Tuning Dual Mikuni VM32s on a 750 Combat

Think I read Victory notes online , the slides were 2.5 and pilot was 25 , as I sit here more comes back , I had troubles getting needle jet and needle set and for now all I can recall was middle groove on both …. take a look around with google and you should find what you need from Victory ….
 
I did mention the issue I had with carb position once I was seated , not sure if that could have been cured by going back to stock seat , I liked Corbin so much , never considered switching , the performance was more right there with the dual setup , fuel use went up and I sat in uncomfortable position so while experiment kept me occupied for a spring , I was soon back to single VM34 and happy , keep my experience in mind ….
 
TomU said:
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
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My point is that the final sizes for both are probably within one or two steps of each other, up or down.

The difference is less than you think because more air flowing means more fuel is drawn. You are dealing with mixture, which is a ratio of fuel and air, not absolutes, i.e, not the measured quantity of fuel or air flowed.
 
TomU said:
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.
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I am going to say that Sudco did not fit an air jet at all, they just left what was in it as it came from Mikuni! Because 2.0 is default.

It's lazy, and a lot of riders will never really notice or if they notice, they will not resolve issues, because they will not change the air jet.

I suspect that 95% of those who never really feel their Mikuni set up is correct and subsequently abandon it, have never changed the air jet.

The used ones I have bought all had 2.0 fitted.

I came by a decent set of 34mm carbs and manifolds from a Commando with a whole box of jets this way. There was a huge range of (expensive) needle jets in the box but not a single (cheap) air jet.

If in doubt go for a 1.0

An individual Commando will only need to change up and down a little from there.

Is there a 159-P3 needle jet? I haven't found one. There is a P5, but that is in fact a half step and not repeated, sequence is P0, P2, P4, (P5), P6, P8
 
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Stephen Hill said:
Does anybody actually have twin Mikunis installed on their Commando and is happy with them?
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I really would not use any other carb on my bikes, 4 bikes, all on Mikunis, one is a 'Commando'.

Recently dumped the 32mm Amal Mk1 from my '56 AJS 500 single, and fitted a VM32: much, much easier starting, it will now tickover as low as I want to set it, no popping and banging from the exhaust, which was bad on the overun, and an extra 5mph easy cruising speed and will pull up hills in a higher gear than before....straight out of the box....

I am sure I could find more maximum speed, but that isn't what a bike on trials knobblies is really all about! So far I have only changed the pilot one size and dropped the needle in the clip one step.

I am sure that the setting could be refined further, and progressively I will try more, I can also assure you that the air jet fitted is not a 2.0! (It is a 1.0, and I have a 0.9 and 1.1 set aside for tweaking)
 
Thanks SteveA. There are many folks satisfied with the single Mikuni.
I am interested in hearing from folks who are satisfied with twin Mikunis. You might think it is just more of the same, but somehow every thread related to twin Mikunis talks about problems with jetting/running, relocating the ignition, uncomfortable riding position, no room for air filters, etc, etc. So they removed them.
I have a set in box, and would be motivated to install them, if there was some positive comments on what you end up with. The positive reviews seem slim to non-existent.
 
SteveA said:
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.
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Steve, on the Commando I assume you are running twin carbs with offset manifolds and air trumpets with no filters?
 
Stephen Hill said:
Thanks SteveA. There are many folks satisfied with the single Mikuni.
I am interested in hearing from folks who are satisfied with twin Mikunis. You might think it is just more of the same, but somehow every thread related to twin Mikunis talks about problems with jetting/running, relocating the ignition, uncomfortable riding position, no room for air filters, etc, etc. So they removed them.
I have a set in box, and would be motivated to install them, if there was some positive comments on what you end up with. The positive reviews seem slim to non-existent.
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I had used twin 34 VM on my 920 , no problem (Sudco jetting, as I was a newbie in 1997 with the miks ) , then put them (920 on standby in the shelf , and still is !) on my Atlas (std tune) ride with them , but feel a bit overcarbed, so fit a pair of used 30mm Concentric (work fine ) and still use them , but there was no problems either on the ATLAS , but the 920 was missing its carb .......... so to conclude have a try yourself and you will see on your own !
 
