WR450 bog w/ low rpm throttle whack

My 03 WR450 has all the free mods and I just changed the following:

Needle from OBEKP #5 to JD Red #5

Main Jet from 145 to 165

Pilot Jet unchanged, 45

Pilot Air Jet unchanged, 70

Starter Jet unchanged, 65

Leak Jet from 70 to 52

fuel screw from 1.5 to 1.75 turns out

Exhaust - from uncorked stock to YZ

disconnected TPS

Before, it had a slight WOT miss. Now it totally hammers, except if I'm cruising at low RPM in 2nd or 3rd gear and then just whack the throttle open. Unless I kick it down it'll bog for a second or two before catching up and hammering again. I'm having a hard time keeping the front wheel down in all other circumstances, I'm really happy with the changes. I'm at sea level and it's about 85 degrees and humid tonight. Should I go back to the #70 leak jet or is it an AP issue. I believe the delay is still at the stock setting, but I don't think the pump squirt isn't hitting the slide.

Someone please help.

Put in a 48 pilot, #40 leak jet and then test.

If a bog is still there delay your AC pump squirt 1/4 turn at a time until it bogs no more. :)

I can't remember which way delays the squirt :):D:D:) ...SC

Quote from Thumperfaq:

"The other part that is often ignored is the adjustment screw that comes on the linkage stock. This is to set the starting point of the accelerator pump. Turning it in delays the start and turning it out advances the pump action to start earlier. At immediate squirt from idle would be zero delay from the "touch point", where the rod just touches the diaphragm. A setting of 1/2 - 1 turn inwards delays it adequately to keep from squirting on the slide. This, I believe, was the .6mm measurement with feeler gauge. The original BK Mod posting from SUnruh also had the pump stroke set to 1 1/4 turns out from touch point to set the flow. This did not say the pitch of the screw being used and so was a little vague."



I tried the 48 pilot and the 40 leak jet. The bog got worse. I turned the AP delay 1 full turn in, in 1/4 turn increments, but the bog is still there. I think the 45 pilot may work better because I was getting good idle at 1 turn out or less on the fuel screw, whereas yesterday it was at 1.75 turns out. The bog was never there with the #70 leak jet, but it also didn't have the YZ exhaust, JD needle or main jetting it has now. I also changed from a 65 starter jet to a 70 because I was having a bit of a hard time starting with the E-start, but that shouldn't affect this, should it?

Any suggestions?


Go back to your 70 leak jet. I have 05 WR450. I had the same problem. I jetted every which way, but couldn't get the bog out. When I went back to the stock leak jet, it worked great. Check out the tech articles on tuning the carb. Read the whole thing. It might give you some suggestions.


Shorty may have a good point. The 05 combustion chamber is detuned and compression is lowered so maybe what works for the 03/04 WR450s does not work well for the 05's. :)

Thanks, that's what I did. Back to the 45 pilot, 1.5 turns out and 70 leak jet. Also put the AP delay back. Kept the 70 starter jet because it seems to work. I'll give it a try tomorrow and update.

I had the same issue with my 04, but AFTER I installed the Boyesen Quickshot----BOG NO MORE!!!! It was worth it to me, I love it.

I had the same issue with my 04, but AFTER I installed the Boyesen Quickshot----BOG NO MORE!!!! It was worth it to me, I love it.

Yep your bike hauls booty :) When we riding again?

The best thing Ive seen on the 05 WR4.5 is the boysen pump cover and a change of the main and pilot. After seeing how well Endurordr's bike worked I pulled one off the shelf and tried it on the shop bike. All I can say is WOW.

Not gonna say how it works on the 03 ro 04 as the engine dynamics are slightly different. Those bikes accept the 40leak jet well, with a tweak on the APscrew.

Remember, this is an 03 WR450 I'm talking about. The #40 leak jet seemed to bog a lot worse than the #52. I'll do a test when I get home tonight and update, but how much delay should I need on the AP with the 40 leak jet? If everyone else is using the 40, maybe my delay isn't enough. I'm a little unsure of how to set the gap on the lever.

Time the squirt. Ill let indy xplain it as my instructions typically suck, but by going to a 40 leak jet you increase the squirt and need to delay the actuation of that squirt.

Anyway, where is the fuel screw adjustment at? the response of the fuel screw will give you an indication as to whether your in the right spot on your pilot circuit.

I went back to the 70 leak jet and 45 pilot at about 1.5 to 2 turns out. The 48 pilot seemed to idle best at about 1 turn or less. The bog is still there. Is it just that I am too quickly opening the throttle all the way at too low an RPM? I'm going to double check that the squirt isn't hitting the slide right now. Any help timing the squirt would be great. BTW, it's about 80 degrees and 125% humidity, I'm swimming :)

Just checked the squirt. I'm 99% sure it isn't hitting the slide. It looks to be a decent stream, I don't know about 8 to 10 feet, but definitely enough to hit the cylinder.

OK, I guess I'll have to ride with this. I went back to the 52 leak jet, 45 pilot 2.25 turns out. As long as I don't quickly go WOT from low RPM it seems to be OK. Definitely room for improvement, but workable. Maybe I'll try the Boyesen Quick Shot in a few weeks? Any other ideas?

You may want to raise the idle a bit. These bikes don't like to idle like an XR. Also, make sure your fuel screw is adjusted once the bike gets hot.

This is an excellent thread guy....... thank you for being so concise!


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