YZ 426, RED hot header pipe+backfiring

Hi, I am new to this site and need some help.

I just put a new piston+ rings and adjusted the valves on a 426. I got it all back to gether and it seems real lean on the bottom end, lots of poping out the muffler at idle and up to quarter throttle. The header pipe also gets CHERRY RED if bike idles for one minute. Is this a common problem on the yamahas? What is the likely problem? ]

Please help this is not my bike I am doing this for a friend. :thumbsup:

I have an 03 YZ250F and my header also gets red hot. I put one of those Boysen waterpump covers and impeller kits on, they go for about 149$ but they make a big difference.

The header pipe also gets CHERRY RED if bike idles for one minute. Is this a common problem on the yamahas?
NO, its not. Oh my gawd, I hope she doesnt blow. :thumbsup::bonk: Just kidden dude. Its very common. :thumbsup: Use the search feature, you can find a zillion threads on this. Tell us more about the backfiring though, when does that happen? Is it just popping?

It pops at idle with choke on or off and pops and back fires on acceleration up to half throttle and pops and back fires on decelaration.

Hey man

I had a simular prob with my 00 426. Dosent sound as bad as yours but it did backfire on decel alot. My carb had a 42 piolet jet and I just bumped it to a 50. It went away immeadiatly. also try adjusting the piolet screw (only screw on bottom of carb) counter clockwise to allow more fuel. You shoud first turn it all the way in and then out in 1/4 turn incriments cracking throttle each time until you reach the point of highest idle. IF That dosent work re-jet. Good luck! :thumbsup:

Also be sure to check the fuel screw has not fallen out!Happens all the time,it will run bad pop and carry on like crazy!We have had many fall all the way out,from guys trying to make up for a lean pilot jet,go out to far and it fall out.Cant hurt to look. :thumbsup:

Also be sure to check the fuel screw has not fallen out!Happens all the time,it will run bad pop and carry on like crazy!We have had many fall all the way out,from guys trying to make up for a lean pilot jet,go out to far and it fall out.Cant hurt to look. :thumbsup:

i agree with crfthumper. other thoughts i have are, does the bike have factory exhaust? some aftermarket exhaust will reguire richer jetting. check the joint between header pipe and muffler if that is loose or leaking it will cause popping.

the exhaust will certainly glow cherry red while idling, it will glow less red when throttle is revved while idling, im assuming more airflow through the system cools the header pipe slightly. once you are moving it cools right down. the bike will also start puking coolant in three minutes or less while idling.

i run my carb float bowls dry after i wash and im getting bike ready for storage, and as the float bowl level starts to get real low the bike starts to pop, so you may want to check your float level, shot in the dark but you can always check.

on my 04 450 the dealer installed a #45 pilot jet before it was even sent home (stock is #42) runs awesome :thumbsup:

well thats enough rambling!

it could have something to do with the hot start

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