Dual Mikuni VM34 Jetting

nomadwarmachine said:

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.

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Given that you have filters and a restrictive exhaust, and race bikes don't! P4 is still likely to be where you end up. Personally I have never been tempted to go to P2 let alone P0, generally they are used in smaller bore carbs!

Thing about P5 that marks it out is that it is a half step. However, it might work fine and just need the needle lower than a P4. Gonna be better than your P8 anyway.

I would not follow the idle jet size of a race bike, I have tried 22.5, 25 and 27.5 and could never tell the difference! But then again, I don't let it idle! I keep the revs over 2000 to lube the cam. The only place it is likely to affect my bike is coming off a fully closed throttle!

seattle##gs said:

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

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You forgot the advice to put paint marks on the throttle at 1/4, 1/3 and 1/2!

Any updates on this jetting?

seattle##gs said:

snipped My AFR gauge 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

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More info about your air/fuel ratio gauge please! Perspiring minds want to know!

seattle##gs said:

Any updates on this jetting?

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Should have an update next week, when I can make it down to Sudco for the jets. Been reading through the Victory manual -- very thorough!

I think air/fuel ratio gauges are usually reverse-calibrated. They find out what works and note the ratio. Then use it everywhere. - If you tried to figure out the optimum from stochiometry, you would never find it.

nomadwarmachine said:

Should have an update next week, when I can make it down to Sudco for the jets. Been reading through the Victory manual -- very thorough!

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Here's some more information to ignore:

If you end up turning the idle air adjustment screw in further than 3/4 to get a good idle with that 22.5 pilot, you went the wrong way with the pilot jet. That's what is says on little sheet of paper that came with the K&N air filters I got with the 35mm FCRs. 1 turn is on the edge of being lean as it is in my opinion. I would not get the 22.5 pilot to fix what could be a needle and rich needle jet issue. What comes with the Mikuni twin 34mm kits (not what you have) is pretty close albeit rich for putt putting around with a stock engine be it 750 or 850. I would not fear a P2 or P0 needle jet with a 6DH3 needle and 230 - 240 main jets on a street bike. Works on my modified 750 engine and I have yet to melt a piston in 7K miles. If I want to see tan plugs, I have to ride harder.

An AFR sensor and gauge makes sense on an EFI application that can be tuned quickly with software. Not so much on a carbureted antique.

I agree with Schwany.

The 34's came with #35 idle jets, they should work. Most people don't realize how much interplay there is between the air screws and the slide height screws. Turn the air screws in and the idle drops, get it back by raising the slides; this is going toward rich; turn the air screws out and the idle goes up, get it back by lowering the slides; this is going toward lean. You adjust the idle mixture to promote a smooth transition to the needle for a smooth take off from rest. You keep at the two adjustments until you have a smooth take off from rest. If the bike idles well, but misbehaves getting on to the needle try first raising the needle and recheck idle setting, a 6DH3 is, IMO, the best bet for beginning, and put the clip in the #3 position to start with.

As suggested do mark the twist grip relative to the twist grip body. This "tool" is most effective during steady state riding, that is with little to no acceleration/deceleration. Once the throttle setting is above above 25% the idle circuit is "all in" and the needle/slide cutaway begin their job.

Get these two aspects squared away and the next component is the needle jet; I think a P-2 is a good starting place.

Best.

If you lower the needles so the clips are in the top notch, as you open the throttle you should get a large flat spot. If you don't get it, the needle jets are too large. Setting the idle is a minor issue. If the idle screws adjust the fuel, you just screw them in until you get the miss, then back them off slightly. You need to make sure the slides hit bottom at exactly the same time.. If you listen when you drop them without the motor running, you should not hear a double-click. I usually back the height screws right off and adjust them last, if the idle is too slow..

acotrel said:

If the idle screws adjust the fuel, you just screw them in until you get the miss, then back them off slightly. You need to make sure the slides hit bottom at exactly the same time.. If you listen when you drop them without the motor running, you should not hear a double-click. I usually back the height screws right off and adjust them last, if the idle is too slow..

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Where is the idle screw on an Amal?

The two adjuster screws I’m aware of are the throttle stop screw and the pilot air adjuster screw, which is more commonly known as the air screw.

Its called the air screw cos it controls the air. So, screwing it in reduces the air and enriches the mixture. And vice versa.

Here is something I learned about needle jets. If you are way off, say at least two sizes TOO SMALL, there will be a big bog just off idle. It may start and idle well but trying to get underway will require slipping the clutch. A quick test is to move the needles all the way up in the rich position and see if there is any improvement at that one small position. You are at P-8 now and concerned about dropping it to P-2 ,where I think you ought to be. Go ahead and install the P-2 and look for the bog just off idle as a sign you went to small.
Yes, you will have some money invested in the brass bits. It is trial and error so you need the extras to do the try-it-and-see process.
**You are running a #25 pilot with a really rich needle jet. When you lean the n-jet from an 8 to a 2 it will most likely require a slight richening of the pilot. That is why I suggest the #35 pilot. Have a pair of #40 on hand.
I have found that using a single 34, a #3 slide works best around town. The spark plugs last longer. It would also be good to have a pair of #3 slides for trial and error but this increases the complexity and also costs at least $30 per slide.
Aside...I recently put a 36 VM on a friends 850 and it did everything very well...start, idle, and run in town with no problem. A bit snug in the frame but no real problem. The jetting is very similar to a 34.....the n-jet was a P-2.

Two needle jets one size smaller do not cost much. If you cannot get the motor to cough by lowering the needles, the needle jets are too big. Get the cough, them raise the needles one notch.
The taper on the needles compensates for the loss of vacuum as you open the throttle. If you wind the throttle on and get a gasp, you might need richer needles - 6 F.

