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

There are a few combinations of jets and needles that work with a Mikuni on a Commando. 6DH3 or 6DH4 needles and P or Q needle jets. 25 through 40 pilot jets are common. Something to remember is to change one thing at a time so you can see if it makes it better or worse.
 
I think you have the right air jet. The air jet will mess with everything on a Mikuni.

Get a color tune spark plug and get your idle dialed in. From there, you can work on your needle and needle jet. Find the needle that works best for your application usually a 6DH3 or 6DH4. With the idle jet set based on the color tune spark plug start testing yoru plugs. Lean the needle out if you get the needle as low as it can go and it is still a black plug and the go from a P0 to an 08 needle jet.

I suspect your idle jet is a bit rich based on what you have listed. Color tune plug will let you know, and dont change things on a whim, pay attention to where the issue is happening throttle position, rpm, speed, etc.

I personally would get idle jets from 17.5 to 50 for a british bike, mains from 200 to 260, needles 6DH2, 6DH3, 6DH4, and 6F9, as well as O6 through P8 needle jets. It does not hurt to have a couple air jets as well, 1, 1.5, 2.0 and you may want to try a 3.0 slide. However, that is a lot of brass and a solid investment, but you can really dial in your carb and get a much more enjoyable riding experience by doing so.
 
When you lower the needle one notch at a time, you should be able to induce the motor to cough. If you don't get the cough, you need a smaller needle jet. When you get the cough, raise the needle one notch. The 6D needles you have mentioned are what I would use. The slow taper gives better performance, but you cannot whack the throttle open in a hurry, it needs to be fed on. I only use plug colour to set the main jets, but if the porcelain inside the plugs is black, you are jetted miles too rich. There should be a black ring on the porcelain, right down inside the plug where it meets the metal. The heat range of the plug determines how fast the heat of combustion burns the carbon off the procelain. Use hot plugs if you are tuning for best performance, then replace them with colder for normal use.
Once you understand what is happening, you can tune very quickly. If I ever hear my motor miss, the first thing I do is raise the needles one notch. If the miss is still there, I check the wiring. Even in the coldest weather, raising the needles one notch is usually sufficient. Always tune with the motor hot.
 
One thing I would say is - do not become frustrated - you are the boss - make the bike do what you want it to do. Understand each of the functions of the metering jets. Unless you are on full throttle, the main jets do nothing - unless they are miles too small. When you are on full throttle it is possible to meter off the tip of the needle, but that does not usually happen.
 
Thank you all so much!
I am very familiar with British Car carbs (like SUs) and am learning about the Mikuni thanks to you.
I tried to tune Weber carbs once and had to give up. It was too expensive to change all the various emulsion tubes and jets in 6 throats and then find it was not improved!

I understand my mission with the pilot circuit and will get three smaller size pilot jets and test (as well as replace the the float needle valve, floats etc).

To help my understanding of the main jet circuit can someone tell me exactly what the main "Air Jet" does?
1) Does it affect part of the rpm range of the main jet action or all? (ie Making the jet bigger, does it lean
the mixture slightly over the full range, or just (say) the lower end?)
2) What is the difference between dropping the carb needle one notch (to lean out) and increasing the main air jet size
by a certain amount?

Many thanks for all the wisdom!

Dennis
Vancouver
 
As @concours wrote idle stabilisation makes it impossible to tune the tickover by ear, as the revs drop the timing is advancing picking the revs up. My Boyer red boxes do the same as the Trispark and also have idle stbilisation. This is where the Colortune comes in, it has a window into the combustion chamber where you can see the colour of the flame, pale blue is week, bunsen burner blue is spot on and yellow is too rich. This way revs are not a part of the tuning except when you get the right mixture you then set the revs with a final check that its still the right blue.

If it stays yellow drop the pilot jet size and try again.
 
I have a coloutune and have used it on my British Classic car. The problem with the SU carbs on my car is the the idle mixture was not stable so flame colour varied... I tried everything air leaks, you name it but nothing cured that so I went by ear
Dennis
 
DennisMo said:
I have a coloutune and have used it on my British Classic car. The problem with the SU carbs on my car is the the idle mixture was not stable so flame colour varied... I tried everything air leaks, you name it but nothing cured that so I went by ear
Dennis
Click to expand...

Raise your idle up around 1000 to 1500, as you get close to 1500 you will get on the needle so you have to be a little conscious of where you are. Listen for the changes.
 
maaseyracer said:
Raise your idle up around 1000 to 1500, as you get close to 1500 you will get on the needle so you have to be a little conscious of where you are. Listen for the changes.
Click to expand...
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
 
maaseyracer said:
I think you have the right air jet. The air jet will mess with everything on a Mikuni.

Get a color tune spark plug and get your idle dialed in. From there, you can work on your needle and needle jet. Find the needle that works best for your application usually a 6DH3 or 6DH4. With the idle jet set based on the color tune spark plug start testing yoru plugs. Lean the needle out if you get the needle as low as it can go and it is still a black plug and the go from a P0 to an 08 needle jet.

I suspect your idle jet is a bit rich based on what you have listed. Color tune plug will let you know, and dont change things on a whim, pay attention to where the issue is happening throttle position, rpm, speed, etc.

I personally would get idle jets from 17.5 to 50 for a british bike, mains from 200 to 260, needles 6DH2, 6DH3, 6DH4, and 6F9, as well as O6 through P8 needle jets. It does not hurt to have a couple air jets as well, 1, 1.5, 2.0 and you may want to try a 3.0 slide. However, that is a lot of brass and a solid investment, but you can really dial in your carb and get a much more enjoyable riding experience by doing so.
Click to expand...
There is some inconsistency here. He has the right air jet, but should have at hand a 1.0, 1.5, and 2.0?