Much more , on my faithfull old 850 (since 1974),I had a pair of 32 mm Amal , for years (no,problem ),, then I fit a pair of new Amal and ...........no problem , then a single VM 34 mm , and still no problem , so I am happy what ever the carbs , may be I am an easy boy ...
 
MichaelB said:
Steve, on the Commando I assume you are running twin carbs with offset manifolds and air trumpets with no filters?
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The frame is a Rickman, so I have a better mechanical installation opportunity than a Commando frame. I have a long fabricated stainless manifold with straight, slanted tubes to get the required separation of the carbs. I do use trumpets, not filters. I have used both 34mm and 36mm twin carbs on this bike/manifold. It is a race bike, so I have never spent a lot of time on tickover settings, but plenty of time on off to on throttle settings.
 
VM32 Mikunis were probably designed for use on two-strokes. In Melbourne Bert Flood was the Bultaco agent. He sponsored Ginger Molloy on a 360 Bultaco road racer - a modified MX bike. When he was in Australia, the motor was always on methanol. I don't know what fuel checks are done in Europe, but when Ginger raced there, he was up with the fast guys in the 350 class. Bert Flood mentioned to my mate, who also played with methanol, that one of the jets in Mikunis was there to provide a spurt of fuel when the throttle was closed at the end of a long straight. If you think about it, that is where most seizures occur with a two-stroke. Probably when the VM is used on a four-stroke, that jet does not do much. It is not something I have ever had to play with.

 
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I only ever use recommended jetting to help me decide what the biggest main jet should be. I fit that and lean-off the rest as much as is tolerable. The main jet does not matter as long as it is slightly too rich. Acceleration up through the gears depends on gear ratios and the needles and needle jets. You don't often burn pistons or valves unless the main jets are too lean.
 
acotrel said:
VM32 Mikunis were probably designed for use on two-strokes. In Melbourne Bert Flood was the Bultaco agent. He sponsored Ginger Molloy on a 360 Bultaco road racer - a modified MX bike. When he was in Australia, the motor was always on methanol. I don't know what fuel checks are done in Europe, but when Ginger raced there, he was up with the fast guys in the 350 class. Bert Flood mentioned to my mate, who also played with methanol, that one of the jets in Mikunis was there to provide a spurt of fuel when the throttle was closed at the end of a long straight. If you think about it, that is where most seizures occur with a two-stroke. Probably when the VM is used on a four-stroke, that jet does not do much. It is not something I have ever had to play with.

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Name that jet and explain how that works please!
 
acotrel said:
VM32 Mikunis were probably designed for use on two-strokes. In Melbourne Bert Flood was the Bultaco agent. He sponsored Ginger Molloy on a 360 Bultaco road racer - a modified MX bike. When he was in Australia, the motor was always on methanol. I don't know what fuel checks are done in Europe, but when Ginger raced there, he was up with the fast guys in the 350 class. Bert Flood mentioned to my mate, who also played with methanol, that one of the jets in Mikunis was there to provide a spurt of fuel when the throttle was closed at the end of a long straight. If you think about it, that is where most seizures occur with a two-stroke. Probably when the VM is used on a four-stroke, that jet does not do much. It is not something I have ever had to play with.

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How many times do you need to be told that the VM mikunis were designed by Amal?????
 
baz said:
How many times do you need to be told that the VM mikunis were designed by Amal?????
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Whilst that sounds good to say, and the basic carb function is of course very similar, it doesn't reflect reality.

Mikuni may well have initially copied an Amal carb, or a range of them, they did remote floats for early race Yamahas for instance. But the Japanese gained a reputation for copy, improve/refine and innovate for a reason.

But the Mikuni has one more jet, (the one people generally ignore!) and the VM introduced a fuel enrichment valve rather than air 'choking' as a start mechanism. (later copied by Amal on the MkII concentric)

Idle mixture can be achieved by varying air flow or fuel flow, Amal and Mikuni chose the opposite method, so Amal clearly didn't design that.

