'04 WR450 Carb on '99 WR400

[Rich_in_Orlando]

I just got the '04 WR450 carb I won off of eBay earlier this week and put it in the bike. I was told the bike had less than 200 miles on it and I believe it. The carb looks brand new. I didn't do anything to it jetting-wise except open up the fuel screw about a half a turn. It took a few kicks to get it started (I had to dial in the idle speed a little bit) but it ran fine once I got things dialed in. I revved it but didn't ride it. So far it seems fine just like it is. Has anyone out there done this before? If so, what was your experience with the jetting? Did you have to adjust it or put the needle and jets from the original carb in the new one? I don't want to do this if I don't have to and was hoping to get some advice from those who have done this already.

BTW, the reason I did this is because the slider keeps cracking on the original carb and I figured a newer design carb would solve the problem. The slider design is different (it seems to have some kind of damper in the throttle to keep it from slamming shut.)

Now that I'm typing this, I have no idea if the throttle stop has been cut so it looks like I need to take a look at it.

[Rich_in_Orlando]

I just went to the exploded parts diagram from the carb and the jetting really doesn't look much different from the '99 jetting.

Here are the standard jet settings:

Main jet- '99 168 '04 165 - not a big deal

Main air jet - '99 75 '04 70- not a big deal

Pilot Jet- '99 42 '04 45- not a big deal

Needle- '99 DTM '04 DWR (pretty big difference here but it does have the same taper angle. '04 is fatter, so leaner but the clip setting height is 5 clips richer?? so it may just about even out)

Start jet- '99 60 '04 65 (this may also be one reason it was a little hesitant to start at first. May do some tweaking of this one)

[Rich_in_Orlando]

Damn lot of help you guys were! ?

I replaced the start jet with the one off my old carb and went from there. (65 down to a 60) It doesn't need the choke to start, which is a little puzzling, but then again, it's 92 degrees in my garage so it wasn't exactly a cold motor. Without the choke, it starts on the first kick. It runs better than it did with the old carb on and the front tire wants to lift up much easier now than it did before. Maybe I won't have the carb slider cracking issues any more. I think I like it.

[Domenic_WRF]

Good job I think going for a 37mm to a 39mm will improve top end if you're YZ timed..Remote hot start an a leak jet are a plus. Good luck and let us know how it does on the trail.. ?

[Rich_in_Orlando]

Actually, the WR4xx bikes all had 39 mm carbs from the beginning. There were no fitment issues as regarding the carb joint between the motor and the carb or the intake between the carb and the airbox, so it was an easy replacement. The main issue was figuring out where to locate the remote hot start. I ended up putting it on the throttle side so I can activate it with the right thumb. There was interference between the decomp lever and the hot start when I tried it on the left side.

I'm sticking with WR timing. Been down the YZ timing road before and I don't think it's ideal for tight trails like we often ride. Harder hitting but bottom end grunt (read: more clutch slipping) is the trade-off when you go to YZ timing. I've said this before, but I think Yamaha engineers know what they are doing with this bike and I try to leave it as close to stock as possible. (Notwithstanding the original carb design issues. At least they learned from their mistakes.)

[Rich_in_Orlando]

Someday I may try an exhaust cam off of a WR450 to have autdecomp to complete the modernization of my old bike. I change out the cam chain every year and a half or so anyway so the issues of different tooth pitch may not end up being a big deal.

[MattyH]

as i understand theres no cam chain pitch difference so you should be able to slot the new cams in without a drama.

i have a set of 450 cams, with the decomp, for sale. i'd much rather sell them as a set but if the right offer is made i'll part with just the one of them. pm me if you're interested in the cams.