yz400f popping on deceleration

I have a 98 yz400f that i race, its in really good shape but recently when im going into corners or letting off hard it pops like crazy. i know its a jetting problem but im stumped. some people i talk to say its running lean, some say rich. any help would be appriciated.



Turn your fuel screw out & if that doesn't work go richer on your pilot jet. My 06 YZ 450 popped like crazy on decell when I brought it home....upped the pilot jet to a 48 & no more popping.

Popping on decel can indicate a lean condition, but also air leaks (exhaust or inlet) cause this as well.

I just got done fixing that anoying popping sound by going up one size on the Pilot Jet, in spark plug and instaled an easy access air/fuel mixture screw. My problem was solved. Good luck

Popping on decel can indicate a lean condition, but also air leaks (exhaust or inlet) cause this as well.

Exactly! If this problem is recent and the bike was running good before hand then the jetting wouldn't be the problem. I had the same problem and traced it to a leaking rubber intake boot. I find it funny that people automatically jump to jetting when a problem arises. Re jetting is required normally when you add performance parts (ie: non stock pipes or other go fast goodies). Another reason to re jet would be temperature or altitude changes. If your ride has remained unchanged and a problem arises, then look for the problem.:banghead:

A quick side note: These bikes (98/99 YZF) in stock form are know to be jetted from the factory on the lean side, making them hard to tune for a lot of conditions, and they also stall easily at low speeds with the stock setup. Lots of info can be found regarding what jets to use but it depends on where in the world you ride. Mine works fine for the climate in TX so i just leave it stock. I just try to remember that when its cool out it stalls easier in the corners, so i just slip the clutch a bit to keep the revs up.

hope this helps...

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