Grinding a carb slide cutaway...

LPYYZ, most successful jetting results from working with secondary effects. Sudco sets up VM34 carbs with 2.0 slides and lean pilot jets--usually 17.5, unless high elevation is involved. There are very strong secondary effects between cutaway, PJ, NJ, and needle position. You might want to consider that before you start cutting on slides.
 
grizld1 said:
...You might want to consider that before you start cutting on slides.
Click to expand...

Absolute ditto. Recommend using the 2.5 slides, and playing with pilot jets and jet needles...

VM-Mikuni-Zones.jpg
 
Gotta remember that the pilot circuit never turns off and the main circuit is in operation even at idle. What the component-and-range charts won't tell you is how strong secondary effects are in VM and older BS series carbs; for example, lifting or dropping the jet needle by a single step will often force a compensating change in the pilot jet, and a change of a single step in cutaway size can force a pilot jet change of up to 3 steps. Sometimes you get lucky, but jetting often involves several rounds of change and compensation; you run in circles, but if you're doing things right they tighten.
 
There are soooo many variables to account. Displacement, cam, porting, valves, exhaust, ...etc. Here's a small list of the various VM carburetor setups out there. One size *doesn't* fit all...

Various VM carburetor setups

MJ = Main Jet
PJ = Pilot Jet
NJ = Needle Jet (emulsion tube)
JN = Jet Needle
Slide = Cutaway number
_________________

Roy Fisk
Carb: VM34
MJ: 190-200
PJ: 22.5
NJ: P6
JN: 6DHS, 3rd slot
Slide: 2.0
Air jet: 2.0?
Air screw: 1.25- 1.75
Float height: 25mm
_________________

Richard, from the Aussie site
Carb: VM34
MJ: 180-190
PJ: 20-25
NJ: P5, P6
JN: 6F9, 3rd slot
Slide: 2.5
Air jet: Removed

Dwayne, from the Aussie site
Carb: VM34
MJ: 180
PJ: 25
NJ: P6
JN: 6F9
Slide: 2.5
Air jet: 2.0?

Bob Hart, from the Aussie site
Carb: VM34
MJ: 180
PJ: 27.5 - 32.5
NJ: P5, P6
JN: 6F9
Slide: 2.5
Air jet: Removed
_________________

HALCO (UK)
Carb: VM36
MJ: 220
PJ: 25
NJ: 159-Q2
JN: 6DH3
Slide: 2.5
Air jet: 2.0

BOB BERTAUT (VM36 CARB KIT)
Carb: VM36
MJ: 190
PJ: 22.5
NJ: 159-P6
JN: 6DP1
Slide: 2.5
Air jet: Removed
_________________

Yamaman
Carb: VM34 with Ramair pods, Triumph silencers ,Halco 1 1/2" pipes.
MJ: 185 - 190
PJ: 25
NJ: P6
JN: 6DH4, 3rd slot
Slide: 2.0
Air jet:
_________________

gggGary's Madness
Carb: VM38
MJ: 260 (220?)
PJ: 25
NJ: 166-P8
JN: 6DP1, 3rd slot
Slide: 1.5
Air jet: Removed.
Float valve: 3.3

gggGary's Pristine, never had gas in them yet.
Carb: VM34
MJ: 180
PJ: 25
NJ: 159-P6
JN: 6F9, 3rd slot
Slide: 2.5
Air jet: ?
Float valve 3.3

gggGary's Unknown
Carb: VM34
MJ: 260
PJ: 40
NJ: 247-Q2
JN: 6DH4, Top slot
Slide: 2.5
Air jet: ?
float valve 1.5
_________________

Grizld1's 700cc
Carb: VM36
MJ: 190
PJ: 22.5
NJ: 159-P5
JN: 6F9, 3rd slot
Slide: 2.5
Air jet: Removed
_________________

TwoManyXS1Bs 750cc Dragon
Carb: VM36 (Vintage 1975, opened to 37.5mm)
MJ: 270-280
PJ: 35
NJ: 159-Q5
JN: 6FJ6, 3rd slot
Slide: 3.8
Air jet: 2.0
_________________
 
Great summary, 2M! I hope more folks get their heads around the idea that performance tuning is a process, and that most of the time, blindly following a recipe won't produce a good final result. (BTW, is Dragon an alkie burner?)
 
Thanks for all the information.

I don’t have much knowledge about the secondary effects on these VM’s.

I’m running pod filters on these VM34’s with a 700cc engine and a 2-1 exhaust.

