VM34's on kz750 twin

A

Armaundk

New Member
So, has anyone successfully jetted a set of VM34's for a 750? I'm having a devil of a time rounding out a lean flat spot at 1/4 to 1/2 throttle.

I've seen a set of VM's supposedly jetted for a 750 being sold on DCC and Niche Cycles and the jetting just doesn't make sense to me. I've got a set I bought from 650 Central, Michael Morse took his best guess but with his jetting I'm lean where I mentioned.

So here's what I've got in there now:

Pilot jet - 25
Cutaway - 2.5
Needle - 6F9 position 4
Needle Jet - 159 P-6
Main jet - 200
Float level is 23mm

The supposed 750 jetting I've seen advertised is:

Pilot jet - 25
Cutaway - 2.0 (doesn't seem right for a 750 engine)
Needle - 6FJ6 (much leaner than a 6F9)
Needle Jet - 159 P-8
Main jet - 200

I've been trying to work out this damned flat spot for a year now to no avail. Makes highway riding quite problematic. Really hoping someone has some experience.

As always, thanks in advance.
 
i would start with

3.0 slide

37.5 pilot

no air correct jet to start

q2 it comes with

6dho4 needle it comes with all the way down to start

200 main

if you were near me i have all that stuff you could try it and stick a sniffer on it and do a test ride
 
Agree. Sounds like it needs a larger needle jet to richen up the mid range and a different needle to clean it up at 3/4.

Q-2 would be a good start.
 
Man, CX, that's way off from where I am.
I do agree I think the needle's wrong. The stuttering starts right at 1/4 throttle, but it does clear up at about 1/2 throttle with the no. 4 needle position, so that part of the needle is too lean. I'll look into 6DH4...

I've got a P-8 NJ I'm going to throw in there and try...let's see what happens.

I feel like my slide and pilot are ok. Everything from idle to 1/4 is really solid.

Thanks for the help, I'll keep it posted.
 
your problem is throttle valve

(changing that will necessitate changing the pilot)

and next needle jet and finally jet needle

if you look at any mikuni tuning chart you will see what i am talking about

you actually diagnosed that by declaring the bog rpm range

here is one of many on the net for vms

http://www.650motorcycles.com/CarbTuning.html
 
Had to take a break from jetting to recheck valve clearances.
I tried richening everything up, just to see what happened. It was not good.

PJ 27.5
MJ 200
C.A. 2.5
NJ 159 P-8
Needle 6FJ6

It choked on fuel at anything above 1/8 throttle. Plugs black as night.

I got 6DH4 needles, threw those in the same jetting above...better but not good. Still choked on fuel.

I have 3.0 slides and 35 and 37.5 pilots on the way. We'll take it from there...
 
Try changing the slides first to clean that off-idle up. See what difference that makes and let's work form there.
 
leave the needles down to start

use the 3.0 slides

37.5 pilot

put the q2 back in

put the 6dh04 back

200 main is ok to start
 
Mmmmm ok, so, I could not even get the bike to start with the 3.0 slides even with the enrichers on...or without.

The slides sit too high, even at the lowest position, idle,screw barely touching the slide. checked throttle cable at all points, all adjusted properly. Is it possible the 3.0's are simply too lean for the engine?

Here's what it's set up with now:
Pj 37.5
Slide 3.0
6DH4
NJ P-8
MJ 200

Didn't have a q2 yet and want to make sure I can get it running before springing for them.
 
If the slides are to high something g is wrong

I use 3.0 slides on 7 of my kits ranging from 650 900 in 2 and 4 cylinders

Pull out the cable and see if the slides drop then or send me some pictures so I can see
 
I hope these pics post...

These are the 3.0 slides at their lowest position, which after putting the 2.5's back in I realize is not much higher than them. For comparison, the 2.5's sit at about the height of a 5/32 drill bit, the 3.0's at an 11/64 drill bit. Which is what I would expect given the leaner cutaway.

Checked spark, all good there, don't know why it won't fire up unless it's just too lean. I tried to cover the intake bell with my hand (and one of an accomplice) and it fired once but didn't start.

Bigger pilot needed, perhaps?
 

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Is the slide all the way down (closed) on the engine side? And if not, are the slides sitting on the adjusting screws or is there no slack in the throttle cables?
 
The slides are down as far as they can be, idle screws are not touching them, there's a little slack in the throttle cables, about 1-2mm, as is standard. The engine side of the slide is not completely closed off, in fact I can put a 5/32 drill bit into the space...hmmm...

Thoughts?
 
like i said something is wrong

the slide should close completely on the engine side or very close to it

here is a pair they are for a cx650 (674 cc)

and the slides are at the set idle height and balance and are running at 1150 rpm



are you sure they are not left hand slides in a right hand carb? or vice versa?

i use a left and right in all my kits so the idle screws are easier to get to

and if they get mixed up they wont sit right as the cut aways and notches are mirror images

the other thing to check is the brass jet needle in and down all the way or is it somehow holding teh slide up
 
I must be losing my mind. Ok, slides are sitting correctly, no obstructions and the front side is indeed closed. Left slide on the left, right on the right. Same jetting as above, mix screws 1 turn out.

It cranks and every once every three or four revolutions thinks about firing in one cylinder. Tested both plugs, spark is good to both.

Shit, at least with the 2.5 slides the thing ran.
 
with mikunis there are 3 or so ways of getting to the same place the quastion is if you will be clean thru the whole rpm range

dcc jets theres like this but i heard they run rich at mid range

etting: 200 mains, 25 pilots, 159 P-8 needle jets, 6FJ6 jet needles, 2.0 slides, and 2.0 air jets.

and with a large engine i would never run slides that rich it will over fuel under party load and cackle

in your place i would pull the chokes and crank it a few times and look at the plug is it dry?

if so i would put a 40 or 42.5 or even a 45 (probably rich )\\

make sure you miuxture screw is at 1.5 turns

i still feel its worth sticking with a 3.o slide to avoid over fueling on the way to wide open

i have much better fuel curves and off idle response
 
Yeah, I can't take that dcc jetting seriously. I've read some threads from people who've put their other supposedly pre-jetted vm's on their bikes with horrid results. I'm sure the 3.0's will prove the answer...once I get them going.

My plugs are wet after cranking with the enrichers on. I can smell fuel in the afterburner, er , exhaust. Only thing I can figure now is I need a richer pilot? Maybe a 40? I tried raising the slides a little, about a millimeter. That didn't work.
 
This is not making sense now if the plugs are wet it should fire and it's not lean

I am brain dead tired right now

Pm me your number I will call you tomorrow and we will see if we can figure it out
 
Turn the enrichment off hold the throttle open and crank it see if it's flooded
 
Held the throttle wide open and cranked...no change.

Plugs are wet, checked again. I'm going to go get some spare plugs and try a new set, see what happens. Man, I've never not been able to get a bike started...frustrating. The air/fuel must be off somehow.
 
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