Questioni have a 1991 yamaha big bear 350. the atv starts fine using the choke and idles fine with out the choke after running about a minute. when i throttle it up it runs ok up to the mid range if i ease into it but then startes to pop and hesitate. it does this through all the gears. also if i go full throttle right away and don't ease into it it will stall. any help?
AnswerJoe, I went to http://www.bikebandit.com/houseofmotorcycles/yamaha-atv-yfm350fwb-big-bear-4wd-1... for illustrations.
From your description, the idle circuit and choke system seem to be working okay, which is unusual as they are often the first things to plug up on older vehicles and ones that sit for long periods of time.
That leaves us with the factors that can affect off-throttle, mid-range and top end....
Float level: Don't know if it is adjustable and what the specification is, though
Float valve restricted due to debris backed up behind the valve.
Main jet partially blocked with debris
Main jet holder/nozzle passageways plugged up.
Diaphragm for either anti-backfire or acceleration pump (not clear to me which one it is from the drawings) is torn causing either an air leak or lack of fuel spray into the intake tract.
Intake manifold leak or any vacuum lines attached to the intake manifold that are connected to components that can fail.
You might also check the valve clearances and verify that the ignition timing/advance is correct.
Fouled/cracked spark plug.
That's it from MrHonda....
Bill Silver