Stopped by Sudco yesterday and picked up a pair of P4s -- which were recommended by the Victory manual for dual VM34 setups and are 3 steps down from the P8s I have now. Reassembled halfway last night and may have time to get the rest together and tested this week.

If it looks like I am still too rich, it is easy to drop down to the P2s. Too lean and step up to the P6. Picked up a collection of idles and mains as well, but will focus on the needle jet first -- easy to swap an idle and adjust the air screw once the needle jet is right.

Was told by Sudco that my 6DH2 is richer than what they normally ship (6DH3), but will hold onto that for the time being -- clip in #3 position with the new P4s. Stay "tuned"

nomadwarmachine said:

Stopped by Sudco yesterday and picked up a pair of P4s -- which were recommended by the Victory manual for dual VM34 setups and are 3 steps down from the P8s I have now. Reassembled halfway last night and may have time to get the rest together and tested this week.

If it looks like I am still too rich, it is easy to drop down to the P2s. Too lean and step up to the P6. Picked up a collection of idles and mains as well, but will focus on the needle jet first -- easy to swap an idle and adjust the air screw once the needle jet is right.

Was told by Sudco that my 6DH2 is richer than what they normally ship (6DH3), but will hold onto that for the time being -- clip in #3 position with the new P4s. Stay "tuned"

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I always thoughts the numbers and letters after the 'D" related to the finish on the needles. And the richer taper needles were 6F. The diameter of the parallel part on all the needles is always the same for Mikuni and Amal. - 3.0 mm, I think.
It is possible to get Mikuni needles with variety of different stages in the tapers. You can adjust the mixture at various positions of throttle opening. For a four-stroke motor 6D and 6F should give enough adjustment.

Picked up a few more jets and a 40 degree adjuster at Sudco this afternoon. The carb cables now run more directly into the caps rather than bending around and down. It's looking good -- but the extra length on the adjuster leaves me with too little of the inner cable for the slides to rest. I need to remove abou 1" or so of the cable housing.

Has anybody experience with shortening the housing? I will have to remove the solder end, shorten the housing, and reinstall the ferule. I am comfortable with this, only I am not sure how to reattach the solder end properly. Can I modify this, or should I just look for a couple of new cables/housing to run from the junction? Motion Pro seems to carry these.

nomadwarmachine said:

Picked up a few more jets and a 40 degree adjuster at Sudco this afternoon. The carb cables now run more directly into the caps rather than bending around and down. It's looking good -- but the extra length on the adjuster leaves me with too little of the inner cable for the slides to rest. I need to remove abou 1" or so of the cable housing.

Has anybody experience with shortening the housing? I will have to remove the solder end, shorten the housing, and reinstall the ferule. I am comfortable with this, only I am not sure how to reattach the solder end properly. Can I modify this, or should I just look for a couple of new cables/housing to run from the junction? Motion Pro seems to carry these.

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Pull the crimp off of outer cable housing
Cut the vinyl away to the length you require
Uncoil the spring and cut off with wire cutters
Re attach the crimp
Sometimes I carefully run a disc grinder along the spring the length I require and just break the loops of the spring off
Cheers

Why not just go with your original cable set up until you have your jetting close to final setting?

For the cost of new cables? order some ready made with angle fittings!

Keep your existing cables as spares!

Fast Eddie said:

Where is the idle screw on an Amal?

The two adjuster screws I’m aware of are the throttle stop screw and the pilot air adjuster screw, which is more commonly known as the air screw.

Its called the air screw cos it controls the air. So, screwing it in reduces the air and enriches the mixture. And vice versa.

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You either screw the idle adjuster in, or you screw it out. There is only one way to be wrong. On some carbs it adjusts the fuel, on others it adjusts the air. Lean it off until you get the miss, then ricjhen it slightly until you get a smooth idle.

If you are using Mikuni carbs, you probably have more hope of getting the needle jets right. The Amal sizes for petrol are 0.106 and 0,107 inch. A one thou of an inch step in needle je sizes is too big. If you are too lean at 0.106 inch and rich at 0.107 inch, you are probably miles too rich. I don't know what the size difference is between P0 and P2 Mikuni needle jets, but it is probably much smaller then the step with Amal needle jets.

When you use methanol fuel in a Commando, the difference in needle jets between too rich and lean is one thou of an inch. With petrol, it is half that, because with methanol, you use it twice as fast. The needle jet sizes for methanol at 9 to 1 comp. are 0.117 and -.116 inch. At 0.117 inch needle jets, the motor becomes slightly sluggish. When you use petrol, that effect is much more pronounced.
Most guys who own Commandos probably have them jetted too rich. The motors are probably less weather dependent that way.
In one of the magazines, somebody suggested turning up a plug gauge to measure the needle jets. I think that is nonsense. My brother has a jeweller friend who can measure them. But I think Mikuni needle jets might be flow rated and reamed- , and not drilled to a size.

acotrel said:

You either screw the idle adjuster in, or you screw it out. There is only one way to be wrong. On some carbs it adjusts the fuel, on others it adjusts the air. Lean it off until you get the miss, then ricjhen it slightly until you get a smooth idle.

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Can’t argue with you there, there’s only two ways to turn a screw!

My main point was your terminology Al. You are NOT describing use of the idle screw, you are describing use of the air screw.

And on both Mikuni VMs and Amal Concentrics, it controls the air. Screwing in reduces the air. Screwing out increases the air. You said the opposite, which will confuse people. It is actually very important to know what this screwing in or out is achieving if one wishes to understand whether or not the correct pilot jet is fitted.

As you rightly say, pilot circuit adjustment screws on some other carbs do work in reverse of this.