400ex Spudder?

[Halter]

OK, well I rejetted it...I think I got some bad gas in it right now too...but I am just wondering..I was coming home today and floored it..well when I had her floored then backed off to 3/4s it kinda like spudders and loses *** for about 3 seconds....u think I got too much jetting and go lower or do they all do that??...whats going on?


also I might go try it again see if I wasn't smoking somthing or not

[GPracer2500]

Most likely your main is a little too rich.

As the throttle on (most) carburators is quickly closed an enrichening condition momentarily develops. This happens because the carb slide is cutting down the air supply but it takes the carb a brief moment to react. The result is an instant of richness that dissappears as soon as things normalize. I'm not sure if I fully understand this phenomenon but I believe it has to do with transitioning intake air velocity and its affect on the vacuum forces that draw fuel through the jets.

I think of it this way: Accelerator pumps are used because when you quickly wack open the throttle the intake velocity drops and insufficient fuel is delivered. AP compensate for this by delivering extra fuel during those fast slide movement transitions. Well, the opposite also happens when the slide is quickly closed partially--the intake velocity momentarily shoots up compared to what is normal for that slide opening. The result is a moment of enrichening.

A "partially-chopped throttle studder" can also indicate a too rich needle. If a hesitation or stumble occurs as the throttle is chopped form about 3/4 to about 1/2 throttle than the needle is adjusted too rich or the needle's taper is too thin (rich).

Where is your needle set? Is it the stock needle? If leaning the main in an effort to eliminate the throttle chop studder results in poor WOT performance than focus some attention on the needle also. Leaving your main as-is (or close to it) and leaning your needle might provide the best all-around air/fuel mixture.

[04TRX400EX]

I have a similar issue along the same lines. When I floor it, in second gear for example, a couple of times my throttle has gone to wide-open, but my quad doesn't react at all - it just doesn't respond as if I let off the gas. Not what you expect to happen when you give it hard throttle. A friend said that maybe I was running too rich which is hard to believe considering that I am only one size up from stock and have a pipe and filter. Any suggestions?

[GPracer2500]

Originally posted by 04TRX400EX
I have a similar issue along the same lines. When I floor it, in second gear for example, a couple of times my throttle has gone to wide-open, but my quad doesn't react at all - it just doesn't respond as if I let off the gas. Not what you expect to happen when you give it hard throttle. A friend said that maybe I was running too rich which is hard to believe considering that I am only one size up from stock and have a pipe and filter. Any suggestions?

Take your airbox lid off. If the condition gets better your jetting is too rich. If the condition gets worse your jetting is too lean.

If you've got a pipe and a filter and your running a Kiehin 150 main then that sounds too lean to me. It depends on your elevation and typical riding tempuratures though.

[04TRX400EX]

Originally posted by GPracer2500
Take your airbox lid off. If the condition gets better your jetting is too rich. If the condition gets worse your jetting is too lean.

If you've got a pipe and a filter and your running a Kiehin 150 main then that sounds too lean to me. It depends on your elevation and typical riding tempuratures though.

That's what I was going to try. I always thought the main jet was small compared to others with similar mods too, but the quad has always run well until recently. We usually ride around Northern Phoenix so temperatures vary. What is coincidental is that this problem started when it got warmer here the last couple of weeks. I rejetted after I did the mods in October, when it was already getting cooler. Doesn't warmer weather make your bike run richer, or do I have that reversed? Also, are all jets/jet kit numbers (ex. 150, 160) the same? Is it possible that my "150" main jet is actually bigger than that?

[GPracer2500]

Originally posted by 04TRX400EX
That's what I was going to try. I always thought the main jet was small compared to others with similar mods too, but the quad has always run well until recently. We usually ride around Northern Phoenix so temperatures vary. What is coincidental is that this problem started when it got warmer here the last couple of weeks. I rejetted after I did the mods in October, when it was already getting cooler. Doesn't warmer weather make your bike run richer, or do I have that reversed? Also, are all jets/jet kit numbers (ex. 150, 160) the same? Is it possible that my "150" main jet is actually bigger than that?

Yes, warmer weather will richen your jetting. Warm air is less dense so there is less oxygen in an equal volume of warm air vs. cooler air.

No, all brands of jets are not the same. Factory Keihin jets use a different scale than Dynojets, for example. I think a Dynojet 150 is similar to a 165 Keihin. If your jets are Keihin there will be a "K" stamped on them in front of the number.

[Halter]

so I take it try it once then try it again without my airbox lid to see if its alittle rich...and if it is alittle rich I am going to have to adust the needle..or air fuel mixture screw or what?

[GPracer2500]

Originally posted by Halter
so I take it try it once then try it again without my airbox lid to see if its alittle rich...and if it is alittle rich I am going to have to adust the needle..or air fuel mixture screw or what?

For your case, I'd adjust the main jet first. Go smaller (leaner) and I bet that will clear up your sputter.

[Halter]

well let me run this tank of gas out..cause I think its that tbh..but we will see man