jetting

[WrAtHs]

Ok, I have a stock 400ex that I put a k&n + Outerwears on. Airbox lid on. I went to a #40 pilot and #152 main. I rode for a while and did the FOT, clutch it and kill it method of checking the spark plug. The plug almost melted my hand and was completely clean and bright white. Obviously too lean still. How big will I have to go up on the main jet to richen it up? I was under the impession that with this minor mod, a #152 main would be fine. I guess I was wrong.

Also, would the air/fuel screw have anything to do with this?


thanks :)

[GrounD PoundeR]

I had my quad set up as listed below and I used a 165 main / 42 pilot. Granted I dont use the lid on my air box. Your right about the screw for air/fuel mix. I though my kit came with instructions about it.

Hope that might help. Did you buy a Dyno kit? If so it comes with various sizes try a 155 and see how it works.

[WrAtHs]

I just put in a 160 main and the plug is still very white. What am I doing wrong? Do I still need to richen it up? All I've done is put a k&n on it.

I'm using a dpr8z also. Is the ceramic part of the plug supposed to be light brown with four strokes? Or bright white like mine?

I'm getting pretty frustrated here :(

[Ryan]

Sounds like you air screw. I would check this first before going any richer.

[WrAtHs]

I know where the mixture screw is, but that's about it :) What is the method I should use to adjust it properly?

[Ryan]

Duncan Racing: AIR SCREW: The air screw is a small (5mm in diameter) slotted brass adjustment screw located on the inlet side (air cleaner) of the carburetor. The airscrew is a fine-tuning adjustment designed to allow the carburetor to be slightly adjusted for variances in atmospheric conditions. The airscrew works with the pilot/slow speed system of the carburetor, mainly affecting the engines initial starting, idling and initial power delivery. Proper adjustment of the airscrew can offer direct feed back on the necessary setting required for the pilot jet. The airscrew is adjusted in a rather straightforward manor. See Figure 1 for application chart

The ideal procedure for setting the screw in the correct position is to warm up your ATV engine to the proper operating temperature. Then turn the idle up so it is idling about 500 RPM’s higher than normal. Next turn the airscrew all the way in until it bottoms out, once bottomed out slowly back the screw out a ¼ turn at a time (give the engine 10-15 seconds between each ¼ turn of the screw, to allow the engine to catch up with the adjustments). Continue backing the airscrew out until the engine idles at its highest RPM. The preferred setting window is between 1 and 2 turns. If the engine idles at its highest RPM from 0-1 turns out this means the pilot setting is on the Lean side and a larger pilot jet should be installed. If the engine idles at its highest RPM at over 2 turns out, this means the pilot setting is on the Rich side and a smaller pilot jet should be installed.

If you get no RPM fluctuation when adjusting the air screw there is a very realistic chance that there is something clogging the pilot/slow speed system. Clean the system thoroughly with contact cleaner and blow out with compressed air. Carburetor must be disassembled.

If the airscrew adjustment process is unsuccessful and leaves you confused. Set the screw at 1 ½ turns out and consult a professional for further assistance.

For further jetting tips, click HERE (http://www.duncanracing.com/techfaq/Tech_keihin-carburetion-jetting.phtml)

[WrAtHs]

Thanks alot for the info. But it seems like that has to do with the pilot jet, and I'm not having idling trouble or anything. I'm just worried that my main jet may be too lean. Will adjusting the air screw make it run richer at FOT?

[Ryan]

Well it could be a handful of problems. You could be sucking in to much air or not enough air which could cause you to rean neither lean or rich. Good luck...

[12gofast]

There is no AIR screw on the 400ex....it is a fuel screw and adjusting this screw will only affect your 1st 1/8 of throttle.

You can seperate the functions of the jets in 3rds
pilot and fuel screw.....1st 3rd
needle and clip............2nd 3rd
main.............................WOT

[WrAtHs]

Well, my whole problem is just that I keep moving up main jets and my spark plug remains white and new looking. Should I keep moving up jets untill I see a change in the plug. Or should I leave it at the 160 main?

I'm just about ready to go back to stock cause this is getting frustrating and I don't really want to blow my quad up.

[toby400ex]

Also, how long does the new plug have to be used before you can read the plug correctly?

[WrAtHs]

Well, I ran mine all day with a #160 main in it and there was no change. Still white.

Also, why did you go to a 146 main jet after piping it? Isn't that a bit lean? What does your plug look like?

[trxex]

one would think a 160 main would be rich for just a k&n.iam running a 160 with my mods and my plug dont realy get much color also.i beat the crap out of my 400,maybe i should try a # 9 plug.because when my 400 was 100% stock i also had no plug color

[WrAtHs]

Won't the 9 run hotter though? I have a dpr8z. Should I go to the 9?

everyone i talk to says run a 9 becuase the motor will run cooler

[hon265mxer]

if u are having a lean problem u obviously mess with the easiest stuff first which is ur MIXTURE SCREW i dont know why everyone finds this so hard to believe, they must like spending money on jets they do not need, try adjusting ur fuel mixture screw out 1/2 turn at a time from the original stock setting until ur spark plug has a good color to it, but hey dont take my word for it im just the moron who has his honda certification, if u have a serious problem like this dont ask people that run a site b/c they obviously dont have a clue what is going on and if they did they would be sitting in a shop makin money on it, ur best bet is to find a mechanic that has some drag racing experience, its far from our field but in all reality dragsters have the most perfect engine and they know how to tune them

[WrAtHs]

Hmm, according to Duncan racing the mixture screw only affects idle to 1/8 throttle. I'm not really concerned about that. Does the mixture screw on a 400ex affect the full range of throttle.

By the way, I'm not doubting anyone on this forum. I'm the new guy here and I'm just trying to get as much info as I can :)

thanks

duncan has alot of expireince doing motorwork , honda tells u that about the air screw becuase thier probaly assuming its a stock motor and the mixture is off becuase of diff elevation not that youve added a performance exhaust and filter

[toby400ex]

Originally posted by WrAtHs
Well, I ran mine all day with a #160 main in it and there was no change. Still white.

Also, why did you go to a 146 main jet after piping it? Isn't that a bit lean? What does your plug look like?
I'm running a dyno jet main, which is a whole different numbering system, and its what it told me too run with my mods and elevation. It runs perfect:D

[WrAtHs]

Well, thanks for all the help, but I've decided to go back to stock. It's just too much too go through for something that won't even make much of a difference. I'd rather mess with my sprockets at this point.


thanks guys :)

[Poyntman400ex]

Can anyone help me figure out what the elevation of Dayton , Ohio. I too am installing a Dynojet kit. I'm going with the Stage 1 instructions. Too scared to run with no air box lid.

[TagRider]

Wraths, part of the problem you're running into is the gas you're using. Many of today's fuels come with oxygenated additives (ethanol) and detergents which help clean fuel systems. A byproduct of this is the inaccuracy of plug reads. The best way to figure out what main jet size you need is to put a jet in you know is too big (say a 180 Keihin jet). Get the quad warmed up and go ride. Pin the throttle in 4th or 5th gear and if the main is too big your quad will bog and run terrible. If this happens step down one main jet size and go ride again. Keep repeating this procedure until the quad stops running terrible on the top end.