The power of the airscrew.

I have a problem with fouling. I run 175, 45 pilot, DVP needle in #2 position (which I hastily moved to #1 the hightest clip groove yesterday after fouling at the track). What I've read is that alot of WR's are running the DVP in the 3-5 range. Which would be richer yet then what I am running. The only thing I can think of is the air screw which I have at 1 turn out. Does the air screw have enough impact to cause fouling of plugs? My plug reads rich but my jetting sounds lean except for the airscrew. The bike runs great except for these occasional fouled plugs.


Just a heads up. The air screw you are referring to (on The bottom side of The carb) is really a fuel screw.

Thanks for the heads up guys...It still seems as though my jetting should be lean but the black plug reading shows rich. 175main, dvp - w/clip in the #1 - top position (leanest position) and a 45 pilot and pilot screw out only 1 turn out. I am running an E-series muffler with with no disc's, YZ timing, airbox lid off and riding in temps of 70 deg F. elevation of 1000ft. Are you guys that are running the DVP getting good plug readings? Could the fuel pump be out of adjusment? Thanks for any help on this matter.


Are you SURE you don't have an OBDKP needle??

If you did have a DKP the top clip would be equal to a DVP 5 clips lower.

DKP#1 equals DVP#6.... a rich situation needing a lean pilot setting, matches what you have done.

The Yamaha parts fiche is wrong for the YZ426 and lists a DVP where the part is actually a DKP. Look carefully at your needle. This would explain it.


[This message has been edited by James Dean (edited 10-27-2000).]

James Dean's thought is the only thing that makes sense to me as I use a similar setup, with out any fouling plugs, and your corrected remedies sound like it would be so lean that it wouldn't even run.

I use the following setup

180 main, 48 pilot, 100 air jet, YZ timing, DVP #4, DSP pipe and header, airbox lid removed, about 1 3/4 screws out.

If you truly have a DVP needle, I can't even imagine how your bike runs, let alone running rich.

Kevin is correct. Turning the fuel screw out(counter clockwise) enriches the pilot circuit. Clockwise leans out the mixture. Moving your needle clip higher causes a leaner mixture on top because you are moving the needle lower into the main jet nozzle which restricts gas flow at the same throttle postion as before moving the clip. If you look at your manual where it has the diagrams of how the jets and needle clip position affect mixture as you open the throttle, you will see how they overlap. Changing the jet size and/or moving the needle clip position changes the nature of that overlap. Clear as mud, huh? :)

[This message has been edited by Boit (edited 10-27-2000).]

Last night I pulled the carb to check the needle and it was in fact a DVP. Then on page 7-9 of the owners manual it recommends checking the overflow lines from the out to the garage I go and sure enough one of the lines is plugged up despite being slit on the end. Hopefully this will take care of the rich condition. I'll update after my next ride.

I checked the needle and it was in fact a DVP. So last night I noticed in my owners manual on page 7-9 it recommends to check the overflow tube on the out to the garage I went and sure enough one of the tubes was plugged despite the slit on the end. The outside of the tube was clean but the inside had a glob of mud wedged inside the I.D. Now I wonder how long that has been like that! Hopefully this will take care of my sooty plug and fouling. I'll keep you posted after rejetting and my next ride.


It will be interesting to see whether your jetting will be way off now or if something else is also causing problems. Be sure to check float level and all the passages such as pilot air jet.



You said 'no discs" are you running the end cap? White Bros recommends you run the discs (8-14) for optimum performance. I think from discussions on this board, most of us are running between 8-12. 8 if you need the bike quiter and 12 if you don't.


86TT225, 98CR80, 99WR, WR timing, throttle stop trimmed, air box lid removed, White Bros head pipe, silencer and air filter. Odometer and headlight removed. Moose hand and mud guards. YZ stock tank and IMS seat. Renthal Jimmy Button "highs" and Renthal Soft half waffle grips. AMA, SETRA.

I run my E-series with no disc's and just the endcap. I've seen pictures of Hawkins bike with an E-series running no disc's. But on the other hand I've read magazines that said that their testers preferred the muffler with disc's. My personal impression is that there was not much if any difference and most other silencers have straight thru designs...either way the WR has great power delivery.

would you let me know what you find out I'm having trouble fouling plugs my self.. like every ride...

Well I thought the fouling problem was behind me after finding the plugged carb vent line and cleaning it out. I even richened up the DVP needle to 3rd slot from top instead of the top groove which would put my jetting in line with what other people are using (also in anticipation of the colder weather I would be riding in this weekend). I turned the pilot screw out 1/2 turn further to 1 1/2 turns out. So I loaded up the bike and headed for a test ride...within 1 1/2 miles the bike was popping and runnning rough. I thought I probably just needed to lower the clip (make it richer) and everthing would be fine. So when I pulled out the plug expecting to see a lean condition I was surprised to see the same old sooty black plug... Is there anything left to do short of going back to the old jetting that worked marginally? How difficult is it to check/adjust the accelerator pump? Thanks for everyones help so far.

if you have a check valve on your gas cap hose take it off..

run a regular hose


Does this only happen at 1/4-1/2 throttle. This indicates clip position. There is always the possibility the needle was mislabeled or mismanufactured.

If it is rich below 1/4 throttle it is probably the pilot circuit giving you trouble. Maybe choke too. Your pilot screw setting says it is still rich at idle because you have been keeping it close to 1 turn. (not a needle problem at all?) The float level could be high. Pilot air jet clogged. Lots to consider...

Check that the accelerator pump starts to squirt at 1/8(or a little higher) throttle. Just after the slide lifts up out of the way.

Plug readings are difficult unless you are sure of the throttle position giving the reading.


[This message has been edited by James Dean (edited 11-04-2000).]

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