I had the bike tuned on a dyno by a reputable and experienced guy once I had it running after the rebuild. The pilots and main jets all worked out great and the performance was excellent. It wasn’t until I started putting some miles under the tires that I was getting fouled plugs after 100 miles. So bad that I couldn’t start the bike without cleaning the plugs. I tried going up and down with the jets and needles but everything I tried only caused the bike to run too rich or too lean. Nothing was as good as my initial settings were. I figured out that I spend the majority of my time between 1/4-1/2 throttle which has led me to believe that it must have been the 2.0 slide causing this problem considering everything else was excellent.

Right now with the 2.5 slide the bike operates normally but I have to advance the throttle fairly smoothly in order to avoid the engine dying or coughing. A lot slower than I remember operating it with the 2.0 slide. If I snap it open at all the engine can’t keep up.
 
I've seen this before: plugs black, owner leaned out the jetting past any reasonable level, grief persisted. Got my hands on the bike at a rally and checked for voltage drop between battery and ignition--nearly 2V. Fabricated a jumper lead from battery straight to ignition and the "rich" symptoms vanished. The main switch was passing enough current for the bike to run, but not enough for a good hot spark.
 
grizld1 said:
Great summary, 2M!
(BTW, is Dragon an alkie burner?)
Click to expand...

Thanx, Griz. The Dragon was broke-in on 130 octane avgas, later run on period 104+. Not a normal setup, kinda schizophrenic. Sub-13 in the quarter, 70mpg @70 on the highway. Used some principles of that period's CAFE (Corporate Average Fuel Economy) contests, plus some other tricks. Remember the winning Honda 50 that got 1,500+ mpg?
 
'Scuse me while I pick my jaw up off the floor and try to reattach it. "Some other tricks" had to involve sorcery, can't think of any other explanation!
 
2M, The Bob hart set up is from the northern hemisphere. Those set ups are posts from the Vertical Twin Site...........posted into the Aussie site after enquirers.........VM distributors in Aus didn't like to give out their set ups for the VM series......... they didn't know........
 
Thanks Grizld1,

Last fall I developed a problem with my reg/rec and it would only produce a charge above approximately 1500 rpm where the voltage would then follow rpm. In traffic or at idle the battery would be discharging. I wonder if that may have had something to do with it? That being said if I was cruising around 2500-3000k I was always developing a decent charge so it shouldn’t be related I would assume?

The reg/rec was replaced this winter and I’m currently running a motogadget M-Unit which supplies power to a kill switch and then to Mikes ultimate high output coil.

I will check to see if I have an appreciable voltage drop. Do you have an idea what minimum voltage I should be seeing going into the coil?


0B2BFD24-6985-430F-9639-E3A953708212.jpeg
 
Add up the number of switches, relays, and connectors in your ignition circuit. Multiply that total by 0.1V. You shouldn't see greater voltage drop than that. Think about mounting your reg/rec where it gets some air; those fins are there for a reason.
 
Thanks, I’ll take a look on Monday.

It’s mounted to the front of the fender which is open to airflow, it’s hard to tell in that picture.
 
Short update...

My ignition system checked out okay.

I left all of the jetting untouched and switched back to my original 2.0 slides from the 2.5’s. What a change in performance. No hesitation at all and a strong pull regardless of how fast the throttle is opened. You could barely advance the throttle at a normal pace without a severe hesistaion or complete loss of power with the 2.5 slides. I found it hard to believe what a difference going up one slide size had.

Unfortunately I think this has confirmed my original suspicion that I’ll need a ~2.25 slide to avoid the fouled plug syndrome.

I’ll post an update once I attempt to file down a pair of 2.0 slides.
 
Filing down the slide will reduce the cutaway, making it richer. If you're going to modify the cutaway, work with the #2.5s; don't try to raise the radius of the #2s. But I'd be inclined to work more with needle position and pilot jet combinations. You haven't mentioned it, but what pilot jets, needle jets, and needles are in place?
 
Ok, I’ll try to file the flat bottom of the 2.5’s like 5twins mentioned instead of trying to enlarge the 2.0’s.

I have been running..

Main: 200
Pilot: 25
Needle clip: 3rd detent

I can’t recall what needle and needle jet are in there currently. They came “tuned” from TC Bros with their stock xs650 set-up in place.
 
Can't comment meaningfully or give any responsible advice on your jetting, as the NJ and needle comprise the heart of the carburetor. I can say that I'm not at all surprised that you're fouling plugs with #2 slides and #25 PJs. I have no idea what TCB installs.
 

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