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View Full Version : 400ex w/ white bros. which jet size to stop the popping?


starcrossed010
08-29-2005, 01:59 AM
put a white bros. slip on exhaust on my 400 and i was wondering what size jet i should bump up to from stock size... it pops and back fires on deaceleration because its running lean, i know i could take it apart get the stock jet out and bump it up a few sizes but what the hell if someone else has experienced it already, share your jet size!
prepracing
08-29-2005, 02:05 AM
Its just pretty much just test and tune, but you will need anywhere from 155 to 165 main jet, maybe adjust air/fuel mix screw some to fine tune it.
FoxMxRiders
08-29-2005, 04:59 AM
I dont know for the main jet but when you are in deceleration,this is the air screw that make you lean,adjust it and no more popping.I put a pilot jet 42 in mine yesterday and he was popping with my pipe and filter and now no more popping :D
<DRS>GPF
08-29-2005, 05:58 AM
Originally posted by prepracing
Its just pretty much just test and tune, but you will need anywhere from 155 to 165 main jet, maybe adjust air/fuel mix screw some to fine tune it.


agreed... i have a 160 w/all of the pipe's discs on and it feels strong with no popping..

i dont believe the pilot jet will have anything to do with more than idle, but since your gonna be there...
replace it with a #42 and itll help with cold starts..
phildogg
08-29-2005, 09:47 AM
the main jet has nothing to do with popping on deceleration. the main is only used with 3/4-full throttle. I do think the pilot is the cure for the popping on deceleration. which pipe are you running?
phil
FoxMxRiders
08-29-2005, 02:19 PM
Originally posted by <DRS>GPF
i dont believe the pilot jet will have anything to do with more than idle, but since your gonna be there...
replace it with a #42 and itll help with cold starts..

when you left the throttle,the needle comme back all the way in the main jet then this is your pilot jet that take the command.That's why when you decelerating,this is the pilot that help for the popping
jamienlogan
08-29-2005, 09:40 PM
Is it bad that it is popping? Mine pops to when I decelarate. It runs great other wise.
FoxMxRiders
08-30-2005, 04:54 AM
popping mean that your lean on the air/fuel screw.Just open a little your air/fuel screw,that suppose to correct the problem,but if you have like 3.5turn on the air screw and it pops,change your pilot to a 42 and you will be fine