temp change affecting jetting?

hello, this is my first post but i've been visiting the site since I got my 01 wr426 about 2 mos. ago. i think the site is a God-send, but up until know have had no reason to post. I bought my bike used and it is Yz timed with a white bros. full exhaust (7 discs), and air box lid removed, all of which the original owner performed. the only thing I have done is clip the grey wire, remove the coolant overflow, and change the oil to mobil 1 5w-50. I live in the piedmont of south carolina(<1000ft.) so when I bought the bike early August the temps were in the mid 90's with pretty high humidity. The bike absolutley ripped, all my 250 ridding friends were talkin about trading theirs in and how much power the bike had. But now the temps have become much more fall like with early morning lows in the low 50's and highs in the mid-high 70's. now the bike runs like crap. Every time I let off the throttle it spits and pops and sounds plain awful. It still has good power if you can get the throttle past half-way w/o it dying. it also will not idle unless the choke is pulled out, and then very rough and high. I've fouled 3 plugs trying to start it in the last 2 weeks, 0 in the 1+mos. prior. My question is will this amount of temp change have this big of an affect on my jetting. I know it's getting more air because cooler air is more dense than warmer air, thus the need for more fuel in the mix, i've gone a clip down on the needle(from 4 to 3, stock I think)and it did'nt seem to help. I know the same jetting questions have been asked and re-asked but I could'nt find anything that related exactly to my situation. what do you guys think? Everyone whose answered these questions before please show patience with a jetting newbie, and prove to my two-smoke buddies that my high maintenance powerhouse is worth the money. Thanks

go look at your weather forecast.

the bloke will say "i've got good news, we have another high pressure wind coming in so things will be warm n' sunny".

cold weather is low pressure so jet down. warm weather is high pressure so jet up.

you went the wrong way.


[This message has been edited by Taffy (edited September 30, 2001).]

[This message has been edited by Taffy (edited September 30, 2001).]

I had the exact same problem with additional humidity thrown in for good measure. I fiddled with the left hand hidden screw on the carb (pilot air jet I think) about a 1/4 - 1/2 turn out and it worked wonders for me.


The Missile

'99 WR400F

Airbox Lid - gone

Throttle Stop - shorty!

Plastics/Tank/Seat - One Industries

Street Legal


You've got the idea, now just turn out the fuel screw until the popping stops. If you get to 2.75 to 3.0 turns out, it's time to go up one, on the pilot jet.

Try to make only one change at a time and keep notes. If that works and your still having problems in other throttle positions. Post it and someone will help.



99 WR400f, YZ timed, MX-Tech suspension, Scotts steering damper, White Bros E-Series (12 discs), tapered header and a/f. Kouba T-handle for the fuel screw. Works Connection billet throttle tube and frame guards. Cycra Pro-Bend, triple clamp mount handguards. Thumper Racing rad guards, Renthal Jimmy Button highs, YZ Tank and IMS seat, YZ number plate, odo removed, de-octopussed (Thanks Clark) EKP #4, 50PJ, 175MJ, 100PAJ and 65 SJ at 500-1000' Thanks James Dean!

thanks for the replies.

What do you mean by jet up? Move the clip down therefore the needle up and vice-versa?

also, I just ordered an owners manual, so I don't know which is the fuel screw you guys are talkin about turnin. Is it in the bowl or on top w/ the needle. again, thanks for any help.

the pilot screw is underneath, yep.

yes,you should drop the needle. i looked at your write up and you say that you "dropped" the clip when infact you dropped the needle (the opposite)

the trouble is that one circuit affects another and they're all over the show.


[This message has been edited by Taffy (edited October 01, 2001).]

Taffy, I'm sorry for the misunderstanding. What I did was move the clip on the needle down, towards the "point", .is this going down or up, numerically?

Anyway, it does'nt really matter right now b/c I went to adjust the recessed screw on bottom left of carb and could'nt get my driver on it, got to lookin and much to my dismay saw it had vibrated completly out. So i'm on my way to the shop to see if maybe they've got one layin around or can order one for me.

Thanks again for the continued assistance.

-3 to 90 and no jetting or performance changes in my bike. Ran the same all winter and summer here in NH. We are low in humidity, maybe thats the big difference. Both my buddies 99 WR's had no issued from summer to winter either. Maybe we are at the perfect altitude or something.

What's the easiest way to access the pilot screw on the bottom of the carb? My YZ426 started popping on deceleration and I was told that this might help.


2000 YZ426F

2001 CR500R

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now