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?

Well, my thoughts on this are that the MCM creates a larger vacuum than stock because it wants/needs more fueling. The existing DJ spring/needle combo will fuel it correctly - as per my dyno AFR charts.. We gotta try to avoid any weird carb setups because we won't know if a strange setup is simply masking the issue.

 

Edit: I doubt the enrichener will be the issue, but let's think about it... If you try to plug it do it like Richard has mentioned, in the throat at the butterfly. Otherwise, if it is actually causing the issue it will possibly either draw only fuel if air inlet is blocked, or only air if blocked at the float bowl. Of course that may show in the performance too.

It is such a simple device you should see evidence of damage if you simply pull the plunger out and look at it. It is a simple piston with a rubber (Viton) seal that plugs up the passage. The spring is stiff enough and I'm thinking if there is vacuum it would suck it down tighter, so not likely unless there is clearly something in there holding it open or some damage that should be seen.

It isn't in any way like a regular butterfly choke like Honda uses. That literally would block off air flow and definitely go seriously rich.

I would think it could pretty much be proven the carb isn't the issue if moving the needle up or down a notch doesn't improve it a bit with one or the other setting since that range is in the range the needle affects. Of course if the slide is leaking a bit either with a hole or the perimeter seal, it will stop the slide from lifting, causing that flat spot. I didn't see if you checked it or not. Even if the enrichener was at fault it should get better if leaned out.

Not so sure the MCM would create any larger vacuum than the original cam timing, but even if it did, it should have affected virtually all the bikes using the CVK about the same way and it didn't. You figure if the intake opens sooner it would actually start the vacuum draw a bit lower and smoother than the later opening. As for the amount of draw, it's a 250cc cylinder. I'd think advancing the cam would allow the draw to start sooner and close off sooner, having a bit less blow back in the mid range rpm - that boost it gets. But that's all sitting here thinking about it and I'm no cam specialist, just considering what I know about mechanical aspects and flow.

These are all simply my thoughts and what I would be considering. The hardest part is the electronics. All it takes is the black box to take a dump, you can't tell for sure without another to swap out.

 

I have also read/heard that the choke should not affect anything but idle and low rpm. However, here is my experience. I commute on the bike and use the choke to start it in every day and then take off with the choke on. Then about a half a block or 1km down the road (depending on weather, summer 1/2 block, winter 1k) the bike starts stumbling from 4-6k rpm mostly when on throttle, not cruising. Remove the choke and the bike runs normal. I always took this to mean that the engine was warmed up and the fuel added by the choke was making it run rich. This has happened the same way for over a year with anything from 126 to 132 main at 5k feet ASL. My conclusion: the choke does affect the mid range fuel mixture to some degree.

cheers

 

I think you are right, so I edited my post to reflect that and some thoughts on the enrichener circuit.

 

Based on the enricher circuit discussion, did you drill out that circuit on your carb, and this started after that mod?

 

To my knowledge there is no mod to drill out the choke circuit.

 

Do you mean the starter jet drilling mod for easier starting? That was done before I bought it. Would have to see about the other guys, but I guess they would if they have easy starting.

 

I run around 1250 on the 650 and around 2000 on the 250. I know supermoto racers will set the idle to around 2000-2500 to avoid stalling when chopping throttle and backing into a curve. Singles are easier to stall when braking down hard or stopping quick, always have. Maybe EFI might minimize that, I don't know. They also can stall if the throttle is cracked too far too quickly, lighter fly wheel minimizes that on MX bikes. My 650 will idle down around a grand, but it requires a bit of throttle work to not stall it when that low, so 1250 it is. Could maybe play with the idle mix a bit more to make it better, but just haven't done that yet. It works and I'm not worried about it.

 

I run at around 1250 and I get that stall when stopping and closing throttle suddenly, specially when cold. This sounds normal so it may not be a factor on the mid range sputtering issue.

 

If I understand how to functions correctly, the plunger prevents a vacuum - caused by air passing through the carb. When the plunger is pulled out it allows a vacuum to pull fuel from the bowl. If there is a rubber seal at the end of this plunger and it's leaking, it would allow air to be drawn up (highlighted in red) and engage the enricher circuit. The predominate forces should be working to pull the plunger out if anything.

https://i.imgur.com/c9bNHuu.png


http://web.ncf.ca/ag136/cvCarb.jpg