Z400 won't idle backfires HELP
 

I just bought this quad today and for the first 1/2 to hour it ran like a champ but after awhile it would not idle and it backfired but only when letting of the throttle. When put on 1/2 choke it idled great. I messed with the idle screw but it didn't really help. The guy I bought it off of said it had a max of 25 hours on it. I changed oil, oil filter, cleaned and checked air filter and box, took the spark plug out and cleaned and checked it, drained the gas and put new in, took the fuel line off of carb to make sure it was getting gas (what does PRI stand for in the petcock, what's its purpose?) I guess the next thing to do would be to take off the carb and clean it. Any help would be GREATLY appriated!!!!!! Thanks!!

Z400 won't idle backfires HELP
 

i would just buy a new plug and clean carb, to answer the question on what the PRI stands for is primary. this allows you to run your tank so low and then when u have 2 tenths of a gallon of gas left you put it on RES which is reserve.

Z400 won't idle backfires HELP
 

I agree with cleaning the carb how knows how long its been. THe PRI position stands for Prime. This position is used for checking the fuel supply system. It's also used for hard starting in below freezing. That's what the manual says. The on position should be used for normal use.

Z400 won't idle backfires HELP
 

The PRI stands for prime...and it means the valve does not need vacumn to flow. Both on and res need vacumn from the carb to allow gas to flow. Theres not any difference to ON, except pri doesnt need a vacumn signal.

You say you adjusted the idle screw....how about the mixture screw? Try going 1/4 richer, and that should make it idle better, and stop the backfire on decell.

Z400 won't idle backfires HELP
 

make sure the choke is not stuck.....

then if all is Ok with that turn the black knob on the LH side below the carb...........

Z400 won't idle backfires HELP
 

does it have any mods on it?? it sounds lean, if your pulling the choke and it runs better then its needing some more gas.
what year is it? i know the 05's air/fuel screw is on the bottom of the carb , and has a copper cap on it. you will have to take a little drill bit and drill in lightly till its in good. then just pull and the cap should pop out. see if he's played with the jetting any.