, make sure you lengthen the time of the search. The default is usually one week, try all posts. It will give you much more! Good Luck! He is right on regarding the fuel screw. The popping is from a too lean setting in the pilot circuit. You actually want it to pop ever so slightly during deceleration. If you eliminate the pop entirely, your throttle response wont be quite as good as if it pops slightly. :D You might also want to check for air leaks around your carb boot (both sides). Start it up and while it is idling on the stand, spray some wd40 around the boots, if the engine speed changes at all you have a leak. :)

  • slappie

Posted April 02, 2004 - 05:14 AM


Is it possible to get to the pilot jet by just removing the carb bowl drain bolt on the bottom of the bowl or do you have to pull the carb?

  • jerryls

Posted April 02, 2004 - 07:16 AM


You can get to it by removing the bowl.

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  • skthom2320

Posted April 02, 2004 - 07:21 AM


You don't have to remove the bowl, just the bolt.

  • felony

Posted April 03, 2004 - 05:21 PM


you can get to the main jet by just removing the bolt on the float bowl :) bottom,but youll need to remove the complete float bowl to get to the pilot jet.

  • stroker101

Posted April 03, 2004 - 06:23 PM


air leaks in the exhuast maybe a culprit

  • bigwhitediesel

Posted April 04, 2004 - 07:07 PM


Thanks for the help guys. The air/fuel screw on my 450 is accessible without removing anything. But after getting out an old dentist mirror, I figured out my problem. The shop that rebuilt the motor, removed my air/fuel screw, and never replaced it :). Great service huh? :D

Does anyone think that might cause back firing?

Is it even remotely possible that the screw could've fallen out? It had never been touched before.

  • bigwhitediesel

Posted April 04, 2004 - 07:09 PM


I have had the nut holding on the head pipe fall off, and it most definitely back fired. That was the first thing I checked. :)

  • Fastest1

Posted April 05, 2004 - 03:01 AM


It is very easy to imagine it fell out. They do it all the time especially if they are more than 2 turns out! Glad you caught the problem. Now go buy an extended fuel screw that you dont need tools for! :)

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