73 VM28 carbs

I have a set of 1973 VM28 carbs that Im have issues with. Stock air box, stock jets but runs lean. The bike does not accelerate smoothly through the mid range but takes off around 3500-4000 rpm. Seems to be lean in the mid range. I tried 22.5 pilot jets but still lean. Checked the firing with the color tune and it definitely was running lean on number 4 carb and slightly lean on the others. I finally got close to a blue flame on carb 4 once the air screw was bottomed out. Carbs were ultrasonic cleaned and all passages were open? There was a lot of corrosion on these carbs. Is it possible there is a bunch of corrosion in the passage even though I do have  flow through the small opening in the carb throat ? Has anyone removed the balls to access those passages for further cleaning. Got tired of messing with them so I installed one of my spare sets and the bike runs well with that carb bank. Just kinda thinking if corrosion could be causing my problem, its also possible someone has seated those air screws to tight and screwed up those bodies.

A build up of scale etc in the passages could certainly be the cause.
I've just done a Suzuki T500 that wouldn't run right on one cylinder despite extensive restoration and several carb rebuilds by shops that couldn't find the issue.
The culprit was a restricted fuel supply due to an inaccessible fuel passage cross drilling being partially blocked with fur/corrosion.
The only way to clear it was to remove the brass blanking ball to gain access.



 

Don't mean to hijack this thread but I have a very similar issue with the same carbs (early Z1) and coincidentally on number 4 cylinder. My bike runs nicely though, but the number 4 plug is quite white compared to the other light tan colored plugs and the exhaust pipe is discolored as well. 
Long winded way of asking a question - how do you go about removing (and replacing) the brass balls ?

You have to carefully drill out the brass balls.
Depending on the size you can drill a hole in it and then use the plain shank of the drill bit, a pin or jewellers screw driver etc to wriggle the ball in it's socket until it's loose enough to remove.
Difficult or small ones are just drilled out and the remains removed with a pick.
You can replace them with a suitable sized ball bearing, blanking plug made from brass or aluminium rod or even a small grub screw. 
Brass ball bearings are readily available in various sizes online etc if you need to retain the original look.
 

Great - thank you very much Zed.

   Two different air screw lengths and tapers are available for the same PN# 16014-012.  Carb body number and manufacture date determine which taper / length of air screw is correct.  Early air screws are 22.5mm long w/ blunt taper.  Late air screws are 24mm long w/ sharper taper.  A short / blunt screw will bottom without seating in a body designed for long / sharper screw.  John Brooke's Feb. 2025 book goes into detail far beyond the scope of this post on VM28 carb body variations.

   We had a VM28 carb body many years ago in which both types would bottom.  It didn't occur to us at the time to test seating with Prussian Blue.  We just replaced that carb body.

   When we see carbs corroded / varnished badly enough, we clean each fully-stripped body individually in an ultrasonic tank in all six possible orientations over six individual cleaning cycles: upright > then upside-down > then lying horizontal in each of four possible left > airbox > right > head rotations to help ensure no air bubbles are present.  The only way ultrasonic cleaning can work in small passages is if they're flooded.  A passage containing an air bubble will not be cleaned.

   If a carb body has been vapor blasted without being sealed to prevent bead from entering the internals, glass bead can remain adhered internally.  It's easy to spot externally using a flashlight in dim lighting.  Water / detergent solution will break surface tension & cause bead to fall from carb bodies in an ultrasonic tank.

   We've seen severe corrosion to significantly pit carb bodies.  Seems logical that deep enough pitting could enlarge internal passages enough to change flow characteristics?

Good Ridin'
slmjim & Z1BEBE

Great replies, Im going to remove the balls and see. SlimJim I ultrasonic clean pretty much like you have done. I need to look at those air screws also. Some times after ultrasonic cleaning I have found those brass balls on the bottom of the tank so now I double check to see if they are all in place after cleaning. They are pretty easy to remove.

If it Helps Cra-z1. I've two ultrasonic machines but haven't used them in years. Found them to be more of a hinderance then beneficial. And might give You the shutters slmjim but i Never seal off the galleys/pilot/main passages before blasting! But correct media has to be used, can't be playground sand sized. I although have a dedicated power washer that shoots 1000psi+ thru all jet openings after. You can optically witness exit of water and it's pattern/strength. Waay different then just blowing air/carb cleaner thru. I remove those brass balls all the time especially when it comes to carbs that share a singular air correction air opening. The ball hides an air correction jet not seen and often ignored because you wouldn't know it's there. Lots more info available if needed... 

Like this shared air supply for Pilot and Main fuel systems, note   one ball removed already. Like Zed mentioned, often missed. Super common on two strokes.