Well, we know he has a 2.0. We should also know that the 2.0 is the standard fit to a new Mikuni. Personally, I would get a 0.9, 1.0 and 1.1.

Checking other threads on here will show that most find a sizing between 0.9 and 1.1 to work well on a 4 stroke. 2.0 is mostly suitable for 2 strokes. Some 2 stroke racers even run with no air jet fitted at all.....just the carb body hole.

I have Mikunis on 3 bikes, one is a 2 stroke, and it does indeed have a 2.0 fitted. The 4 strokes don't, one 750 twin set up, two 500 singles, one a race bike with a 38, the other a '56 plodder with a 32.

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.

The problem being it is more a 'rate of change' adjustment than a fixed throttle opening adjustment. It doesn't really relate too throttle opening like the other jets, and it mostly affects higher rpm performance, the risk is you can suffer high rpm lean out if it is too large. This is why a lot of riders never really realise their air jet is wrong; they don't ride at high rpm than much of the time. Those inclined to run a single Mikuni on a Commando may never run in the range that would show a problem. Personally, I think this why vendors leave the 2.0 in. I find that lazy, but that's just my opinion.

The Australian member 'Fullauto' ran a long thread dealing with getting the air jet right on an 850 road bike, single 34mm. His reference was mostly Jim Comstock. They got there in the end and I think settled on 1.1.

I am currently running 36mms on my 750 short stroke, with I think a 0.7 or 0.8, with 34mms, it was a 1.1

And, I think the issue of black plugs and not getting a good response to the mixture setting is most likely too large a pilot jet......

And....if you can, get a copy of the Victory Manual for tuning VM Mikunis on British motorcycles.
 
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Steve, Frank and everyone... Thank you.

Every reply really helps me along !!

Frank, just so I understand. Is the hesitation on acceleration just when you start to accelerate? If yes, would this not point to the pilot circuit jet not being big enough, or the pilot circuit air bleed screw is not adjusted for maximum idle speed, but is actually running the circuit too lean?
My problem is that with a 35 pilot jet, I get max rpm at about 2 1/2 turns out, but unscrewing further does not drop the idle speed (ie via leaning) which I understand means I need a smaller pilot jet

Thanks. Really I mean it. I have learned a lot from this thread

Dennis
Vancouver
 
DennisMo said:
Steve, Frank and everyone... Thank you.

Every reply really helps me along !!

Frank, just so I understand. Is the hesitation on acceleration just when you start to accelerate? If yes, would this not point to the pilot circuit jet not being big enough, or the pilot circuit air bleed screw is not adjusted for maximum idle speed, but is actually running the circuit too lean?
My problem is that with a 35 pilot jet, I get max rpm at about 2 1/2 turns out, but unscrewing further does not drop the idle speed (ie via leaning) which I understand means I need a smaller pilot jet

Thanks. Really I mean it. I have learned a lot from this thread

Dennis
Vancouver
Click to expand...
Thanks DennisMo. I'll certainly go higher with the main jet. In the meantime, I'm trying to get the plugs to last more than 100 (one hudred!) miles which with a 35 pilot was what the plugs achieve.

Frank
 
DennisMo said:
Steve, Frank and everyone... Thank you.

Every reply really helps me along !!

Frank, just so I understand. Is the hesitation on acceleration just when you start to accelerate? If yes, would this not point to the pilot circuit jet not being big enough, or the pilot circuit air bleed screw is not adjusted for maximum idle speed, but is actually running the circuit too lean?
My problem is that with a 35 pilot jet, I get max rpm at about 2 1/2 turns out, but unscrewing further does not drop the idle speed (ie via leaning) which I understand means I need a smaller pilot jet

Thanks. Really I mean it. I have learned a lot from this thread

Dennis
Vancouver
Click to expand...
I usually adjust the idle screw until it is lean enough to get a miss, then richen it slightly to get good idling. It helps when you start the motor with the throttle closed. I think all my carbs have had adjustable throttle-stop screws , which set the idle speed. If the motor is idling too fast, the cables might be holding the throttles of their stops. I have never had that problem but I use a quick-action twin-pull twist grip. Junction boxes can have problems.
 
I don't know what revs my motor does when idling, but with the high crank balance factor the bike rocks backwards and forwards, so it couldn't be 1000 RPM.
I use methanol fuel, and when I bought the carbs from Hemmings I requested methanol-kitted. I did not need to change the air jets and I haven't on any bike. My brother has had to do it about once, and he races all sorts of bikes.

If you open the throttle wide, then let go of the twist grip, you should hear the slides hit bottom with a single click - NOT a double click.
 
acotrel said:
I don't know what revs my motor does when idling, but with the high crank balance factor the bike rocks backwards and forwards, so it couldn't be 1000 RPM.
I use methanol fuel, and when I bought the carbs from Hemmings I requested methanol-kitted. I did not need to change the air jets and I haven't on any bike. My brother has had to do it about once, and he races all sorts of bikes.

When the motor is not running, if you open the throttle wide, then let go of the twist grip, you should hear the slides hit bottom with a single click - NOT a double click.
Click to expand...
 
I think regardless of the air jet or the main jet, the tuning sequence is the same. The main jet needs to be rich enough to avoid burning pistons, but if is too rich, it usually does not matter unless you race.
I would not change the air jet unless I wanted to get the needle jets and needles into a tunable range. I make my own needle jets out of brass hex, so for me it is not a problem. I would not know a convenient place to buy jets in Australia, these days.
 
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
Click to expand...
try a #3 slide
 

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