dual mikuni vm32 750 combat (2008)

Craig

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this is my first question...doubtful if it is my last
I switched from single vm34 mikuni to dual vm32's
the new pair came pre jetted, engine starts first kick then idles erratically
until it finally quits I'm guessing because of too rich a mixture
can someone suggest a start point for me re-jetting
1972 combat ,boyer, unifilter pods,standard exhaust, NGK BPR7ES, good compression,good coils,good wires, and timed to 31 degrees with hotter cam but unknown grind
I suspect I will need to change all jets and slide... maybe I should revert back to single HELP PLEASE
Craig
 
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carbs

hi craig did it run ok with the single carb if so i would compare your settings of the single carb to the ones from your twin carbs and if they are different i would be tempted to set the twin carbs the same as the single one just as a starting point
 

illf8ed

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poor idle with Mikunis

Craig,

If you're working only on the idle, this should be adjusted with an idle mixture screw. I don't have memory of how many turns out for a starting point with Mikunis, but the Amal concentric is 1.5 turns out from fully closed. There may be idle jets, but if pre-jetted for your machine these are probably correct. Play around with the mixture screws.

I'm using sleeved Amals on my '72 combat and they have been working great for 10 years.

Good Luck

David
 

Craig

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dual mikunis on 72 combat

Thanks to both responders,you gave me a couple of useful tips now if it will just stop snowing.
The reason for the switch from single 34vm to dual 32's... is the port side ran perfectly but the right side was always a bit rich.I really have no experience with this type of thing and any more ideas will be greatly appreciated.
This bike was purchased as a running basket case late July 04 and I stripped it down to frame and cleaned and replaced as I put it back together.I'm sure to need much more advice thank you for any help in advance

Craig
 
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I ran a single 34mm Mikuni and then switched to approximately the same setup you have now on my '72 combat. It can be done. It was a long time ago and I remember changing needle jets I think ... but it was not too bad a battle. I would try contacting Sudco for one but there are other very good shops now with a lot of experience with Mikuni conversions.

I'm back to the stock Amals now and they are fine as long as you keep feeding them needles and needle jets.
 
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Sleeved Amals

David How did you get your Amals sleeved, you or shop? I may do it on my lathe
 
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Bruce Chessel at Triton Machining in Canada does them. Did mine and they work great.
 
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hi again craig some bsa a10s had that problem with one cylinder running richer than the other they solved it by fitting a biased carb to manifold gasket which effectively pointed the carb at the weaker cylinder ive never heard of one to fit a norton or how much bias was used, but it was,nt much
 
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dual mikunis on 72 combat

Thanks to both responders,you gave me a couple of useful tips now if it will just stop snowing.
The reason for the switch from single 34vm to dual 32's... is the port side ran perfectly but the right side was always a bit rich.I really have no experience with this type of thing and any more ideas will be greatly appreciated.
This bike was purchased as a running basket case late July 04 and I stripped it down to frame and cleaned and replaced as I put it back together.I'm sure to need much more advice thank you for any help in advance

Craig
Mikuni 34 vm has the choke /enrichener dump into the right side of the throat so.... it will generaly show more richness .I have seen the enrichener plunger worn on the tip or the opening link lever ass out of adj. and wont shutoff entirely. Thinking about doing DUAL my own self. Cheers
 

Craig

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Wow that was a while ago , been back to single 34VM since a year after that post , rebuilt the 34 put a new Boyer mk4 on and better than ever , so last 10yrs or more
 
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carbs

hi craig did it run ok with the single carb if so i would compare your settings of the single carb to the ones from your twin carbs and if they are different i would be tempted to set the twin carbs the same as the single one just as a starting point
If the bike performed well with a single Mikuni, there is very little difference in jetting between dual cards and the single carb. Probably only the position of the needle - up or down one notch.
When I had Triumph 650s on the road, I had both single and dual carbs, and could never tell the difference in performance. There probably is one if you ride the bike on full throttle with the engine revving above 5000 RPM.
 

Craig

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Back in the day I did mirror the duals jets to the single , they worked great but due to mount position I was having issues with knee room , plus used more fuel the single seems better in low and mid range too .... plus no balancing , haven’t touched it since I put on all those yrs ago
 

Craig

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Still got most of my setup Tory , someday maybe I do it again or someone else will want to try them ... not sure that I have seen a white Commando before , not bad at all !
 

Fast Eddie

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... not sure that I have seen a white Commando before , not bad at all !
There were loads of ‘em... tended to be ridden by policemen ...!

That one in the clip does look kinda nice, albeit not my cup of tea. And it sounds bloody lovely !
 

RoadScholar

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Hard to make an informed suggestion about the dual VMs without knowing what the jetting package consists of. If, for example, your carbs came out of a factory box they would be very rich off idle, if they have been around-the-block for some time they may have succumbed to the effects of fuel with alcohol content and, good chance, improper calibration. Most well intentioned newbie carb tuners god straight to the main jet to cure perceived rich/lean conditions where needle/needle height and needle jets are the culprits.

No one can advise you properly without knowing what the jetting package(s) are, good idea to strip both. Need to know:

Idle jet #
Main jet #
Needle jet #
Needle #
Slide cutaway #

While you are stripping the carbs look for signs of corrosion, slime, detritus, archaeology...If these are new carbs then just the nomenclature on the calibrated parts.

Best.
 

Craig

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Back when I ran the twin 32 Mik’s , I had bought them brand new through Sudco ,supposedly pre-jetted for my bike , so I fitted as received and they were not great , so took advice from this thread and setup both exactly as my VM34 single was setup .... after some testing I had them bang on for my bike ..... my biggest issue was their placement on bike , cramped my leg shin space and I was always preoccupied with them protruding into my space when on the move .... still think on putting them back on for a season , just for “shits and giggles“ though .....
 

Tornado

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Why dont we hear of dual mik setup more often? Seems one of the downsides to fitting the single mik is some performance loss. So the duals should eliminate that plus give the greater reliability of the Japanese build.
 
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