Flat spot after big bore
 

There was a thread on this exact subject just recently, either here or somewhere else. I searched for it with no luck, so here goes.

I'm running the stock carb, with DJ parts, 124 main, needle 3rd clip, don't remember where the a/f screw is. After installing the 300 cc kit, I notice a stumble from 3/4 throttle onwards, with the rpm's over 6000. It feels more like a lean than a rich condition, power surges like the engine is about to run out of gas, but it isn't actually stalling out on me.

I'm going riding at higher elevations this weekend (hopefully I'll make it over 5000') so that should clarify whether it's a lean or rich condition. The bike carburates fine under 3/4 throttle, and at below 5500 rpm.

Any ideas? I'm going to drop Bill Blue a line and see if he can shed any light on the subject.

Bigger main jet.....

What do you have for an exhaust?

David :)

Probably should have added that, the setup is K&N filter, KDX snorkle, stock header, and FMF Q4 slip on.

I should also add I live on the west slope of the Sierras at 1600 feet, but I occasionaly ride down to the valley floor, and as high as 8500 feet (Carson Pass), my favorite riding areas are in the 4000 - 6000 foot range, so I dont want to jet too rich.

If it feels that bad, I would go two steps up on the main. nothing else. I am curious how it turns out.

I keep thinking what a bike it would be if I made it a 350.

David

I was running a 128 main on my 250, I think the 124 is a bit small.. I would go 128 at least.

On my 351 I am running a 132 I think it is... I will be lowering that one though to a 130 or the 128 since it is running a little rich - but it runs awesome, I just like to be on the safer side :)

Good luck./

Bigger bigger bigger, you need a bigger main jet!!!!!

I have damn near the same setup as you except I don't have a big bore and I run a 124 main jet ( probably could step up to a 128, I am at 3200' elev.). When I go big bore I will probably step up to a 132 main.

If yours is just a 300 kit, try a 128 main first, then maybe a 132. I promise you that is the problem. You are starving it for fuel the higher the rpms. More fuel will cure the surging problem too.

The stock KLX300 comes with a 128 main jet.

Ride on
Brewster

+1 on bigger jet. At least a 128

Good deal, I got Bill's response and he said the same thing, try the 128, leave everything else the same, and see how that works out.

While I have the carb out, I might finally remove the backfire screen as well.

250 CC * 10,000 RPM (5k intake strokes) = 1,250,000 CC/minute

300 CC = 1,500,000 CC/minute

124 Main jet limits out before you have used all the air/fuel the engine can now take in.

This is good, I was wondering my self.

David :)