RPM drops after 1/4 throttle

I'm having trouble with my new 81 KZ440. I'm also sitting at work where all I can do is ponder about it. So here's the symptoms:

Starts fine, idles fine, as I throttle up (in neutral), it sounds great to about 2500ish, then around quarter throttle it starts to stumble and RPMs drop. It will continue to run but poorly and occasionally stall if I maintain or increase throttle. If I let off the throttle, it either stalls or more often it revs back up to 2500-3000ish for 3 or 4 seconds (even though I've let off the throttle) and then returns to a smooth idle. If I repeat the above while riding, the outcome is very similar except it always stalls after the throttle is released. Very little power after quarter throttle.

This is after warmed up, no choke. I recently rebuilt the carb. All parts inside looked great after removing the gunk. The throttle cable and butterflies open and close smoothly. As I'm typing this I'm wondering if I have a vacuum leak at the intake to the engine. Sound like a vacuum leak? I will shore that up first tonight because it's low-hanging fruit, but anything else I should look for?

TIA!
Don in CT
81 KZ440 LTD belt drive

check to see how freely your slide diaphrams move up and down it could be that they are not installed properly or worse senario have a hole in them.they should move up fairly easy and return on there own smoothly when you lift up on them with your finger. one other thing do you have the stock air box or are you using pods

timebomb33 wrote:

check to see how freely your slide diaphrams move up and down

When I put the carbs back together I made sure they moved easily and returned smoothly. They were definitely in good shape, no holes. I'll check again if sealing up the intakes doesn't help.

I'm running the stock airbox, not pods.

Thanks!

So I found the problem! Everyone remember to lubricate your timing advance mechanism! Mine is rusted solid. Now it must be replaced because while trying to losen it I broke one of the weights (at the weak point where it is supposed to turn the sleeve and cam).