Yes, another jetting topic, just looking for confirmation

2003 WR450 - all free mods done, de-corked, gray wire, high flow air filter, YZ titanium silencer, 05 starter upgrade, and more...

I re-jetted and at first the bike was stuttering around 6500rpm(~1/2 throttle) so I moved the clip down a notch, now it's stuttering at 8500rpm (~3/4 throttle.) I'm thinking I need to go up a size on the main jet. Thoughts?

Edited by g17jimmy

So I guess we are supposed to guess the jets you are using now?

Opps -- I guess so. I shouldn't try to post when it's so late at night after a long day. I'll update when I get to work later...

I'm going to make an educated guess:

You are still using the original Needle jet (emulsion tube) and it is worn out and ovaled. Replace it.

Try a NCVS needle or a JD Red needle when you do that.

I'm an idiot, that's the problem. I forgot to tighten the clamp on the engine side of the carb. I'll tighten it when I get home today and test again, hopefully that solves the issue. Just in case I still have the problem, here's my thinking on the issue:

I picked up a set of FMF stage 1 jets (includes needle) for cheap. I've read a lot of stuff that people don't like FMF jets, or so it seems, but they were cheap. If they don't end up working out I won't be worried. I can see a lot of difference in the factory needle vs the FMF needle, but the FMF needle doesn't seem to have a part number on it, so I can't tell any specifics other than it came with the stage 1 kit.

I installed the new needle instructions say to install the clip on notch #2 and for the WR450 say to use a 140 MJ , or a 148 if the air box is modded (mine is.) Initially it was stuttering around 1/2 throttle, so I moved the clip to #3 and that's resolved. If I understand the graphs (link in the original post) of where the different carb functions come into play, >3/4 is the MJ. The issue I have is that the largest MJ the kit came with is 150, so if that isn't big enough I have to figure out where to get the right jet for the needle.

I must say just with changing the MJ/needle power down low is much improved. I always thought this bike was strong before, on a hard acceleration from stopped I'd be riding the tank just to keep the front tire close to the ground in 1st/2nd and the tire would come up a little in 3rd. Now 3rd is almost like 1st/2nd. I know the "seat of your pants" dyno isn't terribly accurate, but 3rd gear told me there was definitely an improvement.

I tightened up the clamp on the engine side of the carb and now it's stumbling at 6500 (1/2 throttle) again. I contacted FMF and they said to contact DynoJet. Great service at FMF /sarcasm....

As it is right now, 148 MJ, FMF needle(no identifying marks on the needle), 3rd clip position, all other jets factory.

FMF / dynojet jets and needles are notorious for being 'imperfect'.

Genuine Keihin jets only...........

After realizing that the loose manifold issue was actually helping, I realized that the needle was rich and moving the clip to position 3 only made things worse. I backed that off to position 1 and the bike is smooth now, but the low end power is missing. I don't race, and should probably not accelerate as hard/fast as I sometimes do, so that may not be a bad thing. The only thing that bothers me about moving the clip up is that it's not as strong at the very top end, either (70-90mph,) and since this bike is about 50/50 on/off-road, sometimes you do have to push it a little. I guess I'll play with jetting more when I need to take the carb off again.

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