1998 YZ400F backfire and won't idle...please help!

I have a 1998 Yamaha YZ400F that I bought new in 1998 and never ever had a problem with the bike until now. It backfires on de-acceleration and it will not hold an idle without the choke...I have done no performance modifications to it which remains bone stock. It just started having this problem one day, it's not like it started gradually and got worse. If anybody has an idea what may be wrong please respond I would greatly appreciate it.

check the TPS sensor, it sounds like the problem, we had a few of them at the shop with that same problem.

Is there an air leak.....air boot or air filter prob?????

TPS could be a suggestion...thank you, also I do not believe there is an air leak past the filter, the airbox is cracked all the way around the seam but that should not affect it that much to make much of a performance loss, and the intake boot looks sealed all the way around some I'm not sure if that's it...But thank you guys for the suggestions ill check into it some more.

Thank-you for both suggestions i'll have to double check that stuff

I'd look for an air leak from the description you gave. Check the seal for the air box to boot seal, the spigot seal at the head as well as the o-rings on the carburetor. The intake bell on the carb has a nitrile ring that can be sucked out by the intake pressure or move if not replaced carefully. I had this happen to my 426 in 2003. Just a thought and the TPS suggestions are good gouge to follow.


had the same problem on 250f checked everything called yamaha they said to replace my needle & nozzle problem solved they were worn out

The '98 had a hot start that consisted of a frame mounted air valve in the middle of a hose that runs from the air boot to the intake port on the head. Be sure of the connections and that the valve closes.

Thanks for the suggestions everybody I'll give them a try and see what happens.

my yz400f did the same thing (wont idle and backfireing and runs good with the choke on and the idle wound down) and it started when i was riding with the fuel tap off, could it have sucked up some grit in the bowl? whick jet should i concitrate on cleaning ? also i broke my tps cable casing at the top but im still using it, its just not adjusted rightbut it has run fine for the last 4 rides.

Typically a bike that only will idle with the choke on has a restricted pilot jet.

sounds like an air leak too me aswell

Typically a bike that only will idle with the choke on has a restricted pilot jet.
Or a stuck hot start, or an air leak, especially with the hot start set up a 400 has.

I have a 99 YZ400 that was running great in the spring and it sat for 6 weeks. The last ride the bike would not idle and backfired when the throttle was chopped. I am guessing a partially clogged pilot jet but wanted to know if Tommyyzf369 had a resolution to his problem?

Yha, i agree with the rest.

just check the carborator for grit, and that all air leaks are fixed (including hot starter) and just do a general check of all the needles and jets to make sure they are all set right.

This is a long shot but check to see if your air/fuel mixture screw at the bottom to the carb fell out.

This happened to my sons bike. Sometimes the screw will just vibrate out.

Hope you figure out what it is. Good luck

i cleaned my carb and check and rechacked for air leaks, took it to yamaha and the adjusted my tps, checked my vavles and put in a new sparkplug for $110.

runs like a champ!!

but its a good thing i took my carb apart cause it was nasty in there.

Right on.

i found a blocked pilot jet. its all sorted now, runs perfect! thanks heaps.

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