Dual Mikuni VM34 Jetting

Rebuilt engine on a 1974 Commando Mk2 850 running twin Mikuni VM34 carbs and K&N filters. The engine/head has not been modified and is running a stock cam. I live near sea level in Southern California (Los Angeles). New NGK B7ES plugs are black but do not foul. Valve clearances are within spec, New Pazon ignition, with timing dead on 31 BTDC. MPG is down from where I was before the rebuild -- it is definitely running rich.

The motor starts easily on choke andl falls into a comfortable idle soon afterwards. However, once warm, the idle decays and the motor will die at lights, exit ramps, etc. I have adjusted the idle air screw, which seems to be most comfortable at 1 turn out. Turning it out further does not have any appreciable effect on the idle.

Here are the current specs for the carb internals:

• Needle Jet: 159-P8
• Needle: 6DH2 (1st position/highest)
• Slide: 2.5
• Idle Jet: 25
• Main Jet: 250

Single VM34 owners seem to coalesce around larger idle jets (~35) and smaller main jets (~220) but I don't know how applicable single carb settings are to dual setups like mine. I like the Mikunis and, though rich, I don't think I am very far off from where I need to be. Have read through the Mikuni tuning guide and other tuning how-tos (http://www.vintagebikebuilder.com/mikuni-tuning-and-jetting-guide.html).

Before I start tinkering with the jetting, I want to get a sense of the baseline Mikuni setup for the VM34s on the Commando 850. Does anybody know what those are, and how far off my jetting may be? The PO installed the VM34s with pod air filters that probably breathe like my K&Ns. That may explain the settings, but this is definitely too rich for the motor. I have already moved the needle clip as high as it will go and expect I am now at the point of tinkering with idle/needle/main but it would be helpful to know what is a proper baseline for a 850 in standard tune with dual VM34s and pod filters.

I believe that you may be woefully over carbureted for a stock engine; your knees probably won't thank you either. A pair of 32s is pushing it, a set of 30s may be just right. (keep in mind that the Mikunis are much more efficient than equivalent sized Amals) Good luck with this. I'm not trying to rain on your parade.

Best.

On a Triumph, when a single carb is fitted (of the same size as twin carbs) the main jet is several sizes bigger in the single carb.

You‘re running larger jets in your twin Miks than the single Mik application. Sounds wrong to me.

If you have the clips in the top notches of the needles, there should be a large flat spot as you wind the throttle on. You probably need the next size smaller needle jets. As you progressively lower the needles, you should induce the cough (flat spot), then raise the needles one notch. It should not matter to you if the main jets are a bit too rich - at least that is safe.
When the jetting is too rich, the motor should be sluggish. When the jetting is too lean, you should get the miss or cough. Best jetting is just above the miss. If you cannot get the miss, you are jetted too rich.

I only use plug readings to check main jets. There should be a black ring on the porcelain right down inside the plug where the porcelain meets the metal. If you use plugs for tuning which are one range too hot, you will stay safe, then fit the colder plugs.

acotrel said:

I only use plug readings to check main jets. There should be a black ring on the porcelain right down inside the plug where the porcelain meets the metal. If you use plugs for tuning which are one range too hot, you will stay safe, then fit the colder plugs.

Click to expand...

You can see right down to where the porcelain meets the metal? .... get outa here Willis!

robs ss said:

You can see right down to where the porcelain meets the metal? .... get outa here Willis!

Click to expand...

Ask yourself, Grasshopper, why is it you do not see?

That needle jet is HUGE, and most likely the source of your problem. I run twin 34 Mikunis around these settings on my 750 short stroke race bike:

• Needle Jet: 159-P4
• Needle: 6DH2 or 6DH3
• Slide: 2.5
• Idle Jet: 25
• Main Jet: 190 to 210
• Air Jet 1.0 to 1.2

You don't mention the air jet, so you probably have a 2.0 which they come with as standard and is only good for 2 strokes!

Getting it right will prevent lean out at high rpm! In effect, this jet runs smaller for less air and richer running.

You will probably end up at the richer end of these if you are running a restrictive exhaust and air filters. I would start by checking the air jet and coming down 2 sizes on the needle jet, put the needle in the middle.

The suggested starting settings in the VM tuning manual for the 750 and 850 twins err towards being too rich, so expect to end up several steps weaker than those figures.

The problems you describe are not related to main jet, but I think you know that.

RoadScholar said:

I believe that you may be woefully over carbureted for a stock engine; your knees probably won't thank you either. A pair of 32s is pushing it, a set of 30s may be just right. (keep in mind that the Mikunis are much more efficient than equivalent sized Amals) Good luck with this. I'm not trying to rain on your parade.

Best.

Click to expand...

Over carbed yes probably, woefully, no I don't think so.

I have also run 36mm twin Mikunis on the 750 short stroke, with very little noticeable difference to the 34s. In fact the 36mms are now back on simply because I wanted to use the 34mms on another bike. On an 850 race bike some years ago the 36mm Amal MkIIs were better than 34mm MkIIs and 36mm Mk1s. The difficulty in running Mikunis back then would have been finding a supplier and identifying a base set up, working it from scratch would have been a nightmare with no dyno access.

