help with jetting for freinds 2004 400ex

[ratso]

My friend just got the bike. It pops like a popcorn machine on gas off gas then falls flat. Bike has a k&n filter with a yosh slip on and a dial a jet. Help!!! it had a 165 main and he put in a 155 but still pops. It acts as if there is not much differance. I don't know what the dial a jet does but have adjusted. I think it is like the mixture screw under the carb. There are no leaks. Any help will be appreciated Thanks.

[ratso]

ttt

[BigBore24]

when does it pop? at idle or what throttle position. if it is idle then adjusting the main jet wont really help. i would say you are probably lean on the pilot jet. do you know the size of the pilot thats in it? if its stock i would bump to a 40 or 42 keihn jet.

[ratso]

the pilot is 42 it pops when you start to get into mid end power. Also on decelleration

[BigBore24]

generally backfiring on decel means you are lean. when you say it pops i assume you mean backfiring out of the exhaust. are these keihn jets or dynojet. the screw on the bottom of the carb up by the intake boot is the fuel to air mixture screw. if you back it out it will richen the mixture. after backing it about 3 turns you are no longer affecting it. I wold think a 42 pilot and a 155-160 main jet would be fine (as long as they are keihn jets) you also definately need to find out what position the clip is on the needle. it mostly contributes around mid throttle. the clip shold be in the 3rd slot from the top.

[ratso]

Thanks for the info I will find out what jets he has. What is wrong with dyno jet??? I have a 250ex with a k&n and yosh with dyno jet kit and it runs fine 96 main 2 clip.

[BigBore24]

nothing wrong with dynojet. they just are sized differently than the keihn jets and i am not familiar with the sizing comparisons. somewhere on this site there is a chart to compare them