Can you give me advice about tuning/jets etc on my single Mikuni VM34

Calling out random jet numbers is irrelevant. Conditions and mixture are relevant. The air jet makes different jetting relevant, the slide makes different jetting relevant as well. Not every bike will be the same. I have tuned dozens of bikes with Mikunis, I have tuned mikunis on a dyno. Getting a series of jets, slides, needles, etc is. Every bike is different, and while most of these run well on a good common setting, not every bike is the same. I tuned a single carb triumph last year which you would think the kits would be perfected on. Ended up changing slides, needle, needle jet, idle jet, and air jet. Helped a friend a couple months ago the slide that came with the carb from a reputable Norton supplier was not right for his bike. When I tuned on a dyno I thought the bike ran well before, and we changed everything but the slide.

Tune for your bike, and use others jetting for commonalities, I personally would buy that range of jets. Change one thing at a time, focus on your idle first. I do agree with whoever suggested buying the victory library book.

Change the air jet and it 'messes with everything'. Well, to a degree, yes it does. You certainly don't want to put a lot of effort into changing needle jets and main jets until you have it in the ball park on the air jet.

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The air jet controls the "slope of the full throttle curve" to quote the victory library book:

This jet controls the "slope" of the full-throttle fuel curve. As engine speed increases (with fixed throttle opening), mixture will gradually become richer by itself, unless some provision is made to compensate for this: air correction is the compensation. It may be the least-understood circuit in any carburetor, but it's necessary for proper function, and is present (but may be invisible) in all carburetors. The correction is done by metering the amount of air entering the needle jet.

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The air jet can change everything. A larger air jet will flatten out the mixture curving making it richer faster, and a smaller air jet will make the curve longer and richer as rpm increases. For perspective a 2.0 jet is the biggest air jet I have seen available. So, I will take back what I said earlier, and based on what you are saying it is a good call to start with the air jet.

"
I have had my 1973 850 for a year and refreshed everything except the Mikuni and hope you will share your advice.

I have not had the Mikuni to bits but here is what the old owner says is the setup."


OP, a disassembly and inspection is a good place to start.

frank73x89 said:

If it may help, I have a VM 34 Mikuni round slide on a 750 commando.
Here goes:
Slide 2.5
Needle jet 159 P-0
Jet needle 6DH 3 (set in the middle groove)
Main Jet 230
Pilot jet 25
Air jet 2,0
I run a Boyer ignition.
Now, I,m not done tuning it. I have a 1.2 and 1.5 air jet coming to replace the 2.0 in it right now.
If not completely warm, it will hesitate on accelaration with choke off.
The plugs with a 25 Pilot jet jet are overly dark grey/black but with a 35, there are pitch black.

Frank

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I found that using a #3 slide and a # 40 pilot jet kept the plugs cleaner for a longer time. I have tried different air jets...1.0...1.5..2.0...and no air jet. Never seen or felt a difference.

My '66 N15CS 750 is stock except .040 over 7.5:1 NOS stock replacement pistons and the single Mikuni.

Needle jet 159 P-2. It's what i had, and it does fine, but P-0 is the recommended one.
Main 230.
Idle 27.5. I had it at 32.5 and same issues you are describing. Go down 2.5 or 5 I'd say. That was the last thing I had to do plus getting a 'real' Mikuni rubber spigot. Turned out I had a cheap copy and it leaked air, really messed things up. Now runs sweet.
Needle 6F15, set to middle it's what I had and it works.
slide cutaway 2.5
air jet 2.0

Good suggestion to replace the rubber manifold... I assume that's what a spigot is!
I am going after the pilot circuit first and replacing the needle valve, floats etc and giving
it a good clean.
Thanks
Dennis
Vancouver

Here is a progress report.
I gave the carb a good clean and replaced the 35 pilot jet with a 30.
When I unscrew the idle air screw (when it is rich at one turned unscrewed), I now have success with the engine speed rising to a peak and then dropping as it gets too lean. Maximum revs and smoothness is at about 2 1/2 turns out

But I found a 230 main jet with the 2.0 main air jet.
From what I read, it seems most find the 240 main jet better.
The engine seems to run fine and pull well at 1/2 to full throttle.
Should I go to a 240 or should I perhaps raise the needle a notch and see how the bike performs?
or should I drop the needle until I get surging at 1/2 throttle and poor performance then raise the needle one notch

With thanks
Dennis

DennisMo said:

Here is a progress report.
I gave the carb a good clean and replaced the 35 pilot jet with a 30.
When I unscrew the idle air screw (when it is rich at one turned unscrewed), I now have success with the engine speed rising to a peak and then dropping as it gets too lean. Maximum revs and smoothness is at about 2 1/2 turns out

But I found a 230 main jet with the 2.0 main air jet.
From what I read, it seems most find the 240 main jet better.
The engine seems to run fine and pull well at 1/2 to full throttle.
Should I go to a 240 or should I perhaps raise the needle a notch and see how the bike performs?
or should I drop the needle until I get surging at 1/2 throttle and poor performance then raise the needle one notch

With thanks
Dennis

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Unless you are using more than about 3/4 throttle, the main jet should not make a difference unless it is miles too lean. At lower throttle openings down to about 1/4 throttle the fuel is metered by the needle and needle jet, and below 1/4 throttle the slide cutaway and idle circuit operate. Most carburettors use a slide with a 3mm cutaway. The mixture is usually adjusted with the fuel metering jets, not the air supply. The shape of the needle compensates for loss of vacuum. For performance, a lean needle and feeding the throttle on, usually works better than a rich needle and whacking the throttle open.
I do not usually need to change air , pilot jets or slide cutaway. Most bikes are tunable with needle, needle jet and main jet.

Thanks to everyone.
This has been a success.
I checked my spark plugs and there is now a nice tinge of brown to them. Clearly now hotter and no black on them.
I will leave the main jet at 230 as the bike pulls well with throttle.
Dennis
Vancouver