carb issue

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old fart

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Hi all, New to board. Been lurking for months. This retired professor could use some help. My rebuilt 1981 Special has been fitted with brand new Mikuni VM26-8074 carbs on Mikuni VM 28-200 intake manifolds fitted with UNI UP 4182 foam filters (Speed Moto kit). Starts and idles fine. Air screw and idle screw set at 2 full turns out from seated. Problem: engine will not rev. It smells rich. It has a manual petcock, dual fuel lines with filters, valves are set correctly. Anyone have any experience with these carbs or the Speed Moto kit?

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No replies??? Surely one of the many experts I have been following for months has something to contribute. Forgot to include that changing the clip position on the needle has absolutely no effect.

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There's not many people on here that mess with VMs, most run stock carbs so that's probably why you are not getting any response. There are a few people that have put VMs on, but it seems that once they put them on they don't come back here to report. I'm not hip on round slide carbs, I prefer flats, but if they are having issues right off idle I would look into the slide size
 
And its also good to note that these style carbs don't like the throttle being opened quickly, you need to do it smoothly otherwise they will fall on their face
 
Yup, what Wolf said. The day I have to replace the stock carbs on my XS is the day fuel injection comes into my motorcycle world. And it is unlikely an XS will be involved. I'm afraid you are on your own with this adventure.
 
I have speed motos kit and it's okay. Raise the needle one spot or too and you will be able to ride it. I've been emailing them for a year now. I dont like the needles they send with it. One thing I suggest is getting the 90 degree elbow for the throttle cable on top of the carbs. This will make sure the throttle doesn't bind like it does for me. If you have any questions feel free to ask.
 
Thanks for the reply. Looks like it might be just the two of us. Adjusted the clip - second from top - no change. Adjusted the air screw and the idle screw - no change. You can smell the fuel in the exhaust. Plugs get so black they stop sparking and must be cleaned. Just ordered some new leaner main jets and pilot jets, perhaps that will help. Probably won't get them for several days.

Are you running 190 main and 40 pilot? I'm at 4000 ft. and high heat (90-100) daily right now with very low humidity (10-20%).

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Slide size matters. Try a different size slide. If you try adjusting jets, and end up having to change the slide anyway, the jetting will be off again. Try the slides, then adjust for them.
 
I have the same kit for my xs 360. Min idles correctly and if i slowly give it throttle it seems to run pretty well. But it does fall on its face if you rev it. Could this be a sync problem? And if so how do i do it on these carbs? Only one manifold came with a vacuum port and the cables just sit on top of the carbs with no adjustment screw. Please help
 

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