Hey guys i have a2002 400ex it has a full Yoshimura exhaust brand new ITP tires and rims the original owner poke a hole in the screen of the muffler tip and it backfires don't know if its that or if it could use rejetting runs really good just back fires at high rpms please need help

 

DirtDude,
Your new exhaust gives more power and may need larger jets.
It is such a cheap experiment ...try it.
Backfire is going to happen if you run too lean.
A hole in a screen is redundant.
Poote

 

Ill try that could a little exhaust leak in the front header were it mounts could that cause it to backfire to

 

Dirtyriderboy
From Clymer shop bible,
Back fire occurs when fuel is burned in the exhaust.
1. too lean
vacuum leak
not enough fuel pressure
dirty carb
2.Loose exhaust...like u suspect
3. leaks in intake
4.timing
5.stuck or leaking reed valves
A 400cc two stroke is a monster good luck.
Poote

 

Its a 4 stroke new to the website it will only do it at high rpm's

 

my boy has the same set up . bought the same yoshi full system. started to backfire........... as suspected. replaced main jet with a dyno jet 170 main. also removed air box lid. bike runs amazing now