I have 400ex with FMF Powercore IV, K&N filter and Jet kit. I'm running stage I and the 146 main (I'm at 400 ft). I adjusted the Fuel mixture out 3 1/2 like the instructions. Runs really strong low and mid but seems to sign off on the top end early.

There is black soot on the screen of the pipe and plug looks rich. Here is my question:

Should I drop down one size on the main? The kit came with (134,138,142,146 which I'm using, 155,160,165,170). Is the 142 Main the one I should use to run more lean or is there something I should try between the 142 main and the 146 main.

Second, would a slight adjustment to the fuel mixture screw without changing the jetting make it run leaner? My understanding has always been that the pilot or fuel screw only had a very slight impact on full throttle and mostly impacted idle to first 1/4 turn of throttle so I don't think so but thought I'd ask.

Thanks for the help.

 

Actually turning the pilot screw affects the whole throttle range. Thats why it sould be dialed in first. What you are doing by turning it out (counter clock wise) is opening an air screw, letting more air passed the throttle plate. The pilot fuel circuit is always on, so it will affect everything. Although it is not as critical as main jet size and jet needle length.
Hope this helps

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The Keihin carburetor maintenance article now accessible from the "What's New" section of the ATV Connection home page contains a useful URL reference at the end, suggesting jetting techniques.

While Thumper4X4 has a very hot and well-tuned quad (he won the Rocky Mountain Mud Bog with it, against ALL comers), I was under the impression the pilot screw of the Keihin controlled fuel flow, not air as he says, and turning the screw counterclockwise made the mixture more fuel-rich. I agree with the Hard-Packer; when the throttle is wide open, the pilot jet's effect is overcome, and the mixture richness is largely controlled, by the main jet.

I don't believe you need to split hairs, finding intermediate jet sizes in the assortment you have--the changes from the jet in your carb to the next number in your possession appear quite conservative.

I replaced the Phillips-head screws on my float bowl to Allen-head screws, then hex-head machine bolts, so I could more easily drop the bowl in the limited space available without removing the carburetor to change main jets while experimenting.

Good luck, and may you find that coffee-with-cream hue you seek soon!

Tree Farmer

 

bshiels,
Being more familiar with Mikuni carbs than Keihins, I would think there is a jet between the 142 & the 146, even tho your kit didn't supply it. That's quite a big drop. Of course, you obviousy know how the bike is running & what the plug looks like, but you might check on an inbetween size to try.
Thumper is correct re the pilot: it's always open but would have no affect at full throttle: in fact at that rpm, all jets are open.
400' at Dallas? Is that a typo? I thought Big D was higher than that.

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THE AZ HARD PACKER, calmiller@kachina.net

save the public land FOR you, not FROM you..join the Blue Ribbon Coalition online @ www.sharetrails.org

 

Thanks for the responses. I'll give the 142 a try and see what happens.

I could be wrong about Dallas being about 400 ft above sea level, it may be higher but I know it is still under the 3000 ft mark where I would use the next jet.

Changing the phillips head screws is a great idea. I think I will do that. Sure would make quick main jet changes a snap..

 

bshiels, I have a '00 400EX with a similar set-up. Here is my set-up for my region (IL at 743 ft above). I am running a FMF Powercore IV-sa, K&N filter w/outerwear, and Dynojet kit (junk!). Also, I drilled 11-1" holes in my airbox lid and had little, blue filters in them. I have taken the filters out and am just running the screen-like holders that the filters were in now though. The filters would get wet and throw off the carb's mixture (would not let enough air in). I have the Dynojet needle set on the 3rd clip (from the top w/the needle pointing down), a #160 main jet, stock #38 pilot jet, and the fuel mixture screw turned out 3.5 turns. This set-up really works well. But here is what I suggest you do. If you are not running in wet/winter conditions, take off the airbox lid. Run about a #160 (+/- a size depending on sparkplug color), run a #40 pilot, set fuel mixture screw to about 2.5-3 turns out, and raise the needle one notch (e-clip to the 4th hole from the top). Getting to the needle can be a pain! There are two phillips-head screws that hold it in place that have a tendency to strip out. These mods are good only if you take your airbox lid off! Be sure you are running an outerwear to help pre-filter the air before it gets to the K&N. I would not recommend buying a Dynojet kit (same kit that is in the K&N Powerkit). These mods that I mentioned will give you better performance than that overpriced jet kit offered by DJ. Just get yourself a pocket full of mains and start jetting. For a good source about what to look for in sparkplug color, go to www.motorcross.com and look under sparkplugs. Also, when it comes to adjusting the mixture screw here is a tip. When the screw is between the carb and the airbox, it is for adding air (turn out to lean) and when the screw is between the carb and the manifold/cylinder, it is for adding fuel (turn out to richen). One more thing to consider, the stock main jet on a '99 400 EX is a #140 and the stock on a '00 400 EX is a #148. I don't know why.

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a Fast Boy from Illinois -(FBI)- Riding a 2000 Honda 400EX