Tuning Dual Mikuni VM32s on a 750 Combat

Couldn't find a thread specifically on tuning twin Mikuni VM32s on a 750 Combat so posting this. I bought the Mikuni carb kit from Sudco right after i acquired my Norton in '94. Problem is i essentially parked it and never rode it since then so i have no idea if the carbs were ever set up properly. I'm now in the process of restoring the bike and about to rebuild the carbs. Before i slap them back on, i'd like to get them set up as correctly as i can. So question on tuning.

They came set up from Sudco as follows.

Main Jet - 250
Pilot Jet - 35
Needle Jet - P0
Needle - 6DH3
Needle Clip Position - P4 (2nd position from the bottom)
Air Jet - 2.0
Slide - 2.5
Float Bowl Needle & Seat - 3.3

This seems a little on the rich side.

Niche shows the following specs for a 750 (they don't say Combat though, or twin carbs for that matter)

Main Jet - 220
Pilot Jet - 25
Needle Jet - P6
Needle - 6DH3 (same)
Needle Clip Position - don't say
Air Jet - 1.0
Slide - 2.5 (same)
Float Bowl Needle & Seat - 3.3 (don't say)


Is anyone running dual VM32s on a Combat that has dialed in these carbs that can weigh in on the specs? (and not looking for any "switch to Amal" comments, thank you)

FWIW - There is a listing in the Norton Tech Digest for twin VM34's:

Main Jet - 220
Pilot (Idle) Jet - 20
Needle Jet - P0
Needle - 6DH3
Needle Clip Position - Middle (to start)
Air Jet - 2.0
Slide - 2.5

Thinking out loud - standard Combat Amal jetting was a 230 main jet, so may want to bump up to that with the Mikuni's since the units of measurement are the same - I actually have 240's in mine, seems right for my bike.

Not sure if this helps or not - you get what you pay for - lol

Derek Wilson said:

FWIW - There is a listing in the Norton Tech Digest for twin VM34's:

Main Jet - 220
Pilot (Idle) Jet - 20
Needle Jet - P0
Needle - 6DH3
Needle Clip Position - Middle (to start)
Air Jet - 2.0
Slide - 2.5

Thinking out loud - standard Combat Amal jetting was a 230 main jet, so may want to bump up to that with the Mikuni's since the units of measurement are the same - I actually have 240's in mine, seems right for my bike.

Not sure if this helps or not - you get what you pay for - lol

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According to AMAL, the Combat used 220 as did the 71/72 750s. In 73, all 750s used 230, again according to AMAL. If you buy a Premier set today for Combat, they come with 220. Counterintuitive! The Combat and 73 750s were 32mm carbs; the 71 and other 72 were 30mm. The parts book doesn't state a size but does agree with AMAL on the carb number and sub-number. The workshop says 220 with mute and 230 without.

I often recommend 230 for Combat, especially with E5 or E10 gas but I can certainly understand 240s as well.

Without knowing the air cleaner and exhaust, I would recommend leaving the 250s in and see if it's too rich.

For my 920 /4S/9.5:1 (Pete Lovel) head tuning ,in 1997 Sudco supplied to me twin VM34 : 250/159P0/6DH3/35/2.5.
From a well tuned 750 (belongs to a racer grand son of the Norton french importer) a pair of VM 34 : 220/159P5/6DH3/35/2.5 , I had bought only some parts of his bike at the end of the nineties ( frame , engine in bits etc...)

I tried the 2 VM32’s briefly , went back to single VM34 , first of all it was limited shin room with the carbs right where I wanted my legs …. the kit was jetted way rich , took the 34 jets out , bought a bag of doubles for one size under one over and tried that , never really got them dialled in close but still off , plus was burning more fuel …. Put the 34 back on and rode for 20yrs with no issues ….

Craig said:

Put the 34 back on and rode for 20yrs with no issues

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I agree - my low compression 750 with a 3S cam works great with a single 34 - I am pretty sure it will out run my Combat.

I'd venture that he jetting that's installed is pretty close. I'd start with the needle clip in position 3 (middle groove). Given the nature of the Combat cam a set of #40 idle jets would be good to have on hand unless you can achieve a smooth transition from the idle circuit to the needle. Think vacuum signal.

When the cam starts to control the action you engine will need more A/F. Mikunis and Amals although fundamentally the same are functionally very different.

Best.

The 6D needles you have are probably the slowest taper (lean) - which is good. Start with the clips in the centre grooves and run the motor. If it runs ragged, you should raise the needles one notch, but it probably won't. If you lower the needles, you should be able to make the motor cough as you wind the throttle on. When it does that, raise the needles one notch. If you cannot get that to happen, the needle jets are too big. Main jets are not your problem, because you only use them when on full throttle. If 230 mains are used, I would use them. If you ride flat out the lean needles will get you there quick, and when you are on full throttle, you motor gets a reprieve. Usually the only way you burn pistons or valves is by riding using full throttle for longer distances when your main jet is too lean. A larger main jet should not increase your fuel consumption, if you don't ride like an idiot. You will probably find with lean needle jetting, you use less throttle.

