I recently bought a 99 400 ex. It has a jet kit, fmf powercore 4 slip on pipe, k&n and outwears, and a wb rev limiter. My question is it seems to sputter at low rpm ranges. When I ride wheelies it tends to sputter more than other times. It runs very smooth and strong at higher rpm ranges. Has anyone had this trouble; could it be caused by the pipe or jet kit? I read the booklet that comes with the jet kit and noticed that there were two stages. I think he probably installed the more aggressive of the two. Also it seems to rev quicker as it gets hotter. i.e. After riding it for a while it seems to have less top end, but gets there much quicker. I contribute this to me getting used to the power and speed. I have rode four wheelers all of my life, but this is my first sport quad. On a side note I think I got a pretty good deal. It is as clean as any new one I have seen( the guy is very wealthy and has lots of toys), It has all of the above and nacs graphics, seat cover, cross bar pad, handlebar mounts, headlight covers, thumb throttle cover, ac front bumper, and renthal handle bars. I got it for $3500. What do you guys think? Thanks ahead of time for the help. CJ

p.s.- At times like this atvs and riding seem meaningless and insignificant, but we have to move on. My thoughts and prayers go out to all of those involved directly and indirectly with this tragic event.

 

I have the very same problem! The bike acts a little like a 2-stroke. It bogs for a second, then the "powerband" hits and it flies. It began when I put on the pipe (Powercore IV-2 and PowerBomb Header). At the time, I was running a White Bros. (foam with stock insert) filter, and after endlessly fiddling around with the jetting, the problem was finally cured. Then I put on the K&N (couldn't leave well enough alone!) and it reapeared. But, no amount of playing with the jetting has made it go away now. I even had the dealer (I really respect their guy who does jetting!), and even he can't make it right.

So, here is what I think. There is just TO MUCH AIR for this engine/carburator/pipe/filter combination, and no amount of jetting compensation will make it go away. I could put the foam filter back on (I hate those things) or I think increasing the bore and compression would solve it. Another fix I am considering, is the Edelbrock Carburator.

 

That is exactly what mine does. The guy I bought it from ran it with the airbox lid off. When I frst got it I didn't notice the sputtering. I put the airbox lid on, because a friend told me that it would probably take in too much dust for the kind of riding we do. Have you tried running it with th lid off? Is it o.k. to do that? thanks CJ

 

My airbox is drilled with about 16 1" holes, with airbox vents. The vents have filters in them, so it's cleaner than just running with the lid off. If I ran with the lid off, I would have a nice little sandbox in there in no time at all!

If it ran OK with the lid off, you may have changed your mixture by putting the lid back on. So a simple rejet may be your answer!! I think my problem is more complicated.

 

How complicated is a re-jet? And what size jets should I try? I am definitely going to leave the lid on. I am fairly good at working on machines (cars,etc), but I have never messed with a four wheeler engine. CJ

 

Hey Cbrown;
I have played with numerous jets also. I may be able to give alittle advice, but am not sure the exact cause of your problem.

First, take the carb apart completley and clean all passages and jets and inspect everything. Alot of times, people put in these stupid power up kits and make everything across the spectrum way to rich, which will bog on low end (I know, because I did the same thing).

Second, It is hard to say exactly where you should be, but I can suggest a ball park. ONE WAY to test is starting with the air filter. Tape a plastic bag all the way around 1/3 of the air filter and ride it, is it better or worse (better means it is too lean, worse means it is to rich) because you are restricting air intake.

I am changed to a 45 pilot as asdvised from Curtis Sparks and I had a similar problem (bog on low end when a threw the quad in to a corner), changed to 42 pilot and I am fine (-1 1/2 truns on screw), the needle is in stock position and I am running a 158 main (which based on plug reading looks almost perfect, but it is running a little hot, so I may go back to 160 main.

My motor mods TC cam (low to mid), 11:1 piston (406cc) full Sparks exhaust, lightened flywheel, K&N filter w/ large square cut out of airbox lid. I have talked to Tom of TC and he doubted anybody would ever be above a 170 main unless the carb bore was increased, even with head porting because the stock head out flows the carb anyway.

Hope this helps. Anybody else have some views on this?

Nick

 

Here's my combo: 2000 400EX with White Bros. slip on E-Series(18 discs), no airbox lid, and White Bros. air filter. Jetting is: 42 pilot, 165 main and the needle clip down one notch. Bike runs perfect at all R.P.M. ranges. I think all your problems stem from the needle position being too low. The main jet only effects 3/4 to full throttle. If your bike stumbles below that, it is too lean. As mine did until I moved the needle up. Part throttle response is awesome now.

 

mine ran like crap on the bottom with the air box lid on, remove it and its a raped ape.

 

I messed around with it a lot this weekend, but it still wants to sputter on the bottom end. It seems to do it with the lid off as well. The guy I bought it from had drilled 5 ports in the lid, so I tried covering up different comb. of them. Nothing really fixed the sputter. Some made it worse. Covering up all five ports was the worst. Another question Is the WB rev limiter shorter tahn the stock one? The guy gave me two and said one of them was the WB. But it seems to run better and smother with the stock(longer one). Thanks for all the help. CJ

 

What does the porcelain on your plug look like? What jetting are you running?