Mikuni Adapter on a Cool /2 Custom.

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Mikuni Adapter on a Cool /2 Custom.

Post by ferrarirobinson »


Nice Custom !
I’d like to ride it. Sounds Excellent !

I am mostly interested in the Mikuni Adaptor(s). (8 minutes in)
Can anyone identify them and a source ?


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Post by ahistand »

Just check EBay type in Mikuni carb adaptor there are tons of different sizes available.

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It doesn’t mean that there

Post by ferrarirobinson »

It doesn’t mean that there are not more on eBay and elsewhere, but I had only found one that matched mounting stud spacing and other dimensions for the VM 26 8074 carbs. It was slotted and you replaced the stud mounts and the throat was 28mm which was 4mm larger than head intake. It was from OZ (Australia) and was $69 plus each. It would work and probably what was used in the video.

Just looking for options.

I have designed a manifold that will address those issues as well as match the Bings built in angle exiting the head and will price to see if not ridiculously priced over $69 each.

All this has been educational and will even be fun if the goal is achieved; easier Cold Starting. This thing about dumping Fuel on the Ground is nostalgic but way too frustrating when you should be riding. I rode /5’ ticklers for 20 yrs and never had starting issues as severe. I’m sure there are those out there who can start their tickled vintage bike with two fingers at 0° Celsius. I can’t. If the Germans had tickled bikes during the War, i can’t imagine starting your bike on a cold Russian Winters morning.


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Couldn’t find proper stud

Post by ferrarirobinson »

Couldn’t find proper stud spacing from the multitude of Mikuni Adaptors. Closest fit was from Oz (Australia) and required pulling carb head mount studs and had a 28mm throat at the mounting end. The head opening on my R60 is 26mm. Maybe a step is not too bad but smooth is smooth, so my design is 26mm. It also is bolt on and take off using original mounting studs.

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Mismatched bores

Post by Captonzap »

When the BSA engineers were developing the BSA Gold Star engine, they put a one and one half inch GP carb on a head that had a one and three sixteenths inch inlet bore.
Some one noticed, and they opened up the head to match the carb, and the measured horse power went down.
I don't know if the same outcome would apply to a BMW, since it is not as highly tuned as a GS, but it is something to think about.
The turbulence created by the lip might cause an increase of fuel atomization, causing a more efficient burn. ???

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Are the stock carbs/heads

Post by schrader7032 »

Are the stock carbs/heads "mismatched" as you discuss? I don't remember.
Kurt in S.A.
'78 R100/7 '69 R69S '52 R25/2

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Mismatched bores +1

Post by jwonder »

Mismatched bores are actually a good thing and we use them on our race car engines that have carburetors all the time. The physics is as follows:

1) Picture the column of air that is traveling down the carb throat when the intake valve opens, it is moving at quite a fast rate and is laden with fuel.

2) When the intake valve closes the column of air slams into the closed valve. "An object in motion tends to stay in motion"

3) The air now wants to "revert" back up the intake track. This is what is commonly called "reversion".

4) The amount of reversion, time, length, etc is quite dynamic and changes with throttle position, RPM, barometric pressure, etc etc.

We have found after years of working on racing engines that we can put this to work for us! A step in the intake manifold works in two ways:

1) A smaller manifold creates a lower pressure area under the step (in the intake tract) and the lower pressure resists going "back up" into the higher pressure area. We have also found that a larger manifold created positive turbulence and has the same type of anti-reversion benefit, but not as much.

2) It creates turbulence in the incoming charge and this is REALLY good. Yes, I said turbulence IS GOOD for intake tracts!

Years ago many engine builders were flowing their heads with complicated flow benches and new ways to make intake tracts as smooth as possible in order to (so they thought) get a better flow rate. We have one in the back of the shop collecting dust right now! This thinking is wrong. A bit of roughness on the walls of the intake tract breaks the boundary layer of the air. I.e. If you look at any aerodynamics book worth its salt you will see that when a flow is right->left (for explanation purposes) there is a VERY THIN boundary layer actually going the OTHER way (left->right in this case) on the surface of the object (in this case our intake tract). This reverse boundary layer is actually what creates disruptive turbulence and having a non-smooth surface helps break the boundary layer and increases flow rate.

I will not go further into the physics behind it and the "amount of roughness", and the "amount of turbulence", because it depends on a myriad of factors.

In other words, if you have a step where the manifold is SMALLER than the intake tract that is the best scenario. We have also seen that using a manifold that is larger than the intake holds benefits but not as much as the smaller one.

In other words, don't worry about it! :-)

P.S. your adapters came out really nice!!!

James Wonder
Long Island, New York

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Re: Mikuni Adapter on a Cool /2 Custom.

Post by Riderg »

Charlie, are you selling your adapters by any chance?



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Re: Mikuni Adapter on a Cool /2 Custom.

Post by jimstravlin »

Sudko in CA sells the Amal to Mikuni adapters. you can also get them thru Rocky Point cycle...

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Re: Mikuni Adapter on a Cool /2 Custom.

Post by ferrarirobinson »

Sorry for delayed response Riderg. I’m still in the prototype stage and still have to go back to the drawing board to satisfy my needs. I’ve installed the Mikunis and are in the process of tuning the carbs. You and everyone are welcome to use any information that I may stumbled upon.

Many thanks to Captonzap for your/informative post.

Bottom line is that I still need to get the new carbs out of the way of the foot controls. As close to the Elegant design of BMW and Bing as I can come.

Here is what my thoughts were/are and forgot to post earlier. Keep in mind that I am a shade tree mechanic and spend a fair bit of time waiting for an apple to fall on my head. (All while addicted to the feet up time on two wheels):

Be glad to PM the drawings but will post here with update to where I am now.
I bought the VM 26 8074 Mikunis from Rocky Point. Bill has been great to deal with.

Mounted the manifolds onto the stock bolts and gaskets. The area of the manifold from the mounting studs rearward should have been shorter in length. Maybe 12mm-16mm. The length with the carbs mounted was way past the stock Intake piping to the stock air filter chamber so I used a couple of K&N RC-1070 Air Filters. The problem now is you can’t get your feet on the pegs and use the foot controls without repositioning your feet. Obviously I should have beveled the manifolds head connection end but didn’t know how much due to (poor R&D skills ?) and not believing that the (Elegant)stock Intakes for the Bings was @ 10 plus or minus degrees.

I made 18 degree Adaptors from PVC pipe and fit them between the manifold and carburetor and found angled radiator hoses that supported that angle to get the carbs (filters) inboard as much as possible.
(This may help with the turbulence that Captonzap mentioned)? The End

Here is the drawing of the Adaptors and a few extra photos of my progress.You may copy or do anything you like with the design drawings. They are for anyone and everyone.

PM me with any questions you have, I’ll be glad to share my opinion of what I have learned so far.

And as for Accelerator pumps. These carbs don’t have them but the no throttle necessary during choke may be very nice.

There is still a note that the carbs may leak under certain conditions. OH GREAT ! Something to remind me of leaky Bings and hard starting.


p.s. Sorry for photo composition as I’m still learning the new forum and adding attachments. I do like the new forum. Thanks to the person and persons who did this for us. (The Members)
Will post another photo of angled set-up to date that still needs above post modifications.

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