Zephyr 750 Sticks at 4000rpm

 
So we've pretty much finished my sons Zephyr... looks good, runs like crap. We're trying to get the last problem sorted out so we can get it roadworthy and registered:

It starts fine. When it's warm it idles OK (carbs are balanced, idle screws at stock settings), although it does sound like it misfires ocassionally at low revs (the odd clunking noise that disappears when above 1500rpm). When you open the throttle up it sticks at 4000rpm and doesn't seem to come back down again (at least, not in the 5-10 seconds before I shut it down). This happens with the throttle cables off as well. 
If I push the butterflies on 1 & 4 closed it drops back down (pushing down on 2 doesn't help as the throttle plates are all returning properly to the idle position). Basically, forcing 1 & 4 closed brings it down and when I release them it idles nicely again. I'm thinking one or two of the sliders are sticking, but they all seem fine. I wouldn't think there would be enough air getting through when the butterflies are closed but they are definitely closed.
There was a couple of air leaks but I think we have them under control. No noticeable jump in revs when I drench them in brake cleaner.
There are aftermarket needles and jets as the carbs came with a dynojet kit and we wanted it back to stock. I think they're from a Keyster kit.

Just a note on the choke: I need it fully open to start but after a minute or so I can start to close it. At this point the revs go through the roof if I open the choke even a little bit too much.

[EDIT: The vacuum numbers when balancing are quite low - about half of what my KZ750 has]

I'm really not sure what to try next. Any help would be greatly appreciated

Low vacum levels suggest air is getting in somewhere as vacum is generated against a closed throttle, is the carb linkage clear of the choke "fast idle" mechanism once the choke is off ?
Also if it was previously fitted with a dynojet kit some of the stage three kits come will a drill bit to open a passage in the carbs (my Harris needed this and so does a 1200 Bandit) can't remember the details it was over 20 years ago but if that is the case it may not be possible to go back to standard jetting if the carbs have been drilled ? 

Thanks, would that be the hole drilled in the bottom of the slider next to the needle?

wrote:

Thanks, would that be the hole drilled in the bottom of the slider next to the needle?

 

Could well be, somebody must have more recent experience than me or you could try a google search for dynajet fitting instructions

Tried google before and it seems that hole is drilled out with most of the dynojet kits. Would that be enough to keep the sliders up and the main jet open? Would enough air get in on a closed throttle? 
The Zephyr uses 32mm Keihins with choke plungers so no fast idle like the CV34's. 

I guess the next step is to put the needles and jets from the kit back in.

wrote:

Tried google before and it seems that hole is drilled out with most of the dynajet kits. Would that be enough to keep the sliders up and the main jet open? Would enough air get in on a closed throttle? 
The Zephyr uses 32mm Keihins with choke plungers so no fast idle like the CV34's. 

I guess the next step is to put the needles and jets from the kit back in.

Probably the easiest and cheapest way to check if this is the problem 

Did you have separate the carb bodys during a rebuild or cleaning?
Looks like a misalignment of the 1-2 and 3-4 sync adjustment mechanism.

 

I did seperate them, but I have double checked the linkage and they are bench and vacuum synced. I can also bring the revs back down by forcing the 3rd throttle plate down, but that gets a bit hot. When I force the butterflies closed I'm basically pushing against the lower of the two springs in the sync linkage. The Zephyr carbs have a single screw with a second spring instead of a lock nut.

 

Ok, so did you syc/calibrate all of the vaccum gauges on one vaccum port before you did the carb sync?
Don´t trust cheap china fire cracker tools!

 
 

My Beohm sync tool i have to use for more than 25 year. Expencive but best investment ever for ever....

 

I Have the same gauges and have just recently checked the calibration and then synced my 750H with them. All good on that front. 

I'll swap out the needles and jets on Friday and post back.

I will give you a call tomorrow and we do some brainstorming, okay? I have a Zephyr 750 carb tuning odyssey story for you, we have to talk about.

Sounds like what my KZ750 With CV34s did when it was air leaking. Any throttle at all past idle and it would jump to 4k-4.5k RPM and just stay there. It would also only happen when it was warm, exactly like yours.