KZ750 Twin Mikuni VM34

Hi,
I got a set of Mikuni VM34's in good shape and trying them out on my ´78 750 twin. I rebuild them with new jets, needles and float valves, i kept the 2.5 slide since i found others uses it with succes while others say 2.0 or 3.0 slides.
I tried starting the bike yesterday with size #30 pilot, #190 main and needle in middle position, but it did not start. I did make sure to check ignition and the bowls both had the same amount of fuel in them. I sync'ed the carbs using thin string to set a idle starting point, do any of you have a good idea to what hight the slides sit at idle in these ?
I will get it started and i am able to get the bike working on my own but i would really like to learn from the more experienced here!
How whould you do something like this ? The jetting i am using right now is based on what severel shops selling the carbs for this bike specs. The carbs are setup as the manual says and the bike ran perfect with the original carbs and i have not made any changes other than carbs. I am just looking to learn more.

I am changing the carbs out for Vm34's because i am not much for the vacuum carbs and was offered these carbs for cheap. They are originally from a snowscooter so all the internals are changed out. Only downside is no choke, for some reason these carbs came without the plunger system, holes not even drilled. I never really liked the plunger on anything else than a single cylinder engine so i don't mind.

Simon, Copenhagen Denmark.

Mk2Rookie wrote: Hi,
.
I tried starting the bike yesterday with size #30 pilot, #190 main and needle in middle position, but it did not start.
.


Simon, Copenhagen Denmark.

That is why you need the fuel enrichment circuit.
Raise you slides to increase the idle speed.
If it were me I would start with a 2.0 slide and 20 pilot.


.

I think you will find it difficult (nearly impossible) to start that bike with the snow sled carbs that do not have provisions for a choke (or better described as fuel enrichment circuit by Nebr_Rex). KZ 750 twins and their KZ 400 brothers have a reputation for being cold-blooded, I'm using VM30s on my KZ400 and sometimes even on hot summer days it needs the choke on for at least a few seconds to start.

Without the fuel enrichment circuit you'll have to figure out a way to choke the carbs (limit the inflowing cool air) to get them to start. You can begin by trying to cup your hands over them while trying to start, assuming you don't already have filters in place. But beyond that I do not have any advice.

I know the bike will be harder to start without a choke, but sinces the plunger system in the VM's does nok work as normal chokes or fuel enrichment curcuits i still think i can live without it. Or at least i am not giving up yet.
I do know the slides are raised for higher idle.
I think #20 pilots sound lean based on what i read, other bikes i used VM's on, but please tell me how you came to think of that size..
Maybe forget what carbs i am fooling around with and let me know if you have any general tricks to start a bike with new carbs that are not OEM. How to get a good baseline.

Diggerdanh's post gives a good explanation.
You need the extra fuel to start a cold engine. The colder the air temp the more extra fuel you will need.
How do they get the extra fuel to start the snowscooter? Is that snow scooter a 2 stroke? 2 strokes carbs
are jetted differently than 4 stroke carbs. I base my Mikuni vm carb jetting on what Kawasaki used on KZ900/1000
vm28 carburetors. Even thou the fuel has changed over the years it is a good base line to start with.


.

20 pilots are probably way to lean especially with no choke. I can't imagine it starting with no choke, use tape over the back of the carb and see what happens.
Look and see if the needle jet ( the part the needle slides into) has a shroud or if its mostly flush with the carb throat bottom.
2 strokes usually have a shroud around the back side of the needle jet, 4 strokes don't, vm's can use either. one is a 159 series (shrouded) the other a 169 series (non shrouded) I think…
It changes the way the fuel is picked up but I don't think it will make a difference in starting.

The side stand broke and put a hold on it but got it fixed and got the bike startet. Runs awesome with these carbs, pulls harder and have better response. I still have to play around with jetting but #200 main jet and #35 pilot works very good.
I installed a primer pump from a snow scooter, 1 squirt of gas and it starts on first kick! No idle when cold but I am used to that from my Yamaha SR500.
I strongly recommend these carburetors.

Simon

I’m working on the tuning for my 76 750B right now and was wondering if you kept the 2.5 slide on to get it to work? Also how many turns on the pilot screw should I go?

the last post on this was over 5 years ago mate and the last time he logged in was 2018, pitty cause i would love to know more on how these ran.