RoadScholar said:

I'd venture that he jetting that's installed is pretty close. I'd start with the needle clip in position 3 (middle groove). Given the nature of the Combat cam a set of #40 idle jets would be good to have on hand unless you can achieve a smooth transition from the idle circuit to the needle. Think vacuum signal.

When the cam starts to control the action you engine will need more A/F. Mikunis and Amals although fundamentally the same are functionally very different.

Best.

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When the jetting is correct, the cam does not affect it. All the cam does when it comes on song, is take a bigger gulp of mixture. It does not change the fuel's combustion properties. The cam is usually chosen to suit the inlet port diameter and the rev limit of the motor. You can get the motor to make it's power by pulling harder at low revs, or revving higher without increasing pulling power. In either case, how you jet the motor remains the same.
In my opinion twin Mikunis would be better than Amals on a Commando, simply because the needle jets and needles in a Mikuni are less agricultural - you can do more with them. Have a look at the Mikuni needle jets and needles chart. The Japanese change the taper on the needles to suit the way the bike is intended to be used. In the 1970s, nearly every new model of Japanese motorcycle had changes to the needle tapers. It was probably the reason that Japanese bikes performed better than British bikes of the same type.. The needles compensate for loss of vacuum as you wind the throttle on - less vacuum, you need more fuel - bigger port, faster taper. If your tapers is too slow, or the port is too big, you cannot whack the throttle open.

My brother races Kawasaki triple two strokes in speedway sidecars, running methanol. I have almost come to blows with him over this stuff. When he races, he always wins.

I believe the VM34 specs are for an 850. I'm interested in data on a 750 Combat. If there's none, i'll take a stab in the dark and update this thread (if i ever get my bike finished that is )

Interesting in this thread (again, dual 34s on an 850), there are a couple recommendations for a richer needle jet (P4/5) and a leaner main jet (200/220).

TomU said:

I believe the VM34 specs are for an 850. I'm interested in data on a 750 Combat. If there's none, i'll take a stab in the dark and update this thread (if i ever get my bike finished that is )

Interesting in this thread (again, dual 34s on an 850), there are a couple recommendations for a richer needle jet (P4/5) and a leaner main jet (200/220).

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...

www.accessnorton.com

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Wherever you start, you will probably end up in a different place. The main thing is to be able to recognise the symptoms of too rich and too lean, and keep buying jets as you need them. I am in a better position. I rum methanol fuel, with 34mm Mk2 Amals, so I can make my own needle jets. And I know what the wrong taper needle does. The rest is easy. My main jets are a bit big, but that does not adversley affect performance much. Mostly when you are racing, you are accelerating and changing up through the gears - with lean needles you wind the throttle on slower. So you are usually only on full throttle towards the ends of the straights, and with a Commando you often get done there anyway. Most guys back-off sooner for corners. It is not something I worry about.

Theoretically, if your main jets are delivering the perfect mixture at full throttle, you should be able to whack your throttle wide open and accelerate the fastest. However it does nor work like that. You will usually go faster if you feed the throttle on slower, using leaner (slower taper) needles.

TomU said:

Couldn't find a thread specifically on tuning twin Mikuni VM32s on a 750 Combat so posting this. I bought the Mikuni carb kit from Sudco right after i acquired my Norton in '94. Problem is i essentially parked it and never rode it since then so i have no idea if the carbs were ever set up properly. I'm now in the process of restoring the bike and about to rebuild the carbs. Before i slap them back on, i'd like to get them set up as correctly as i can. So question on tuning.

They came set up from Sudco as follows.

Main Jet - 250
Pilot Jet - 35
Needle Jet - P0
Needle - 6DH3
Needle Clip Position - P4 (2nd position from the bottom)
Air Jet - 2.0
Slide - 2.5
Float Bowl Needle & Seat - 3.3

This seems a little on the rich side.

Niche shows the following specs for a 750 (they don't say Combat though, or twin carbs for that matter)

Main Jet - 220
Pilot Jet - 25
Needle Jet - P6
Needle - 6DH3 (same)
Needle Clip Position - don't say
Air Jet - 1.0
Slide - 2.5 (same)
Float Bowl Needle & Seat - 3.3 (don't say)


Is anyone running dual VM32s on a Combat that has dialed in these carbs that can weigh in on the specs? (and not looking for any "switch to Amal" comments, thank you)

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I run twin 34s and 36s, but regardless the Niche set up looks much closer than the Sudco one, though I would suggest P4 in place of P6, or at least having a pair to try.

Not changing the air jet from the standard fit 2.0 is both lazy and wrong and affects the main jet choice. The 2.0 risks high rpm lean out, and I suspect those who leave the 2.0 as it is rarely use high rpm!! If you did, you would know there is something wrong!

