Just bought a '98 YZ400, the bike has about 30 hours on it and is stock except for a smaller countershaft sprocket and a 10 ounch flywheel weight. Sometimes when I blip the throttle it bogs for a split second and then kicks in. I guess you could say it has an intermittent hesitation. Any ideas what this might be?
I didn't know what I was missing riding a 2 stroke so long, Thumpers forever!

Paul in AZ

First thing to check is the Accelerator pump.

Pull the loosen the screw that holds the boot for the air box on the back of the carb. Slid the boot off and fold it back so you can see in the carb inlet. Look at in the carb inlet at the bottom of the slide. Don't get your face too close! Now open the throttle while you look in.... You should see a small stream of gas squirting back at you from the bottom right ( as you look in ). If you don't see a stream, you've found the problem. You need to service the pump. Let us know what you find and we'll go from there....

By the way, if you search the archive's you find info on the pumps....

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Chris in the Mojave
'98 YZ400F

Thanks Chris, I'll try to take a look at it later this week and let you know what I find. I review the archives as well.

Paul in AZ

I have a 99 yz 400 that is also basically stock with the same problem except that it will not only hesitate, but also quit when the throttle is hit wuickly. I've checked my accelator pump, adjusted the valves, tried to jet the carb (173 main moved the needle clip in several position), checked the TPS, etc. I don't know what else to do.
Can anyone help?

Nick

James Dean, do you have any idea on how far the fuel mixture screw should be backed out?

Thanks,
Paul in AZ

Paul,

You've made sure the pump is working, right?
The pump on the '98's has an exposed shaft, the shaft sticks or crap works in to the diaphram an stops the pump from working, this is REAL common on the '98's.

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Chris in the Mojave
'98 YZ400F

Hey Guys!

I ride WR but I figured I'd see what you guys are doing over here. Keep it in the family...

I wanted to add my 2 cents to your problem. I just had the exact same problem. My bike periodically would die as soon as I hit the throttle from an idle. I also thought it was a lan condition, but I was very hesitant to change anything because the bike had run perfectly until this point. So...I went home changed the oil, cleaned the fuel filter and cleaned the carb. I also did one other curious thing. I had recently read in one of the bike rags that said if you keep your gas in a plastic container for more than about two weeks you can really loose fuel quality. The only way to get more storage time is to store your gas in a metal can. So I replaced the gas and guess what...the damn thing runs perfect. I can't tell you which of the changes made the difference, but I ain't bithcin. Change your oil, clean your filter, clean your air filter, and put new gas in your tank. Hey what could it hurt...

JJ in WA

Quote

James Dean:
After working out all the kinks w/my WR250F via TTalk, I thought i'd try to get the old girl('99 YZ400F-bone stock)running right. Your old post seems to be where I need to start to solve the throttle hesitation, bog, when first cracking the throttle or coming out of turns, and jumping that plagued the '99 model. Or, is it a P38 mod I need or what ? A search didn't turn up much. Do you have any more to add on this topic ?

New 98 yzf 400 owner, new to this website as well. I noticed the hesitation problem
the first time I test rode my bike. If I crack the
throttle wide open, it will just kill it! Other than that, it rips! Ive been told that this isnt
a jetting problem(per say) Its the accellerater
pump,?..and theres an after market fix for it?
Its pumping to much gas too quick...? or not enough-lean? Obviously we all have it! four strokes have always had a little "lag" when you
hit the throttle.....but it shouldnt kill the motor!( dirt thumpers have never had accel-pumps
before) and maybe its just not right.

There are some articles by Brian kinney?B.K.?
that I found,discussing this problem-and how to fix it( limiting the accel pump stroke) I havent
done it yet,I just took it apart and came in to
read it again. Theres another fix for the 426s,?
I dont know what it is,(I think accel pump also)
that they call the B.K. mod ? and everybody loves
it.
Then again,I just got this bike,so,...
I dont know fer shur
motohed clothing