Nothing does - tons of questions

3 replies to this topic
  • Math

Posted June 08, 2005 - 08:11 PM


I've tried other jetting settings tonight in order to get rid of the bog. Seems to be a rich bog since 1) it becomes worse as the engine gets hot 2) rising the needle worsen it.

01WR426 yz timed, air lid removed, FMFpc4, stock header, 0-1000', 40-85 deg.

So far I've tried

170mj with combinations of clip 3 to 5 and PJ 45 and 48.

I know that the bike pulls very hard at 170mj, clip 4, 48 pj but the bog kills the engine as soon as the throttle is wacked.My latest attemps were worthless.

The timing of the pump is finely tuned now and the squirt is thrown right under the slide as soon as it clears up. Next step, will do the BK mod.

Can somebody tell me why the BK is effective? I mean why does reducing a squirt from 4 sec to 1 sec would get rid of a bog that occurs within a millisecond after the throtle is wacked. The bog is already happening way before 1 sec. I really don't get it. I rather think the flow is too big. I wish I had a leak jet on this 426. Can somebody explain why 426s don't have those leak jets.

I'm even thinking about checking the valves back again.

  • Gadsen

Posted June 08, 2005 - 09:15 PM


I've tried other jetting settings tonight in order to get rid of the bog. Seems to be a rich bog since 1) it becomes worse as the engine gets hot 2) rising the needle worsen it.

01WR426 yz timed, air lid removed, FMFpc4, stock header, 0-1000', 40-85 deg.....

Math, I have a Candian WR450 and it pulls from idle to WOT and zero bog. I can blip it any time/rpm and it will roost. I have the FMF PC4 also, with the quiet core insert. Have the fuel screw (zip ty) out 1.75, run a #168 MJ, JD blue needle, clip position #3, all other stuff is stock. Starts easy, runs perfect, no backfire etc, no bog, smooth stable idle. Its dialed perfect. And it runs great from sea level to 4000 feet. I start getting only a slight hesitation when I whack the throttle about 4000 feet.

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  • Hamish

Posted June 09, 2005 - 02:52 AM


change your riding style! You don't need to go from idle to WOT. My 426 will also die if I do that, for that matter any well tuned flat slide equiped bike will (ducks for cover :) ) At 3000rpm, your engine simply doesnt need that much air, which kills the port velocity, which I assume prevents the fuel from atomising correctly. If I want full power from low RPM I will give it about 1/3 throttle instantly whick will give the engine the jump it needs, then feed it harder as the rpm rises. Have you ever driven a car with a Holley double pumper?
This is just my opinion, many will probably disagree, but thats the point of the forum.

  • Math

Posted June 09, 2005 - 05:24 AM


Thank you for your specs Toy. I just don't have the JD kit ... not yet. JD told me that my needle is probably limiting the throttle response.

Hamish, the bog occurs even if I don't go to WOT. It has become worse than stock. Just giving throttle 0 to 1/2 quickly will kill the engine. I can't even pop up emergency wheelies in the woods. It is becoming dangerous.

I want a bike that snaps like a CR250! :D

When I bought this bike in 2002 I felt like this was the first 4 stroke available that could give me the snap of a 2 stroke even if there was this off idle bog that I thought I could cure.

Now that I've given it a pipe, YZ timing, better timed AP, new jetting under TT members advise including JD, all I've done is worsen the bog. :) Man, my old 1983 kdx200 had better throttle response than this 426. It looks like unless you drill holes in that carb (bk mod), you can't get rid of the problem.

I'm not an engineer at all but I feel like the the stock mechanism of that carb is very very very far from being to the point.

I know I'd probably cure my bog if I'd do the BK mod and buy a jetting kit from JD but man, I'm a week-end warrior :) Aren't jetting kits and Tim Ferry's mechanic mods a little bit extreme? Should not this bike satisfy me without having to go that far?

I'm so bored this morning that I just think I'll throw back this bike to stock specs including pipe and timing and buy a 300EXC.

Seriously, I'll visit a yamaha racing shop here in Montreal and see what they can do to help me. I'm a mechanic newbie, having pros looking at the bike can't hurt. I'm sure I did something wrong.

One of my friend who's a KTM dealer and mechanic says I'm hard enough on that bike in the woods so that I might need serious racing folks to map my jetting in order to be satisfied. I don't agree with him really, I'm 28, 6'1, 170 pounds, in really strong shape but I estimate myself as an average rider at the most. But if it can help, why not trying.

Thanks for listening :D Makes me feel better. :D


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