I think the major point here is he has 34mms, and they can be made to work well, rather than leaping out to buy more carbs.

If he was buying new and wanted to use Mikuni I would suggest 32mm for the road bike, but would probably suggest, based on most people's recent experience, to go with Amal Premiers which have a less compromised mounting position and feed of off the feedback lots of guys have provided here.

Hi, here is what I had found on the 750 racer with twin vm34 (ex Patrick Lefevre the son of Gustave who won several races in France on his Manx and grand son of Norton french importer)
main: 220
needle jet : 159 P5
slide :2.5
needle : 6DH3
idle jet :30

and in 1997 Sudco supplied me with two VM34 for my 920 with 4S (Pete Lovell tuned)
main 250
needle jet 159 PO
slide :2.5
needle :6DH3
idle jet :35

robs ss said:

You can see right down to where the porcelain meets the metal? .... get outa here Willis!

Click to expand...

You need a light. The heat burns the carbon off the plug insulator for a certain distance which is determined by the plug's heat range. When only thee black ring remains, the mixture is right. But I only use that to get the main jets right. Usually I run a bit too rich on the main jets. The porcelain can be white, but the 3 mm wide black ring must be there. When you use full throttle, you give the motor a reprieve.
The shape of the needles compensates for loss of vacuum as you open the throttle. For 32 to 34 mm Mikunis, I would use 6D Mikuni needles.

The shape of the needles and the size of the carburetors affects how fast you can open the throttle. With big carbs and lean taper needles, you need to feed the thoittle on in controlled fashion. But you get used to it. If you whack the throttle open, you might just get a gasp. When you use petrol as fuel, your jetting probably becomes more weather dependent if you have got it right.

Spoke with Sudco this morning. When they sell the dual VM34s for the Norton Commando, they ship with exactly the settings I detailed above, though they also include additional jets to step the mixture up/down. (Looks like the PO bought the Mikunis from Sudco.) Since I am fortunate enough to live a few miles from Sudco, I will drop down there to collect a few (leaner) jets.

Since the idle is my problem in the moment, I will drop down a single step to the 22.5 idle jet and see where that takes me.

I pulled the air screw and do not see any markings, though I am told by Sudco that they ship with a 2.0. Since all of the other settings are consistent with theirs, I expect this is the same as well.

If Sudco ship with a 2.0 air jet they are ignoring it's effect! I think this is because in most cases they can, few ride in the area it really affects things for long. But, if they don't get that right why trust the rest of the set up?.....

If you search for Fullauto's thread on tuning his Mikuni (single carb), he tells how he worked with the air jet to get better fuelling and excellent economy, so it isn't something that just affects race bikes.

But, I do understand that most suppliers will want to err on the rich side to prevent a whole load of customers coming back with wrecked motors from starting too weak, which is what they have certainly done with the needle jet.

If you have this manual it will give you a better idea what it does and how it influences the needle jet fuel mixing:


Page 15 points out most people don't understand what it does, I find out most people don't know it exists!

I understand your logic regarding the idle jet, but I think you will benefit from getting the needle jet nearer first. The needle jet influence overlaps the pilot circuit, As does the slide, your slide is already better than the 2.0 Sudco supply. The VM manual goes a step further to 3.0! But I am pretty sure 2.5 will be fine.

There is a better diagram than this one out there but I am not sure where I have it, this one is OK, but as most, ignores the air jet!

I am certain the needle jet is way to big. Put in P-2 and see what happens

The idle circuit and slide cutaway usually do not work above 1/4 throttle. The needle jet works all the way down. If it is too rich, it will be too rich everywhere.

Given that the needle jet is currently a 159-P8 and the bike runs well above idle (though rich & MPG is not great), I am reluctant to drop all the way to a P0.

There are five steps between the two: P8 : P6 : P5 : P4 : P2 : P0. Based on input from others about their jetting setup, I am inclined to try a P6 or a P5. There are a few of you who run a P5 and below, but this is NOT a race bike and I am comfortable sacrificing some power for ease and longevity.

I think that I will head down to Sudco tomorrow and collect 2x 22.5 idles and 2x 159-P5s. Will try the idle jet only first and see how that goes -- every carb manual I have read advises against making more than one change at a time. Will keep everybody posted on the progress.

It's a Start.
What K&N's are you using?

Modified per Fred's instructions: https://www.oldbritts.com/42_100004.html

NO. I really doubt that you need to go below 35 for the pilot jet.
Buy a pair of #35 pilot jets.
Buy a pair of P-2 and P-4 needle jets
Buy at least a 4-pack of BP7ES spark plugs. Two packs are better. get out on the freeway where you can open up the carbs AT LEAST 1/3 THROTTLE and later 1/2 throttle and hold it there. See how it runs. Check the plugs. I took my Atlas out on the freeway in third with a 21 tooth sprocket. I was quickly doing 80 up the freeway about 5000 rpm. My AFR guage was reading about 12.5. You will have to do it with the seat of the pants method. Be careful, but that is where you need to be to test the needle jet