But the key is that Mikuni supply a vastly wider range of jet sizes and needle tapers.

No one sat down at Amal and designed the VM Mikuni for them. Nor did Amal as a company achieve the same manufacturing standards or develop the range of needles and jet designs and sizes that Mikuni has.

Burlen have improved things in recent years with Premiers, but they still have hiccups with those that are well reported here.

The VM Mikuni isn't an ideal mechanical fit for a Commando twin carb installation, the world isn't perfect, so if you want decent running, good performance and the original look, buy Premiers and inspect before fitting.

For simplicity and acceptable road performance, fit a single Mikuni. If you have the space and want the best performance, fit a pair.
 
SteveA said:
Whilst that sounds good to say, and the basic carb function is of course very similar, it doesn't reflect reality.

Mikuni may well have initially copied an Amal carb, or a range of them, they did remote floats for early race Yamahas for instance. But the Japanese gained a reputation for copy, improve/refine and innovate for a reason.

But the Mikuni has one more jet, (the one people generally ignore!) and the VM introduced a fuel enrichment valve rather than air 'choking' as a start mechanism. (later copied by Amal on the MkII concentric)

Idle mixture can be achieved by varying air flow or fuel flow, Amal and Mikuni chose the opposite method, so Amal clearly didn't design that.

But the key is that Mikuni supply a vastly wider range of jet sizes and needle tapers.

No one sat down at Amal and designed the VM Mikuni for them. Nor did Amal as a company achieve the same manufacturing standards or develop the range of needles and jet designs and sizes that Mikuni has.

Burlen have improved things in recent years with Premiers, but they still have hiccups with those that are well reported here.

The VM Mikuni isn't an ideal mechanical fit for a Commando twin carb installation, the world isn't perfect, so if you want decent running, good performance and the original look, buy Premiers and inspect before fitting.

For simplicity and acceptable road performance, fit a single Mikuni. If you have the space and want the best performance, fit a pair.
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As I understand it
(And I'm probably wrong)
Amal hawked the design to BSA , Norton, triumph etc as they were at that time and had no takers
Because Amal wanted payment for development?
So they sold the design to mikuni ?
I have had a single 32mm CV mik on my bike
Also a single 34mm VM
And a single 36mm VM
And a single 32 MK1 concentric
But now I have the best setup a single 40 mm mikuni flatslide TM pumper this is light years away from a VM IMO
 
Craig said:
Think I read Victory notes online , the slides were 2.5 and pilot was 25 , as I sit here more comes back , I had troubles getting needle jet and needle set and for now all I can recall was middle groove on both …. take a look around with google and you should find what you need from Victory ….
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I think Victory Library has gone belly up and i can't find their manual on-line. Does anyone happen to have a copy?
 
Sorry for that , should have checked before suggesting , my start point was setting for single VM34 , once I got Sudco provided jetting out , that where I began , then found Victory , still if you have the jets that have been suggested here , you should be able to figure this out yourself , nothing ever cast in stone with carbs on old bikes …. dig in , painful , but you starting close to finish line …
 
Craig said:
Sorry for that , should have checked before suggesting , my start point was setting for single VM34 , once I got Sudco provided jetting out , that where I began , then found Victory , still if you have the jets that have been suggested here , you should be able to figure this out yourself , nothing ever cast in stone with carbs on old bikes …. dig in , painful , but you starting close to finish line …
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"Nothing ever cast in stone with carbs on old bikes" is so true
It amazes me that a small change can make a huge difference
And sometimes a big change makes no difference
Some of the frustration can be taken out with a gas analyser
If you are prepared to weld a bung in your downpipes near the head
Mark your twist grip so you can see what position your throttle is in
Make sure the engine is fully warmed up
Then when you ride you can see from the guage if your cutaways are too big/small , needle jet size,needle position,main jet etc
I did this when I still had twin amals that were brand new but unfortunately they just wore out
 
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