Need help with Jetting carb


17 replies to this topic
  • ButchB

Posted December 31, 2007 - 08:06 AM

#1

I read the sticky but couldn't find a definite answer. Hoping somebody will give me some advice. I just rebuilt my 01 426. Motor is all stock with the exception of the 03 cam and a wiseco stock bore piston. Bike seems to be running a little lean now. It pops pretty bad and header glows bright red after only maybe 20 seconds of idle. Carb is all stock now. I'm not sure of my elevation (Rockingham, NC). Can anybody suggest which jets to try? Or is there an adjustment on the carb I can make? Thanks

  • Wes Woodin

Posted December 31, 2007 - 12:15 PM

#2

I read the sticky but couldn't find a definite answer. Hoping somebody will give me some advice. I just rebuilt my 01 426. Motor is all stock with the exception of the 03 cam and a wiseco stock bore piston. Bike seems to be running a little lean now. It pops pretty bad and header glows bright red after only maybe 20 seconds of idle. Carb is all stock now. I'm not sure of my elevation (Rockingham, NC). Can anybody suggest which jets to try? Or is there an adjustment on the carb I can make? Thanks

The elevation in Rockingham, NC is 358 ft. I would richen up the pilot jet. I would buy a few of them and keep trying to see what works best. Don't forget about the fuel screw to fine adjust. All carbs are different so unfortunately it will be trial and error. I don't know what is stock, but you need to richen up the circuits. Good luck

  • ButchB

Posted December 31, 2007 - 01:46 PM

#3

Thanks, i'll give it a try.

  • Wes Woodin

Posted December 31, 2007 - 01:47 PM

#4

Thanks, i'll give it a try.

When you take off the carb tell us what the stock jets are in there.

  • 642MX

Posted December 31, 2007 - 05:13 PM

#5

I read the sticky but couldn't find a definite answer. Hoping somebody will give me some advice. I just rebuilt my 01 426. Motor is all stock with the exception of the 03 cam and a wiseco stock bore piston. Bike seems to be running a little lean now. It pops pretty bad and header glows bright red after only maybe 20 seconds of idle. Carb is all stock now. I'm not sure of my elevation (Rockingham, NC). Can anybody suggest which jets to try? Or is there an adjustment on the carb I can make? Thanks


Turn the fuel screw out about a 1/2 turn.

  • ButchB

Posted January 02, 2008 - 01:32 PM

#6

Pulled the carb today, it had a 178 in it. I went and got a 185 i'm gonna try but it'll be friday before i put it back together cause i ordered some new throttle cables to put in too. I set the screw at 1 3/4 turns.

  • grayracer513

Posted January 02, 2008 - 01:45 PM

#7

Are you changing main jets? The problem you're having is more likely the PILOT jet, as was mentioned earlier. Don't overlook that. You'll end up with your overall jetting way out of balance.

Also, be sure your hot start (red knob) is closing completely.

  • ButchB

Posted January 03, 2008 - 04:47 AM

#8

I got the jet that goes in the bottom of the carb, not sure which one that is? I don't know anything about carbs so its all new to me. SHould I leave the bottom jet stock and change a different one?

  • grayracer513

Posted January 03, 2008 - 06:59 AM

#9

The lowermost jet (the one with the hexagonal body) is the main jet. It has almost nothing to do with performance in the range you're talking about. I'll guess you don't have a manual. Get one here (free):

http://www.yamaha-mo...ice/manuals.jsp

The pilot jet is in a recess just forward of the mainjet (see pages 4-13 and 4-11). You will need a screwdriver that has a straight-ground blade running the width of the bore the jet lives in instead of the typically diamond shaped blade so that it will engage the slot correctly. Custom fit one if you have to.

Visit the ThumperTalk Store for the lowest prices on motorcycle / ATV parts and accessories - Guaranteed
  • Wes Woodin

Posted January 03, 2008 - 11:52 AM

#10

I got the jet that goes in the bottom of the carb, not sure which one that is? I don't know anything about carbs so its all new to me. SHould I leave the bottom jet stock and change a different one?

Butch, what is the year of your bike? And did you buy it stock and do the mods yourself? What cam did you put in it?

  • ButchB

Posted January 03, 2008 - 01:25 PM

#11

Just got back from picking up a pilot jet. It had a 47 in it and I got a 55 i'm gonna try.

My bike is an 01. I had the motor rebuilt, but the only thing changed from stock was a wiseco stock bore piston and the 03 450 cam.

  • ButchB

Posted January 03, 2008 - 06:41 PM

#12

Forgot to tell you guys my bike doesn't have an airbox lid. I'm sure thats gonna lean it out some too.

  • 642MX

Posted January 03, 2008 - 06:43 PM

#13

Just got back from picking up a pilot jet. It had a 47 in it and I got a 55 i'm gonna try.

My bike is an 01. I had the motor rebuilt, but the only thing changed from stock was a wiseco stock bore piston and the 03 450 cam.


A 55 pilot? I didn't know they made them that big?!? Why would you need such a huge jet for?

  • 642MX

Posted January 03, 2008 - 06:44 PM

#14

Forgot to tell you guys my bike doesn't have an airbox lid. I'm sure thats gonna lean it out some too.


None of the YZ426F's had air box lids.

  • ButchB

Posted January 03, 2008 - 09:05 PM

#15

wasn't sure. it looks like one may have been on it before. i wasn't sure if they came with them or not. As far as the 55 jet, I don't know thast why i'm asking. I just got the next size up because the bike seems to be running lean.

  • 642MX

Posted January 04, 2008 - 07:08 AM

#16

What is your fuel screw setting? The fuel screw setting will determine the need for a different pilot jet.

For example: If your screw is set 3+ turns out, then you need a larger jet. If your screw is less than 3/4 turn out, you need a smaller jet.

You need to adjust your screw out until it runs properly. Once you get it running properly, then turn the screw in until it seats while counting how many turns it took to seat the screw.

  • ButchB

Posted January 06, 2008 - 06:10 PM

#17

I put the carb back on today. Set the screw at 1 3/4 turns out. Bike seems to run better. It idles much better, no popping at all. Couldn't ride it much in my neighborhood but it seems to be better. Only problem I noticed is it seems like it doesn't wanna idle back down like it should. Could that be something that needs to be adjusted in the carb? Or is something with the throttle cables? Moving the cables around doesn't affect it.

  • grayracer513

Posted January 07, 2008 - 07:24 AM

#18

Not idling back down is a symptom of a lean idle. If turning the screw out farther doesn't resolve this, then check for free play and smooth operation of the hot start plunger, and then the valve clearance.





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