Rejetting question for experienced......

[mikev7]

I needed to re-jet my 00 YZ 426. It was backfiring at the bottom end of the deceleration cycle. Say i rev it out to the rev limiter in 1st gear than close the throttle, at the very last 1000 RPM or so it starts going poppoppoppop. Not a loud gun fire backfire but a constant one.

I richened out the "jet needle" (not main jet) and it stopped a little but not enough. My spark plug is within range as far as coloring goes, so it's not running lean/rich at mid-full throttle.

What's the best next move? Should i richen out the pilot screw a couple of turns? Should i richen out the main jet needle? Should i richen out the jet needle one more position?

Only experienced peeps reply plz?

[ovrrdrive]

The first things I would have done is to try backing the fuel screw out a little. It doesn't take much to make a big difference.

I have a zipty and I adjust it almost everytime I ride.

I wouldn't go out more than 2 1/2 to 3 turns on the screw though, it can get loose and fall out. If you find it needs to be more than 2 1/1 turns out from seated, go up one size at a time on the pilot jet.

[YZ250F_Rider]

2 1/1 turns??? ?

[ovrrdrive]

???????

You know what i meant... grr...

?

[Pothole]

Yep, fuel screw adjustment needed. When I do my local ride, the elevation starts at 2500' and goes up to around 7000', and the temp this time of year changes 10-15 degrees, so very slight 1/8-1/4 turns of the fuel screw are needed to keep the bike running at peak performance.

[mikev7]

Thanks, i'll try turning out the furl screw 1/8 of a turn.

[yzf450jack]

Yup i have to turn the fuel screw everytime i ride ?

[ovrrdrive]

Thanks, i'll try turning out the furl screw 1/8 of a turn.

As lean as it sounds like you are, 1/8 of a turn probably won't help.

First, I'd seat it and count the number of turns in as you go to find out where it is now. That way you can make sure you don't go too far out. If you're below 1 1/2 out now, I'd probably take it to between 2-2 1/2 turns and see how it runs from there. If its better but still popping bad, go ahead and order the next size pilot jet.

The 1/8 turn increments are for after you get it tuned right and need to make minor adjustments.

[mikev7]

I'm on my way outside now to see if that fixxes it. In the words of the governer arnold swatsanigilililillllller. "I'LL BE BACKK!"

[mikev7]

I looked in the manual and didn't see anything labeled fuel screw. Is their another name for this? Maybe someone can point it out here http://www.ronayers.com/fiche/501_1989/carburetor/carburetor.cfm

[John_Lorenz]

If you have to move the Fuel/Air screw out moe then 2 - 2.5 turns chances are you wil need to up on the Slow Jet in the carb

Standard is 40 I believe you may need to go 42

Then start the Pilot Screw to 2 turns out.

My 2000 does the same thing, I have winter jetting and summer jetting.

item 24 is the Pilot screw sometimes called Fuel Screw

Item 21 is the Pilot jet

[YZ250F_Rider]

It's number 24 on the IPB, look at the bottom of the float bowl. The fuel screw is located directly in front of the nut drain towards the engine. It's recessed, so you wont actually be able to see it. It takes a small screwdriver to turn it.

Its located inside that little bump to the left of the guys pinkie.

http://www.thumperfaq.com/images/tmx_ap_05.jpg

[mikev7]

I read in the manual that you're not supposed to adjust the pilot screw unless you recalibrate the the jet needles. Whatupwitthat?

[YZ250F_Rider]

crazy japineers. Thats whats up with that. ?

I rarely screw with mine, and I even glued one of those knob tips to them. Once it's set it (the pilot jetting) wont need to change unless the temps change radically. And if the temps change enough to "require" a fuel screw adjustment then you had best be in there tweaking the main and the needle as well.

There are lots of guys that love fiddling with it as it's easy to tell when you muck it up, and it's just as easy to bring it back to where it's supposed to be. I figure their a bit figity. ?

[BC3]

If your bike never did the "Back fire thing " before and all of a sudden it does now......It could be and air leak at the exause system "fit up" Back fires aren't always fuel system related...If it started after you fiddeled with the fuel screw

then Yeah thats the cause and you need to adjust it back to where it was..

Generaly the backfire is caused by a combustable mixture in the exause pipe

and with heat you get a BOOM ...take away that mixture and it should stop..

I usually get a little back fire when the bikle is really hot...on long decell...

I think thats what your describing....try turning your idle down a bit if its too high or richning up the fuel screw a touch (1/8 turn).....Check your exaust system for leaks and also a tight exause valve can cause it.....Good luck..

BC ?

[mikev7]

It's the fall weather conditions that was making it do that. The bikes carb was set so borderline lean that when i went ride it (about 30 miles away) in the summer it wouldn't backfire at-all. I decided to change it cause with the changing conditions, the backfiring not only got louder, but it's doing it everywhere.

I took the bike on the street and did some wheelie,, i mean tuning/road testing and i knocked the problem right out of their. Since i richened the jet needle up a groove the bike pulls a lot harder in the midrange. The front wheel always just popped up in the air in the first 3 gears on the street, now it does it a lot more ?, and theirs no backfiring to be herd all the way down from right before the rev limiter.

I wasn't sure that the pilot screw was tight enough in the carb. I only turned it maybe one turn out, so i put a rubber plug in the hole just in case the screw decides to disappear one day.