Just to say you did, run it in the rev range where it's running worse for 10sec or more, then shut it off and pull the plug to see what if anything the plug can tell you about whether it's lean or rich at that point

 

Have you checked the carb's diaphragm for a pin hole?

 

...and the only way to prevent it from stuttering so far has been to lean it out so much that the engine begins to lose power and the plug reads very lean - this doesn't help with the torque dip as you can imagine.

That kind of eliminates an electrical problem.

Check float level ?

 

1. Yes
2. Yes, while cams were out, changed valve shims and removed KACR from the cam. Also removed the carb and installed a #132 main and raised the needle to #4.
3. Since completing the MCM and discovering the terrible flat spot from 4.5K to 5.5K RPM, the carb has been removed and futzed with many times attempting to cure the issue. Per your earlier suggestions, I ordered the new throttle shaft seal, and was very optimistic that was the issue. Unfortunately after installing it, no change. I have not done ANY troubleshooting on the electrical/ignition circuits. However, shortly after getting the bike, I did add the extra ground to the regulator (long before the MCM).

And don't get me wrong in thinking that I'm 100% sure it's carburetor related, though that's still my primary suspect. I'm totally open to other causes. Will be interesting to see if this thread yields any results.

I find it interesting that several members seem to be complaining about this issue, and it seems one common denominator is the MCM. Has anyone experienced this midrange flat spot running the original cam timing? I'm not faulting the MCM, as I know most have great success, and even with the flat spot, I can feel the change in power delivery. I've considered undoing the MCM just to see if the midrange flat spot goes away, but haven't taken that step yet.

 

And just to be clear, I'll try to describe the symptoms I'm experiencing again just to make sure I'm actually troubleshooting the same thing as the OP. I don't want to lump us together if we're actually experiencing two different problems.

 

Did you actually adj the air/fuel jet on the carb? Or just set it as to recommended?

 

Shot in the dark for trouble shooting. When the bike is stuttering, pull the "choke" lever out. It will slightly richen the mixture. If it has a positive effect the mid is lean, if not, nothing was wasted in the attempt. I had a flat spot and that was what gave me the needed test to take care of the lean spot.

Problem is that no jetting or air intake mods have done anything. Richard had a point I didn't see in your work - pin hole in the diaphragm.

Definitely do a new plug if you haven't already - a cheap way to eliminate one possibility and easy to do.

MCM is such a fixed and proven thing there is no actual experimentation with it, like jetting. I don't see how it would only affect one bike out of the probably few hundred running it. The problem has to be either electrical or carburetion related.

Wish I could be of more help. If you had a friend with a 250 willing to swap, it would be interesting to switch carbs, coils, etc. and see what happens.

 

Ok, Snowdrifter.. Seems your issue is identical to PWJM..
New shaft seals did not help PWJM either..
Replacement coil/plug wire combo did not solve the problem - however it did significantly modify it.
Removing and reinstalling the original coil also significantly modified the characteristics of the problem.

The MCM correlation certainly bears investigation - What and how could the MCM cause this? Is there something "bad" that can occur once in a while - while performing the mod..?

Thinking out loud: Increasing TRQ on the engine makes everything work a little harder - increased cylinder pressure, increased vacuum signal, increased demands on ignition, increased exhaust gas flow, etc..

 

I've tried pulling the "choke" while running in the flat spot, and felt no change. This originally led me to believe I was feeling the carb going way to rich, so I tried leaner jetting and different spring/needle combinations. Going leaner only made the problem worse, so I went back to richer jetting. Klxster suggested I might be feeling the carb actually going really lean due to an air leak. I was optimistic when I replaced the throttle shaft seal as that seemed a likely culprit, but again no change.

I can't rule out a pinhole in the diaphragm, though I've visually inspected it and found nothing, as well as verified the slide operation using a shopvac. Though I'm no expert, from what I observed, it appears to operate correctly.

I haven't messed with any of the electrical systems yet, including the plug. I'm running an Iridium plug and will have to pull it to check it visually or replace.

I haven't yet messed with any of the electrical systems, though it would be easy enough to remove and refit the coil and check electrical connections.

As far as MCM correlation, at this point my best guess is that the increased vacuum being drawn is messing with the carburetor some how. Either a small air leak is being developed, or some weird harmonic condition is created with the slide/diaphragm at that narrow engine RPM range. I can't, however, explain why only a small number of bikes run into this while most are fine.

To continue thinking out loud, one has to remember that neither Kawasaki nor Dynojet had modified their cam timing when they developed the original carb components or the Dynojet carb kit. Perhaps a stiffer diaphragm or spring is needed to deal with increased vacuum?

I'm by no means ruling out an electrical issue, but in my experience those tend to either affect the entire RPM range, or be more intermittent. This flat spot issue is very repeatable and constant, and as such should be easier to troubleshoot. Unfortunately it's proving to be anything but easy.

 

The carb has been checked over numerous times. Going so far as to replace the throttle plate shaft seals. There's nothing wrong with the operation of the carb so far as I can tell.

On the advice of a friend who is a bike mechanic, he also suggested pulling the choke out while the bike was in that range to point us in the right direction. Pulling the choke didn't seem to make any discernible difference in the stuttering under WOT load.

I made a point of riding the bike in that range all day yesterday so that i can do another plug read, but as I've mentioned in the first post (Richard!!) I've already replaced the plug. plug cap, and coil in attempts to troubleshoot this. I've also checked the gap on the new plug. The bike also starts within 1 revolution of the starter, so the spark at lower RPM's is strong and healthy.

As Richard asks, I've already checked the float (it's in spec) and the Slide/diaphragm. Neither of these things would cause misfiring at light / part throttle cruising.