So I'm experiencing a flat spot at around 4000 RPM to 5000 RPM. Anytime I am under or at this RPM on a slight to moderate incline, the engine lugs. As soon as I can break over 5000, I shoot off with tons of power. This is my set up:

KLX250
Marcelino Cam Mod
Stock Carb (DJ 124 main, DJ needle 3rd notch & spring, KLX300 needle holder/emulsion tube, 35 pilot, 2 1/2 turns on the AF screw)
KDX200 snorkel
HMF pipe

I had the cam mod done first and rode briefly before re jetting. I noticed a slight pause at 4000, very minor. I ordered the Pipe and the DJ stage 1&2 at the same time. I re jetted before the pipe showed up (128 main and their needle) and this was were I first noticed where it would bog down at 4000, Very noticeable. Almost to the point on steep inclines I would almost stall out even in 1st gear.

The pipe was delayed by 2 months, so I jetted back to stock (put a DJ 120 to compensate for the cam mod). I still had same problem. The pipe showed up and I jetted to this: DJ 124 main, DJ needle & spring, KLX300 needle holder/emulsion, 35 pilot, 2 1/2 turns on the AF screw.

I have plenty of power now but know instead of it bogging down at 4000, the power starts to drop at 4000 till 5000 where it bogs. As soon as I can break 5000, I'm off. This will happen in any gear but I don't notice it in 1st very much unless on a very steep incline.

I'm thinking of undoing the cam mod. Thats the only logical place for me to start with as I've played with the jetting a bit. On the DJ instructions in the trouble shooting they talked about "cam overlap" on certain bikes (not the KLX) and a fix is keeping the stock slide spring. I have the problem with the DJ and the stock spring, but maybe the problem is a result of the Cam mode.

Whats your thoughts guys?

 

Have you tried moving the needle clip down a notch to richen the mixture at mid range?

Ride on
Brewster

 

Yet another case of butterfly shaft seal leakage, no doubt.. The effects of which are a potpourri of symptoms but have the midrange "stumble" as a common denominator.

 

Check the butterfly seal. It can be hard to diagnose as it's only apparent during HIGH vacuum conditions (MCM!!).

I checked mine by blipping the throttle via the throttle plate several times. Do it a few times without spraying fluid on the leak to see how it should behave. Wind it out in neutral a couple times.

Spray it with WD-40, or anything that will impact the idle negatively (preferably something that won't damage the seal). Then blip the throttle a couple times in quick succession. Did it change? Rev it up to 5k and leave it for a few seconds. Blip the throttle a couple more times in various RPM ranges and then try again.

If you start getting a sluggish response or your exhaust smells different, it's almost certainly leaking. It also helps if the bike is good and warm ahead of time.

 

I've always had a flat spot at that rpm range but no bogging except when I was rich or when I had the shaft seal leak. You shouldn't be rich with that jetting but I would try removing your lid while keeping your jetting the same. If it improves you know it's rich.

I don't think the cam mod is the cause, unless you did it wrong. My flat spot got better after the cam mod.

What is the purpose of the klx300 needle holder and emulsion tube? Is it supposed to perform better than the stock parts? If different, I would also switch back before diagnosing.

Last tip, don't take shortcuts, do one change at a time and test. I've tried changing several things at once and just manage to confuse myself.

good luck, cheers.

 

Ahh, hey guys, this thread is two years old.. Red Brigade probably blamed the MCM, undid it only to find the problem worse - kicked his dog and sold the bike.. I responded to it only to bring to light that this problem has gone diagnosed for years - until Neonarc..

 

oh, right. is there an option to send a post back in time? jaja

 

I referenced this thread over in the MCM thread knowing that it was two years old and the OP is likely long gone. But as his symptoms were eerily similar to mine, I thought it might be pertinent.

I did a quick test with some starting fluid last night. Held the throttle and idled up to 3K-4K RPM and held it steady. Sprayed around the throttle spring and could hear the engine slow slightly. Also tried at normal idle speed but the change was barely perceptible (not enough vacuum?) Sprayed other areas but didn't detect any other leaks.

Shaft seals are on order from Ron Ayers. Is it worth investing in a JIS screwdriver to remove the plate or have others had success with a (modified) Phillips? I saw the trouble neonarc had and would like to avoid if possible.

Thanks again for the troubleshooting help guys. Hoping this is the magic bullet and I'll soon be able to fully enjoy the MCM.

 

You can use the JIS in the toolkit (if you have it).
I usually put a wrench on the handle, so I can keep the driver seated with pressure and use the wrench to turn the screw.