Hey all, I am a new guy here. I just bought an 02 400EX. It has a 12:1 (.10 over)piston, cam, 165 main jet, Curtis Sparks full exhaust and a host of aftermarket stuff. Here is the problem, in the higher RPM range, it acts like it is cutting out, as if I was hitting the rev limiter, however, I know I am not that high in the rpm range. I did buy this bike like it is, so I am trying to fix a condition that I am unfamiliar with. It runs like a striped ape at lower rpms, but even in 5th gear, it will sputter lie it is short on spark. What should I check for first.

Thanks for any input you can give me.

 

If it isn't the rev limitter, I would guess jetting. Depending on the cam and air filter system, you could be running lean. You can read the plug to see if you are close, but the tail pipe is easier. Look too see if the end of the muffler (the soot) is white or black. You want it to stay a greyish brown.

Another thoght could be fuel octane. I have an 11.5:1 in a 440ex and it can get fussy with 92 octane.

 

Thank you for the quick reply. Could it just be me running into the rev limiter? Could my 18 inch tires be causing any of my hitting the rev limiter too early? I have been riding Banshees for years, and may just have to get used to 4 strokes again. I am running 93 octane, and that was my first thought. I'll check the plug.

 

yea the 18s may be it... what about sprokets ???? are they stock ???

 

Gearing is 14/38. I should mention that I was also riding on flat, loose gravel roads, where the tires never stopped spinning. I have nbot had it in the trails yet. Today was the first time I had it out, and it was more of a "gettng aquainted" ride. The last 4 stroke I had was a 250 EX. Been all 2 strokes since. I remember occasionally hitting the limiter on my 250, but never in 5th gear, as I have on thie 400EX. But, the tires never really got hooked up, which leads me to believe that I was able to hit the limiter, wheras I likely never will do that on the trail.

 

I would bet your hitting the rev limiter their only about 9100 rpm
It wouldnt hurt to check plug
I would find good straight away take to wot preferrably Hi gear If begins cutting out back off until that stops
Then
At same time Pull Clutch Kill Engine
Bring to stop
Remove plug and that will tell you if fuel lean or not
You could do same in lower gears if no room for hi gear that will get close
Plug should be Tan to Brown that is good
If Gray to White you are on the lean side
If Dark Gray to Black Sooty Rich
Hope this helps

 

Thank you for all of the replies. I should have pulled the plug today while I was out there. Nothing but miles of open road. I will check the plug anyway to see what it looks like. Thank you guys. Any other suggestions are absolutely welcome.

Craig

 

my 450r did the same thing i took off the air box lid and that was the problem

 

My wife has an '06 400ex and I just put an HMF pipe on it and now it hits the rev limiter very quickly. She was used to cruising in 3rd or 4th gear WOT and never hit the rev limiter with the stock pipe. Now she hits it every time.
If I were you I'd get rid of the 18" rear tires, for the trails you should go to 22"s.

 

With engine mods and 14/38 and 18 inch tires, you are over revving it. You can get new rev boxes with higher ranges. They do cause for harder cold starting, typically, from my experience. If you want to keep the small tires get a 15 or even 16 tooth counter shaft sproket.