400ex back fire

[honda123]

Just installed a HMF full exhaust system and rejetted with a 175 main. running a K&N with no lid no engine mods. bike has good response and big inprovement all around. Only problem is when i let off she back fires is this normal???

[nosliw]

Originally posted by honda123
Just installed a HMF full exhaust system and rejetted with a 175 main. running a K&N with no lid no engine mods. bike has good response and big inprovement all around. Only problem is when i let off she back fires is this normal???


no. 42 pilot jet should fix it.

[the Z Man]

You got an exhaust leak at the head pipe for sure, put a new donut inner and don't tighten er too tight... Z Man strikes again!!!

[Old Head]

According to the info on the dirtwheels web page the back is caused from your fuel screw not being out far enough. This screw is located on the bottom of the carb motor side. it calls for 3 1/2 turns out at sea lvl to 3000 ft i think. But you can also go a 1/4 turn out until your backfire stops but no more then 4 turns. If you hit that point you need to go up a size on your slow speed pilot jet and reset your fuel screw back to 1 1/2 turns and start the process over.

[the Z Man]

Exhaust leak!!!!!

[400exdad]

Z seems pretty confidant in himself, and he is prolly right. But the quad definately needs a bigger pilot. The factory is too lean even with the stock exhaust.

[honda123]

checked for exhaust leak found none. Spoke to HMF and was told that 2003 and older have a large enough pilot jet. was told to turn out air/ fuel mix adjustment 2 1/2 turns no more then 3 1/2.

[400exdad]

They're wrong. Mine is a 2000 and it had a 138.... which is too small. Go to a 140 at least. It will start much easier too. Not saying that the air screw won't help out too.

[the Z Man]

I ran a 150 main in mine, she really lit up nice,, I still say exhaust leak, you need to put your hand or something over the end cap before you can getter to hear it..

[honda123]

re- checked for exhaust even held a candel near flange no indication of leak.
as far as the pilot jet is concerned the numbers you guy's are mentioning sounds like main jet numbers not pilot.