Mikuni Adapter on a Cool /2 Custom.

Post Reply
Posts: 18
Joined: Thu Mar 08, 2018 8:43 am

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 ?


Posts: 22
Joined: Mon Oct 16, 2006 10:04 pm


Post by ahistand »

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

Posts: 18
Joined: Thu Mar 08, 2018 8:43 am

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.


Posts: 18
Joined: Thu Mar 08, 2018 8:43 am

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.

ac4a7e23-e103-4487-8668-1dc167a8afe0.jpeg (67.12 KiB) Viewed 215 times
75a9edf9-c193-4214-8af0-a51cd020d07e.jpeg (109.07 KiB) Viewed 215 times

Posts: 85
Joined: Thu Aug 21, 2014 10:24 pm

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. ???

User avatar
Posts: 7125
Joined: Fri Oct 27, 2006 3:00 am

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

User avatar
Posts: 192
Joined: Wed Nov 23, 2016 7:50 pm

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
1967 R69S, 1966 R50/2, 1965 R27, 1963 R27, 1952 R51/3 and 2018 R1200GS

Posts: 2
Joined: Tue Jun 30, 2020 2:21 pm

Re: Mikuni Adapter on a Cool /2 Custom.

Post by Riderg »

Charlie, are you selling your adapters by any chance?



Posts: 93
Joined: Fri Oct 27, 2006 3:00 am

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...

Post Reply