99 yzf400 airbox lid???

I just traded 2 86 honda 50's for a 99 yzf400 and im having some of the idle and backfire problems ive been reading so much about. When i took it apart to clean the carb one of the first things i noticed was a missing airbox cover. I am curious if this could make the bike too lean to idle or if it is just something people do. The carb was pretty much spotless inside but i am going to pull it off and check for leaks. I did notice that when i went to start it the first time there was a smokeytype of puff under the carb but i did not get a good look. Lastly if i do need the airbox lid and you have one please let me know. Thanks for the help i can tell this bike is going to be an absolute beast once it runs right. (last bike was an rt 180)


If you want to find a stage two jet kit for the carb and maybe a slip on exhaust. when i upgraded my 400ex removing the lid was part of the upgrade procedure. the pipe is optional but will help it breath. hope this helps.

I don't think the yz used an airbox lid, just the wr. Check for vacuum leaks around the carb manifold boot and adjust the fuel screw. These things are very picky about fuel screw settings even with just temperature changes. Also the hot start can be leaking internal and causing the problem.

I will try plugging the hot start port on the head tomorrow and see if that helps. The bike has a new top end on it so i know the carb has been off before and im thinking the oring on the intakeboot may be damaged or gone.

Ya no lid on the YZ400, I went through the exact same issue when I bought mine a month ago. If you ever do cure your backfiring on decel issue let me know what you did! I bought a new head pipe, exhaust gasket, pilot jet, cleaned the carb 10x's with solvents and compressed air, adjusted my valves. Still pops on decel, I had to go to a 50 pilot jet also.

Bike runs great except for the slight popping.

Thanks for the lid info. I messed with it some today and actually had it idleing but i think the idle adjut screw was about all the way in to make it work. It sounds better and only really pops when i rev it up and let go while sitting still. I believe this may be from a small leak at the back of the header so im ordering a new gasket for that

My 426 did the same thing till I cleaned out the pilot jet with compressed air. No more popping and idles great.

i turned the fuel screw to 3 turns out and the bike starts reasonably easy and sat ideling for 20 min until i turned it off. Now i cant wait for it to stop monsooning so i can take it somewhere and ride it for the first time

How did you ever get it to idle for 20 minutes without overheating and seizing the motor?

good question !!

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