Vm29/cb750 dohc (I know.....)

Hey all, I just want to start by saying that when a community gets together like this to help others with problems and to share experience, is so great. Props the the people back stage for spending endless hours to keep gas in the tank of the forum scene.
I started with a 1979 cb 750 dohc and rebuilt it from top to bottom with suzuki suspension and a bunch of custom fiberglass. With original cv carbs and air box it was alright but never consistent with throttle response and wot seem flat sometimes. I was living with it by riding nice and easy on it most of the time. I put about 16000 kms on it like this. I was always looking to do a lung transplant to be able to run pods. Finally I found some vm29 smoothbores in alright shape. ultrasonic, replaced everything except floats and enricher plungers. They seem alright. After assembly and mounting I found it running very rich @ idle. Plugs are super black. These carbs are stamped A7. I think these were from kawasaki but sudco also listed cb750 sohc but were from A4 stamping. It has all the internals as listed on sudco as they came off a cb750 dohc with the adapter spigots. I tried swapping pilots from 17.5 and up to 25. Always running rich with black plugs. After a 15 min of idle it wants to stall from fouled plugs. 
with the air screws all the way in to ½ turn out is the best idle but still black. I found the plungers were slightly sticky and #1 and #2 can be pushed back down a slight bit possibly telling me excess fuel leaking by. If I loosen the set screw holding the plunger arm to the choke lever rod the plungers will drop all the way and can no longer be pushed down any further. The rubber on the bottom seems to be alright with out cracks but not to say they haven't shrunk or something. 
float level is set using clear tube when sitting on a bench mounted holder sitting perfectly level. When on the bike they sit slanted forward so the level is high at front and low on rear. To try eliminate any concerns i elevated the front end to level carbs and replace plugs. still black.
for you all who made it this far, good on you!!
It has been a struggle and I can't really find much input on this setup in hondaland but kawasaki has these carbs on many models so I thought I can gain some wisdom here. 
Any help will be greatly appreciated as I am ready to try anything, including drive over them!

Benz1991 wrote:

 After assembly and mounting I found it running very rich @ idle. Plugs are super black. 
I tried swapping pilots from 17.5 and up to 25.
Always running rich with black plugs.
with the air screws all the way in to ½ turn out is the best idle but still black.
After a 15 min of idle it wants to stall from fouled plugs. 



 

According to your statement your idle mix adjustments are all in the wrong direction..
Going from 17.5 and upwards on the pilot jet is RICHENING the pilot mix which will only make the fouling worse.
You need to fit SMALLER pilots to lean off the idle if there isn't enough adjustment on the air screw which also needs to be screwed OUT to lean off the mixture not in..
 

Guess i'm going to have to ask how You mounted those Smoothbores up to a DOHC head?? The stock VB Keihin's had a nominal 43.7mm OD of casting that plugs into the rubber intake manifold sleeve whereas the Mikuni's were about 34mm. Also being angled 1/2 and 3/4 in pairs this can create adapting issues.  

Thanks for the reply, the description I gave is pretty vague...sorry
It came with adapter spigots that have the proper bends. Made of alum w/o-ring seals to the stock intake boots. Pretty slick setup concidering it is probably 30 years old!!
It came with 20 pilots so i tried the 17.5 like sudco lists and it seems to idle worse. I tried a few different sources to find a 15 but not successful. I tried more turns out on air screw but anything more than 1.5 turns it really doesnt like life, starts to backfire out both in and out sides. Thats what is tricking me. I changed the block gaskets and was careful to line up the small holes in the bypass. At first I had overtightened the blocks and one slide was sticky. Took a while to find out why the vac sync was inconsistent. 
 

Should I set float level on bike with carbs tilted downward? And where should I look at tube position (front of bowl has higher level or rear being lower than front) difference from front to rear is 4mm
the plungers look ok but maybe shrunk and not sealing??
Where are the air corrector jets in use, is it ¾ to wot?

Also...I read somewhere the bypass hole beside the slide can be drilled out a bit more to get the vacuum signal a bit higher due to the smaller displacement.
Does anyone know the difference between the vm29 A4 and the A7? As stamped on housing

There's no listing for the Honda DOHC. What slides do you have. 1.5 or 2.0? The air screw adjustment isn't like the other VM's. Could contact Mike for details on your model. 
www.jetsrus.com/

 

It has 1.5 slides. All the specs are the same for the sohc
the screws have small air holes in the tips. George suggested in another post to jb weld the holes shut. I did find a difference after doing so, also ordered another set to return back to original in case. Also got a set of plungers coming.
Don't want to give up now

Benz1991 wrote:

It has 1.5 slides. All the specs are the same for the sohc
the screws have small air holes in the tips. George suggested in another post to jb weld the holes shut. I did find a difference after doing so, also ordered another set to return back to original in case. Also got a set of plungers coming.
Don't want to give up now

Same with my 33's. Do they have a hole in the center if so the turns are 1/4- 1/2 turn. Miller soldered them shut and tapered the screws to act more like other VM air screws. I bought new ones and they work well. I wouldn't go more than 1 full turn. That would be way too rich. I'm running 17.5 pilot jets 1.0 air corrector jets and 125 main jets. It was still a little rich in the mid range so I opened two more holes the moron drilled in them. They were all over the emulsion tubes and that got me in the ball park when I first got them. I kept the stock holes and the two more helped. I'm now going to drop the needles one more notch. PO's suck. 
Steve 

Benz1991 wrote:

Should I set float level on bike with carbs tilted downward? And where should I look at tube position (front of bowl has higher level or rear being lower than front) difference from front to rear is 4mm
the plungers look ok but maybe shrunk and not sealing??
Where are the air corrector jets in use, is it ¾ to wot?

The  main air jets ( air correctors ) are for the off idle mix and fill in the lean spot that occurs on the transition from the pilots to the jet needle which causes bogging/hesitation around 2 to 3k rpm.
Their effect tapers off as the rev rise.
Take the fuel level measurement mid way along the bowl ( halfway between the bowls high and low level ). 
Unless the carbs are at an extreme angle the fuel level in that positon will be correct.

By chance do You have access to a Color Tune or know what that is? If you suspect the choke plungers, try sliding a 1.5mmID x 4,5mmOD x 1.5mm CS oring over the tip of each plunger seating it on existing flat pad. I know i'll get spanked for bringing up such a thing here probably but it works remarkably well. Gives the casting to plunger new life, you want that "port" closed or you're wasting your time tinkering with all the other stuff. Yes those "Air" screws are hollow and unlike the other VM's but Keihin has been using the same thing for years in their slide carbs. Did you verify all the brass floats are pin-hole free? That thin little jet/slide block gasket can be a pain to install and "new" ones available seem to be smaller in diameter then the originals. I always use an appropriate sized twist bit thru the pilot hole on jet block thru gasket into casting to correctly index alignment. Then install the singular M4 screw lightly then emulsion/jet holder. Be SUPER careful not to over-tighten the jet holder into block!! Although you had not mentioned it, check your valve clearance. Couldn't count how many of these 750/900/1100 and CBX's i've done that persons neglected to check valve clearance! They all use the same       25mm shim.   

Hi for your info i am running VM 29 on my Z  1015CC with 117,5 mains, 20 pilots, needle mid position, air jet .9, air screw 1/4 turns out. Big big improvement were the new viton choke plunjers. Now sparkplugs are Coffee Brown, bike pules and rides very good
good luck