I would recommend the Victory VM Tuning manual as essential reading, noting that their recommendations normally end up on the rich side, I think intentionally, for you to drop down from when you have a base and running set up.

acotrel said:

Wherever you start, you will probably end up in a different place. The main thing is to be able to recognise the symptoms of too rich and too lean, and keep buying jets as you need them.

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Yeah i get all that and that's what i plan to do, just want the starting place to be as close to the finishing place as possible (i.e. lessen the number of times i have to tear down the carbs to change the internals)

SteveA said:

I run twin 34s and 36s, but regardless the Niche set up looks much closer than the Sudco one, though I would suggest P4 in place of P6, or at least having a pair to try.

Not changing the air jet from the standard fit 2.0 is both lazy and wrong and affects the main jet choice. The 2.0 risks high rpm lean out, and I suspect those who leave the 2.0 as it is rarely use high rpm!! If you did, you would know there is something wrong!

I would recommend the Victory VM Tuning manual as essential reading, noting that their recommendations normally end up on the rich side, I think intentionally, for you to drop down from when you have a base and running set up.

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Are those on street bikes and are they Combats? Regarding the air jet, there's really not much out there on that (at least that i can find, maybe in the Victory manual?). When it comes into play and it's effects through the entire throttle range. Is it just the idle/pilot circuit or does it effect the entire range? You say a 2.0 risks high rpm lean out, so it must effect the entire range. If that's correct, i don't understand the high rpm effects since the slide has a much larger effect on airflow than the air jet (at high rpm ranges)

I get that for sure , no fun after a while , but no real short cuts , as my ‘72 is in another home now can’t check as buyer asked and got all my notes … I think I remember using 1.0 air jet though …

TomU said:

Are those on street bikes and are they Combats? Regarding the air jet, there's really not much out there on that (at least that i can find, maybe in the Victory manual?). When it comes into play and it's effects through the entire throttle range. Is it just the idle/pilot circuit or does it effect the entire range? You say a 2.0 risks high rpm lean out, so it must effect the entire range. If that's correct, i don't understand the high rpm effects since the slide has a much larger effect on airflow than the air jet (at high rpm ranges)

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Race bikes, and not Combats, 750 Short Stroke!

But it won't make any difference the same jets do the same things, it may just alter the sizes or needle position a little, but really, surprisingly little! And for the air jet, you are going to narrow to around 0.7 to 1.1.

The Air Corrector Jet, to give it its full name, feeds air to the emulsion tube area....around the needle jet.....the bigger the size, the more air it allows in, hence too large a size causing lean out, but again, this is the one jet that is more rpm related than throttle position, which is what you need to track for every other jet-setting. This is why some people never change it, it seems to work up to WOT, but until the rpms get up high too, you may not notice how weak it is! Hold it there long enough and you surely will!

The size tends to affect and interact more with the main jet than anything else, but by default it has to have some effect around the needle jet, but you would have a job setting it for that unless you are someone like Jim Comstock with a dyno to hand, you are playing with 4 variables at once, needle jet size, needle taper, needle position and air jet.

If you have the Victory manual read it again. It says all you need to know about the air jet.

And read the jetting recommendations for British 4 strokes, single and twin carbs and 750/850, 32/34/36mm bore, you will not find one that uses the factory fit 2.0

The 2.0 is only good for 2 strokes, in fact in some of those, people leave the air jet out altogether! My GT500 Suzuki based race bike has 2.0s.

SteveA said:

Race bikes, and not Combats, 750 Short Stroke!

But it won't make any difference the same jets do the same things

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True, but valves (size and timing) make a difference on airflow. You probably don't want to jet a 750 standard the same as a 750 combat. Same thing goes with an 850 or any other bike

My ‘72 Combat 750 with excellent compression on both sides worked excellent with the 1.0 air jet , once I got there the other settings were straight forward , my notes are with the bike and new owner …. I used the Victory settings as a starting point …. sorry I can’t provide the exact info you need for your bike but mine was 1.0 air jet and 220 main jet , the rest will be dependent on trial and error …. good luck !

SteveA said:

The Air Corrector Jet, to give it its full name, feeds air to the emulsion tube area....around the needle jet.....the bigger the size, the more air it allows in, hence too large a size causing lean out, but again, this is the one jet that is more rpm related than throttle position, which is what you need to track for every other jet-setting. This is why some people never change it, it seems to work up to WOT, but until the rpms get up high too, you may not notice how weak it is! Hold it there long enough and you surely will!

The size tends to affect and interact more with the main jet than anything else, but by default it has to have some effect around the needle jet, but you would have a job setting it for that unless you are someone like Jim Comstock with a dyno to hand, you are playing with 4 variables at once, needle jet size, needle taper, needle position and air jet.

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Interesting. Sudco combined the 2.0 air jet with a 250 main jet and a P0 needle jet. So that's lean on air but rich on gas at full throttle (main jet and air corrector jet) and lean on gas and lean on air at part throttle (the needle jet and air corrector jet). Think i might compromise with a 230 main jet, 1.5 air jet, and a P3 needle jet

I have a wideband. I believe there are temporary mounts for motorcycles.