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

the p.o. had these on a 1980 cb750 so they came with the adapter spigots(granted its not stolen). he said there was no problems, plug-n-play! not sure how mechanically inclined he was since the slides were stuck and they look like they were sitting on a shelf for 10 years...my thoughts were since they make spigots to adapt to a cb750 someone had already gone through what i am doing right now. the only difference i found is the vm29-A7 is what i have and sudco lists _A2_
the pilot was a 20 so i went with it for startup but plugs were black and wants to stall after quick rev.
went to 25 and even 30 but it got too bad to want to idle never mind restart hot
i went down to 17.5, then 15, then 12.5
i found 15 is the best of all., least amount of black out the pipe and it will quickly restart, most of the time without touching throttle and idle somewhat smooth but still lumpy
quick blip will still drop below idle but comes back and smoothens out...somewhat...
stop and go traffic is hit and miss if i need to help it idle but it might be the plugs fouling

the spark does not seem to be bright blue, kinda dark and lacking. i do have dyna coils with a relay running to B+. i did try swapping back to the oem coils and found no real difference. but thinking about that, were those plugs fouled at the time.

compression is over 150
found one gauge isn't reading over 90......tried another and it seems to stop climbing at 150 but looks like it wants to go higher. im going to go ahead and guess the comp is good...?
thats what happens when you borrow out your tools hey!

Just some things to consider. Are You sure the floats/float needle seats and needles are reacting correctly? By this i mean, are the brass floats pin-hole free and not filling with fuel. Would seem odds are against this because they all are "blacking" the plugs at an even rate though. Note that unlike the shared DNA with other VM 4-stroke type carbs these had NO overflow tubes/barbs on float bowl. So you won't at a quick glance know if fuel level is too high. This is the DNA of Mikuni float bowls that comes from the Kawasaki triples, they didn't have them either. That off throttle dipping low rpm then coming back up is the sure sign of a too rich condition. Might be weak float seat needle pin springs too. These had a different "bigger" thread float seat. Mikuni N122/032-2.5 and gasket B34/52-xx. Gaskets xx came in different thicknesses .5-.8 and 1mm. You say you did a bunch of engine work. Did You index both camchains correctly?? Are the Emulsion/Jet holder's sealing nicely against the carb casting tower? By this i mean fuel should only be coming thru the main jet not surrounding casting to holder tightening joints.

this is my first "small" engine i have had every single bolt out of. i do lots of diesel work in automotive and ag. i did get cam and crank spot on as far as i know, rolled engine over several times and verified both still spot on. the machine shop had to take off .008" off head to resurface...wasn't too happy but it seems fine. i am not sure what seat gasket thickness is in there. How would that affect the system if too thick/thin? i did have trouble with the seat sealing either between the gasket or the needle. had to polish them w/q-tip on drill and finally got them to hold fuel and even 7 psi pop pressure. How should i check if fuel is getting by the jet block? should i use a camera when running and shove it down her throat to see if gas is seeping by the emulsion tube through the side casting?
 if that is all good, checking fuel level when running w/clear tube is the best way or is there i different way?
I might also be suspecting timing, when i have the timing light on it is about 30* around 1200 rpm. it has electronic ignition so the pick ups are easily adjustable but i found i had to turn plate and pickups all the way to one way......hmmmm

Benz1991 wrote:

the spark does not seem to be bright blue, kinda dark and lacking. i do have dyna coils with a relay running to B+. i did try swapping back to the oem coils and found no real difference. but thinking about that, were those plugs fouled at the time.



 

Was that confirmed with a ColorTune kit or by grounding to the cam cover?

Color Tune is the best way to set the pilot/idle circuit. There are tools that can measure spark length (strength) without using a spark plug.

that was just grounded to frame. also used a spark tester and made gap wider. up to .050" and was very bright and appears strong. i have never used a colour tuner before. maybe i should get it in the ball park first before going that route. anything but black would be a win at this stage. almost wanna put the stock cv's back on but not going to give up now. have too many days into these to turn back now. 

Benz1991 wrote:

that was just grounded to frame. also used a spark tester and made gap wider. up to .050" and was very bright and appears strong. i have never used a colour tuner before. maybe i should get it in the ball park first before going that route. anything but black would be a win at this stage. almost wanna put the stock cv's back on but not going to give up now. have too many days into these to turn back now. 
 

I strongly advise the use of Color Tune kit to set pilot/idle circuit especially when using any non stock parts. Any other method is just guess work which you want to avoid. The kit is not that expensive and is the only way you can "see" the spark and adjust accordingly.

I also recommend riding the bike for at least 30 minutes then doing the test as it has to be done at operating temperature. I always have a big fan running right at my exhaust pipes when doing that test so the engine does not overheat. It is important to have all the tools laid out and available before you do the test so the engine does not cool down too much.
 

I totally agree with you and I am looking into the gunson kit and 12mm adapter sold separately of course. I do find the throttle is somewhat sticky from time to time. Could it be that the jet blocks are not perfectly lined up with the slides. When engine off it seems nice and smooth operation but when running it wants to hang up and wants that little push on the cable pull to drop back to idle. I am thinking the vacuum is pulling the slides and causing that bit of drag between the two

Benz1991 wrote:

I totally agree with you and I am looking into the gunson kit and 12mm adapter sold separately of course. I do find the throttle is somewhat sticky from time to time. Could it be that the jet blocks are not perfectly lined up with the slides. When engine off it seems nice and smooth operation but when running it wants to hang up and wants that little push on the cable pull to drop back to idle. I am thinking the vacuum is pulling the slides and causing that bit of drag between the two

 

I have a set of 33 smooth bores I recently rebuilt and the return spring is not as strong on them as compared to a new set of RS34's.
Maybe look to spray some lube into your cables on the twist grip end then work them a bit. Cable inners can dry out from season to season even if new. Make sure the lube drips out on the carb end.

Did you take the jet blocks out of the bodies? I was warned not to mess with them so I just cleaned with them in place. I don't have any slide issues but they have not been on a running engine yet.

Hanging up at idle is usually a cable stiction or vacuum issue.
 

Yeah I had everything apart and replaced except for floats. I did have them in the u-sonic cleaner and held them under hot water for several minutes then gave them a shake to see if water is inside and all was well. One has a small dent in the side but I didn't think that would be a big issue. The cables are new and I shot graphite until it came out the other end. 

More stuff. If those slides "are" sticking it'll create some weird running for sure. Generally reassembly of the slide block requires the chrome slide installed before tightening the little screw and emulsion tube. This way alignment can be verifying, smooth action of slide thru it's travel is the result. Timing..a couple pictures. First is advancer at idle. Second is the advancer at approx 6000rpm or full advance. Believe this for #1-4, 2-3 automatically follow because pick-ups are 180deg out of each other. I don't know what 30* meant, you might have a readable timing light? The book says 170psi compression but that can vary with type of gauge you are using. That 90psi one would be cause for concern though provided same checking procedure was used on all cylinders. Also a picture of those Gunson Color tune tools. You don't need an adapter for the 12mm, only the 10mm thread. They come in 14 and 12mm ready to use.      

30 degrees btdc timing when at idle. I did not see what full advance was. I don't think there is the double line on my wheel. Just 30 to 45 degree graph on each side. This is dyna 2000 electronic ignition. One thing I found strange, when I removed the cover for the ignition to reseal it I marked the pickup wheel and mount plate. When assembling it using the same marks I found the timing was out about 5 degrees. At this point I had to max out adjustment to get back to 30. Kinda strange in my eyes since the cover is on dowls and I lined up the indexing exactly. Really pulling out hair at this point...I wasn't supposed to be bald back there!!