Access to the bottom of carb?

9 replies to this topic
  • Nevada426

Posted October 11, 2001 - 01:57 PM


I need to do some adjustments to my jetting and was wondering how to get at the bottom of the carb. Also, when I adjust the air screw do I screw it back in and then start screwing it back out? Sorry guys never touched a carb before, but my bikes starting to pop on deceleration and I'm afraid it might damage something.

2000 YZ426F
2001 CR500R

  • sirthumpalot

Posted October 11, 2001 - 02:59 PM


If nothing has changed (elevation, weather, motor mods, different fuel, lots of wear on the motor, etc..) and it hasn't been cleaned in a while then it may just need a good cleaning.

That said, the best thing to do before you start playing with the carb is to write down your current settings so that you can set it back if you goof it up. For the fuel screw, start screwing it in being very careful to count exactly how much you turn it before it is lightly seated. Then put it back to the original location and start your adjustments. As for how to get to the bottom, if you have a really short screw driver then you can get it in there as is, otherwise you may want to loosen the connectors and turn the carb sideways a bit. You can remove the coil by the kick starter and you will be able to turn it a bit more.

  • Nevada426

Posted October 11, 2001 - 03:15 PM


No, nothing is different. It's a little cooler now 80 degress versus 105 degrees, it may have done it before but I didn't notice it until about 2 weeks ago. One thing worth mentioning is I noticed it after an oil change where I had to take off the exhaust system, somebody on TT mentioned that a bad exhaust seal could cause it. The bike runs great it just pops on decel.

2000 YZ426F
2001 CR500R

  • yzernie

Posted October 11, 2001 - 09:33 PM


Be careful when turning the fuel screw in. Sirthumpalot is correct when he says to lightly seat the fuel screw when turning it in. If you twist it to hard to seat it it will damage the needle portion of the screw.

With a 25 degree change in temp., you may be running a tad lean. Try adjusting it richer with your fuel screw to eliminate the pop. If you go more than 2 3/4 turns out you may want to try a larger pilot jet.


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  • Glen_T

Posted October 12, 2001 - 04:48 AM


I'm with Nevada. The original question was how to easily access the bottom of the carb? Can you simply loosen the fittings and rotate it to gain easier access? or do you have to remove it? I just did the BK mod and have had some success, but I think I need to go to a richer jet. The access is discouraging me from playing with the scariest part on the bike. (CARB)
Thanks for any help you can give us!


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  • bbeakley

Posted October 12, 2001 - 06:17 AM


I changed the main and pilot jets on my bike when I went to an aftermarket exhaust and I was able to do it with the carb still attached. I simply removed the float bowl from the bottom and used short tools. I also strained my neck pretty good trying to look upside down at it Posted Image It's really quite simple. I'm dummer than a fence post and could pull it off, just have your owners manual on hand and don't force anything.

  • Jason_in_KC

Posted October 12, 2001 - 07:03 AM


Easiest way to access the main/pilot/fuel screw...take the coil(right side of bike)off, 2 screws. Loosen the the carb at each juncture..head/airbox, then rotate the bottom towards you. Drain the float boal into a container, that gold screw on the left bottom of the float bowl has a pink line attatched to it, the fuel will flow out right there. After the bowl is drained, remove the 17mm bottom bowl screw, dead center on the bottom of the bowl. Remove and look up in there...bam there is your pilot and main, the main requires a 6mm/the pilot is to the left and is small requiring a small flat head screwdriver..I can have a jet change done in 2-3 minutes tops. Hope this reason to take the float bowl off for jet changes. Jason


Posted October 12, 2001 - 08:53 AM


Jason, with the method that you described above, can you also get to the fuel screw?

And while we're on the subject, what does the fuel screw do? ( I'm a long time 2-stroke person, this thumper thing is new. )

What can I adjust by fooling with this screw?

Thanks for your time.

[This message has been edited by (edited October 12, 2001).]

  • Glen_T

Posted October 12, 2001 - 10:10 AM


Thanks Jason! Sounds easy enough.


  • mikeolichney

Posted October 12, 2001 - 11:45 AM


Jason's description is on the money. In addition, while the coil is off and the carb is sideways, the needle is easy to access. Its best if the tank is off, but I did it last night with the tank on. You will need 3mm ball end allen wrenches to take off the two cap screws on the top of the carb. Remove the top plate and activate the throttle so the slide comes to the top. Don't take the slide out, just remove the allen head needle retaining screw. The needle can then be grabbed with a set of tweezers, the ones that come with swiss army knives work well.

The fuel screw is easy to access even with the carb in its normal place. It is in the bottom front of the carb. Turning the screw out makes it richer. You can do this with a #6 screwdriver bit, the (small) one in the 6 in 1 screwdrivers like Moose sells works well. If your bike backfires on decell, back it out in 1/4 turn increments. I was running a 40 pilot in the heat of the summer, last month I had to go to a 42. This morning I had to go from 1 turn out to 3 turns out, the difference was about 20 degrees F. The 426s are very sensitive to changing conditions on the pilot circuit. My WR250F is not nearly as fussy. Its such a pain that I have ordered a Kouba adjuster. Its a permanent T extender screw that lets you adjust it without the screwdriver bit. I hate having to dig out a tool all the time.

[This message has been edited by mikeolichney (edited October 12, 2001).]

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