YZF jetting issues

[Shorty_925]

I did the top-end of my bike done 5 hours ago, and the shop set the jettign perfectly for my standard piep but have put a Ti4 pipe on my '07 yzf450 and the bike runs fine when on the gas, but its back-firing alot when im off it. Ive raised the clip up 1 but should i be going to a 170 jet or 175? Cheers.

[K6K]

Raising the clip makes it leaner I think?...(clip higher - the needle drops lower into the main - less gas) If the bike is breathing better because of a less restrictive exhaust it will be running leaner now. I would think you need to look at the pilot jet circut to make sure it is not too lean from idle to 1/4 throttle. Have you tried adjusting the pilot screw to richen it up?

[LittleJohn]

Raising the clip makes it leaner I think?...(clip higher - the needle drops lower into the main - less gas) If the bike is breathing better because of a less restrictive exhaust it will be running leaner now. I would think you need to look at the pilot jet circut to make sure it is not too lean from idle to 1/4 throttle. Have you tried adjusting the pilot screw to richen it up?

K6K is right on the money. Raising the clip will make it leaner, and you need to identify where the problem is occuring, i.e. where abouts on the throttle. The Pilot jet and air screw (pilot circuit) are from idle to around 1/4 throttle, jet needle and needle jet go from about 1/4 (just before) to about 3/4's then your main jet does the rest. Each different jet overlaps so you need to identify where the problem is before you start changing the jetting.
Once you work out your problem area, you should adjust the jetting. Then take your bike for a ride. Then pull out the spark plug and check it, make sure the colour is tan/coffee not white (running to hot or lean) or black (running to rich). If your plug is fine and all problems solved, you are sorted.
Hope that helps.

[Shorty_925]

Was running too rich when looking at the spark plug, as it was too black. The clip is second from bottom in the needle at the moment. The backfiring is only when off the throttle, as soon as I get back on it, no matter how much or little throttle it runs fine, its just when im off the gas it backfires.

[LittleJohn]

That means the problem is to do with your pilot circuit, which controls 0 to about 1/4 throttle (don't think of the throttle as how much the engine is running, more about the actual position of the throttle). I would start by adjusting your air screw, do about 1/4 turn adjustments (try out first) and see how it goes. If you end up with more than about 2 to 2 /12 turns out in total you need to change your pilot jet and then go through the process again. If you need anymore explanations, PM me and I will answer any questions you have.

[Shorty_925]

Will give it a go, thanks for your help.

[oldguy]

Ive got my eye on a YZ pipe trademe,

http://www.trademe.co.nz/Trade-Me-Mo...-290223868.htm

thinking of puting it on the WR, guy recon's it shold fit a YZ250 so think it should fit the WR.
If it does, will i have problems and have to go throw all this jetting stuff?

[CRF119]

Make sure you have also got you fuel pump/squirter set correctly. The YZF ones tend to lag a bit which gives hesitation from idle.

[barty5]

it will fit mate has a wr250 he races has all the yzf bits on it clutch/ piston etc etc they alll swap over but still gives hiim the 6spd box and keeps the lights to stay in class

[K6K]

Ive got my eye on a YZ pipe trademe,

http://www.trademe.co.nz/Trade-Me-Mo...-290223868.htm

thinking of puting it on the WR, guy recon's it shold fit a YZ250 so think it should fit the WR.
If it does, will i have problems and have to go throw all this jetting stuff?

If it is off a 250 then it should be fine but I didn't see anything in the tardme ad stating which it was off - I think the 450 header pipes are different. Also when I had a steel framed 05 wr250f I think I remember the later alloy framed 250's had slightly different exhausts to the steel frames (when looking at aftermarket part lists anyway). Worth doing a bit of research before bidding...

[camchain]

Hey there shorty, might pay to check header pipe gasket seal as well, - common cause of backfiring on decell. Did you use a new gasket? Give header a little rotate in flange to ensure it's seated square before tightening muffler bolts.