Need help with VM 28 carbs

I have a set of 1975 vm 28 carbs. I installed a v&h 4 into 1 and k&n pod filters on a 77 kz 1075 motor. The problem is off an idle. The carbs were ultrasonic cleaned and run well, except for right off an idle. I have 20 mains and 20 pilots in her right now. When i blip the throttle off an idle i get some black smoke out the exhaust and and the motor cuts down a little. I started with 17.5 pilots which were stock and it did the same thing. I know it acts like a rich condition right off an idle, so my question is, could the needle be set too high and letting too much fuel in during the initial throttle movement? I verifyed the rich mix with a colortune. I checked my enricher circuit and it isn't hanging on during any of this time. I think i need to step down on the pilot, but the carbs came factory with 17.5 pilots. I have played with the air mix screws with no luck. Any suggestions are appreciated.

Are your VM28's the SS version with an accellerator pump?
If so, was it properly cleaned/re-assembled, working?

I have the regular VM28 carbs on a 79 KZ1000 LTD with Pods and VH 4-1 pipe.

My Jetting is 20 Pilots, Needle shimmed 5mm, and 140 Mains, Mixture screws at 2.25 turns out approx.

You say that you verified the rich condition with a colortune plug. Do you have the correct instructions for that tool? The mixture should turn yellow as you accellerate in rpm, then back to a light blue when you hold rpms still.

If your stumble is as you pull away with those carbs, its deffinately the needle with that pilot. Try dropping all your needles slightly, if it gets worse, then go the other way

GL!

oppie501 wrote:

I have a set of 1975 vm 28 carbs. I installed a v&h 4 into 1 and k&n pod filters on a 77 kz 1075 motor. The problem is off an idle. The carbs were ultrasonic cleaned and run well, except for right off an idle. I have 20 mains and 20 pilots in her right now. When i blip the throttle off an idle i get some black smoke out the exhaust and and the motor cuts down a little. I started with 17.5 pilots which were stock and it did the same thing. I know it acts like a rich condition right off an idle, so my question is, could the needle be set too high and letting too much fuel in during the initial throttle movement? I verifyed the rich mix with a colortune. I checked my enricher circuit and it isn't hanging on during any of this time. I think i need to step down on the pilot, but the carbs came factory with 17.5 pilots. I have played with the air mix screws with no luck. Any suggestions are appreciated.

Perhaps not a carb issue.

Maybe an ignition or compression issue.

What is voltage at coil primaries?
Observed spark quality (fat and blue)?
Ignition timing correct?
Advancer properly functioning?

Compression test results?
Valve clearances to spec?

If not already done, would assure all the non-carb components are okay before addressing the carbs.
As and when addressing carbs, would start with clear tube test to assure correct fuel levels.

Good Fortune!

Thanks for the reply guys. I have the non accelerator pump 28 mm carbs. So I gather from what you are saying, the needles do affect the off idle a/f ratio. I don't really like the sync set up on the 28s and have a set of stock 26mm carbs. I was thinking of rebuilding them and givin them a whirl. I hear these carbs have better road manners, but of course lack top end kick. Do these carbs get rejetted same as 28's? I hear they were jetted richer from factory.

oppie501 wrote:

...gather from what you are saying, the needles do affect the off idle a/f ratio....

... which existing a/f ratio may already be perfectly okay, with combustion problem resulting from an ignition and/or compression issue.

As known, even perfect carbs won't compensate for deficiencies in ignition and/or compression.

And no amount of carb fiddling will compensate for deficiencies in ignition and/or compression.

See more about this aspect on wiredgeorge's site, click > wgcarbs.com .

Good Fortune!