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Mikuni carb size


Well Known and Active Forum Website User
VOC Member
Hello all, need someone to help me decide what size Mikuni to fit to my Rapide and sidecar. 28mm are available and with the smaller body size are favourite. 30mm are cheaper but in short supply. Any thoughts on flat slide? John.

Comet Rider

Well Known and Active Forum Website User
VOC Member
Hi John,
The flatslides are not that suitable for road appliactions such as ours, the VM series of round slides are much better.

I know of a number of people who have used the 28mm very successfully, and there is a lot of knowledge out there. Try contacting Brian Chapman who should be able to help.
Also contact Steve Doran of Motocarb (www.motocarb.co.uk) as he isvery helpfull in getting Mikuni bits, and is also one of the cheaper suppliers

Best of luck

Puddle jumper

Well Known and Active Forum Website User
Non-VOC Member
Hi John,
I'm running VM32 on my twin, they seem good, but I'm still playing with the jetting and running the engine in, so can't give you much feed back, I got the carbs from Allen’s performance, who have been very helpful with jetting, at the moment the bike seems a little rich but pulls well and starts very well.


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VOC Member
I saved this from somewhere, but don't know who wrote it:
I have 30mm round slide Mikuni Vm's on my Rapide.("PIP", CRap/D
After some fiddling, they work a treat, set as follows;

Vm30ss As Supplied Now

Main Jet #220 Now #195

Pilot Jet #30 Now #35 (mixture screw 1+1/4 turn out.)

Needle Jet 159P-8 Same

J/Needle 6DP17 (centered) Same

Slide #2.5 Same

Air jet BS30/97 2.0 Same

(2) #002-351, Cable type starter system, conversion kits.

(2) K+N Air Filters

"PIP" is capable of idling cleanly and continuously at 400 rpm, if so
desired. (That just drives the Harley guys nuts)

Motor pulls cleanly from idle to 100+?
(Who believes our Smiths anyway!)

I think 32mm may be a little big for a stock Rapide. I think I'm
pushing it a little with 30's. Your pilot jetting seem very rich
to me. Are you finding if difficult to get your idle mixture lean

Sudco Mikuni has a very complete tuning manual available, well worth
the price.

It might have been on jtan. I also searched here and found Robert Watson's flatslide 34mm settings, but that's all after a superficial look. You may find more with a closer look.
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Well Known and Active Forum Website User
VOC Member
It seems that 32 mm is bigger than anything a Vincent was ever designed for and 30 mm is just under 1 1/4"! For street use a 28 mm would be the way to go as you would still get the torque the engine was designed to deliver in buckets. Not sure of the logic or maths here but 28 mm Mikunis are good for my Aprilia 250 2 stroke revving at 12,000 RPM. That equates to intake of a 500 cc 2 stroke twin at 6,000 RPM or a 1,000 CC 4 stroke twin at 6,000 RPM?! Pretty ordinary logic I agree & fairly poor physics too BUT the intake manifold and inlet ports are still the limiting factor. 28 mm is bigger than 1 1/16 but smaller than 1 1/8 so what sort of bike do you want?


Well Known and Active Forum Website User
VOC Member
Hello, Many thanks to all,the mind is made up and I have ordered the 28mm round slide.
Just ran the bike only to find the the stainless silencer has split round the mounting
bracket. It came from Australia about 20 years ago, when I looked at it I thought that the
bracket will fail. Happily it took all this time for me to be proved correct.
It is stainless with chrome plate on top so it does not go to straw colour and has
been excellent. Have brazed it up and I am hoping for another 20 years use.
Regards John.


Well Known and Active Forum Website User
VOC Member
VOC Forum Website Moderator
Mikuni carb size (28mm jetting)

Dear John,
When I first built my Rapide I persevered for several thousand miles with a pair of second-hand 1-1/8 Monobloc carbs that stuck open over half throttle. I quite like the Monobloc as a carburettor but after much unsuccessful fettling I decided that enough was enough. I considered the options, new original type 276, reconditioned or new monobloc or new Mikuni. I built the bike to ride so originality is not a prime consideration. (Function over form.) Mikuni's are definitely not original but are nicely made with bodies of a good alloy and chrome plated brass slides so they will last. a pair of VM 28mm Mikuni's also cost less than the other options. Motocarb supplied the carburettors with the following settings:
* Main Jet 190
Needle Jet P-6 (182)
Needle 5FL 14 (I put it in the middle notch.)
Slide 2.5
Pilot Jet 30
Air Jet 2.0

The Mikuni's are stub fitting so I made up some adaptors and used a suitable size radiator hose and Jubilee clips to secure them. You will also probably have to make up some new throttle cables. I concocted something that worked using a mix of original cables and some
pieces of stainless bi-cycle cable from Halfords. There are no chokes as such but there is a start lever that in effect uses a seperate set of passages to supply a rich starting mixture. Engage the start levers, keep the throttle shut and it usually starts first or second kick.
The engine immediately had a regular tickover, something I had never quite achieved with the worn Monobloc's. (New monobloc's probably would have achieved the same.)
I have experimented with a few settings. I put in 160 main jets, leaned off the pilot screws, lowered the needle and eventually put the needle in the lowest notch. It was perhaps not quite as "drivable" but still OK. On a fairly steady two up run from Hinckley to Malvern I got almost 65 mpg. Note that this is a corrected figure as the odometer is a little generous with the miles when compared to the Garmin GPS I use. It is generally doing 50 - 60 mpg depending on the type of riding I'm doing. The engine has MK1 cams and 7.3:1 pistons. I use the cheapest regular fuel that I can find locally, usually at Morrison's just around the corner.
The magneto is one of Tony's new BT-H items, sparking twin plugs with the advance set to around 30 - 32 degrees BTDC. The silencer is one from the spares company and sounds fairly free flowing. I have two spare slides that I may make into 3's to see how they work.
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Puddle jumper

Well Known and Active Forum Website User
Non-VOC Member
The Inlet tracks in my heads have been ported out to 33mm; this was done some time in the 50's, long before I acquired the engine, so it seemed best to take advantage of this,
Only time will tell.
I think Steve Hemal fits 34mm carbs to most the engines he builds, but don't quote me on that!!!!
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