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MAZMAN

YZ400f bogging on WOT

11 posts in this topic

hey guys,

thought i would ask a simple question to you guys, while a bit of a novice thing to me.

I have a 99 yz400f, yoshi muffler/pipe/twin air filter.

Just bought the jd kit and had a bit of a play with it.

I have installed the following:

main jet :168

pilot : stock

needle: blue #3 position

fuel screw: 1 1/4 turns

What happends is, when in idle and you go to throttle it fully as fast as you can, it bogs, like it is going to die, but if you do it slowly, it is ok.

i would like to know at what mod would i be looking at to rectify this, would this be caused by the fuel/idle screw, or needle position? When do i use the red colour needle?

Does this sound like it is suddenly running rich until it clears up, or is this a lean out?

Im in australia, NSW from sea level to 3000ft, if that helps.

cheers

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May be running a tad rich, try dropping one main jet size, if it cleans up without losing any top end, thats it.

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I believe this is what the so called "BK Mod" is supposed to remedy. Search for it and you can read a lot of posts about it I'm sure. I haven't done it myself, but my bike runs well and I haven't really felt the problem. My bike was jetted and tuned by a pro race mechanic before I bought it, he may have already done it, I'm not sure though.

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WOT sounds like a main jet issue. I got a little troubleshooting guide from my Dynojet kit that was a little helpful when adjusting, here it is in PDF

Scroll to the second page

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...when in idle and you go to throttle it fully as fast as you can, it bogs, like it is going to die, but if you do it slowly, it is ok.

i would like to know at what mod would i be looking at to rectify this...

Modify yourself. There's no practical reason for an engine to be able to respond to an instantaneous full throttle snap from idle, and there is no realistic reason to expect it to be able to. Learn to roll the throttle a bit more gracefully, as thumper riders have pretty much always had to, and you'll never notice the tendency to bog while underway. If you do, the engine is way off.

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Like everyone else said, you'll never cure the bog entirely. And FYI, it's not a WOT bog, it's an off idle bog. You just happen to be going WOT very quickly. The pilot circuit and leak jets are good places to look to remedy the problem, as well as the BK mod. Not sure which way to go though. Maybe try posting your pilot jet and leak jet sizes too. Someone may have more info for you.

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I have and ride the snot out of a 99 YZF 400 with out a single mod. Stock pipe, jetting, etc. The bog is cured with a simple no cost solution. ROLL the throttle on. If you pin the throttle on ANY bike from an idle you will get a bog. Trust me. I ride the same machine. Just make sure your air filter is clean as these beasts like alot of air and a dirty filter will cause you some headaches.

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May be running a tad rich, try dropping one main jet size, if it cleans up without losing any top end, thats it.

i ran a 185 main in my 98 year round and it was not even close to being too rich.if you have an aftermarket pipe -opened up airbox etc.i would run atleast a 180 main

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Modify yourself. There's no practical reason

Gray - As succinct and to the point, as usual :banghead: . Best advise I've seen handed down in a while :applause:

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I went with the edelbrock carb (no tps, hot start , or choke) and man what a difference. i went from hating my yz to loving it. I started out with the stock 99 fcr carb, upgraded to the 2002 model and the bog was gone. Now with the edelbrock its faster then ever...

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