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Mikuni Carburetors

72r60/7

13278
I’ve been investigating the use of Mikuni Carbs on ‘55 - ‘69 Models (more specifically for a ‘57 R60).
Rocky Point Website list Carbs for 600 cc bikes, the /5, /6 and Slash 7. Note says “Not for /2”. Don’t know what that implies, but will find out.

What I am after here, is information from folks that actually bought and tuned Mikunis on Pre ‘70 bikes. From fitment to starting to running.

First hand experience if you care to share.

Thanks in advance.

Charlie
 
Charlie -

From what I can tell on the Yahoo /2 site, the issue is the spigot that goes between the carb and the head. On the /2, the two bolts are in a vertical orientation while things are horizontal on the /5-on bikes. Possibly people make adapters...I saw the company Sudco mentioned...maybe you can research that. Someone on the Yahoo site said they were running Mikunis on an R51 race bike, so guess it can be done.

Use the search box on the Vintage site...I typed in Mikuni and found quite a few pages of hits. Looks like there's been some past discussion on this.
 
Thanks Kurt, Thanks for the leads.

Yes you are right as to the basic stumbling block is an adaptor between a Mikuni and the Stock Bing connection. That in itself, makes me wonder whether all the ‘55-‘69 Models had the same stud bolt connection and the same spacing. There seems to have been several sources for those adaptors, not including Sudco that I have not searched yet.

Still hope to hear some first hand experiences on using Mikunis.

I’m amazed sometimes (most always) of the searched material that you come up with. The amount and speed with which you find the pertinent matter. If you haven’t, and you probably have, posted tips and pointers as to how to “Search”. I get the most ridiculous amount of unrelated information and quanity from this Forums search feature, so as to be useless and not worth the effort.

Care to post a “Search for Dummies” topic for the Dummies ?

Thanks again,

Charlie
 
Charlie -

I try to help when I can and appreciate the thoughts.

As for the MOA forum searching, I generally agree that the built-in search tools don't work all that well and I have generally stopped using them. Only when I'm desperate will I resort to them! ;) As for an overview of this forum search features, I put together this thread:

https://forums.bmwmoa.org/showthread.php?77843-Tips-on-Searching-the-MOA-Forum

It's under New Members so for long time members, they probably don't think of looking there. I had to park it somewhere. :dunno The search feature that I use mostly for the MOA forums is the google "site" search. It allows use of the powerful search feature of google which can include, exclude, or otherwise narrow the focus of the search. The problem with the google approach is you can't search just the Airheads forum for example.

I've found that with any searches, it's important to try and get inside what might have been posted by someone unknown person and think of what one or two specific words they might have used. That's problematic when one person will use the words "final drive", someone else uses "bevel drive", and another person will use "rear differential". The more detailed words you use, the better chance of finding something. If I remember a post sometime ago and can recall just one of those specific words, that helps a lot.

As for my searching generally, I'm plugged into a variety of forums and have been over the years. When I see something posted by someone that might help me or someone else in the future, I might save it in a Word file on my local machine. You might have seen these threads on the MOA forum...I've been compiling these lists over many years. The internet is a dynamic place and things come and go however.

https://forums.bmwmoa.org/forumdisplay.php?76-Links-and-References

Most of the stuff I own (not just bikes) is vintage or going on towards vintage. As for BMWs, there are several other vintage forums where good information is shared. I read everything that is posted and maybe save something for personal use. But when there's a new request for help being posed by someone, I might (most times might not!) recall that I remember seeing something along those lines elsewhere, so I go check things out. So, my reading all these other posts has helped find things, but I'm sure I've forgotten more than I remember!

In case of your original question on Mikunis, I basically just went to a couple of forums that specialize in the vintage BMWs and used that single word to search. I didn't explore everything that came back from those searches, but I perused the hits and noted a few details here and there. I typically might not come back with "here's your answer" but provide links for you to go explore yourself. I likely don't fully understand all the issues you might be dealing with so about all I can do is "lead you" to the water and hopefully you'll see some things that will give you ideas and places you can dive deeper to ultimately find what you're looking for.

That's about it. Just staying plugged in and sometimes thinking outside the box helps me find things. Hope that helps!

Update -- I keep forgetting about the forum Tag cloud. Along the way, a number of forum users have been adding direct and good tags to a number of threads. Often times search the tag cloud can bring up useful threads from the past.
 
Mikuni Adaptors

1E5D96BC-B67E-4E73-A9D9-AAA77A5C3B47.jpeg

After searching for a long while, I went ahead and designed an Adaptor and had a pair fabricated to match a Mikuni VM 26 8074 Carburetor.

There were a few pics as to the fabricated model and the design but nothing serious. I couldn’t visualize the strength of a 3mm wall thickness and now realize I could have had a little more of a Venturi effect from the 35mm carb throat to the 24mm head intake. I also could have shortened the center section maybe 5mm + - as the original mounting studs have plenty of room to spare.

This is just the first step and the Air Intakes need to be solved. I would rather pipe the Intakes from the stock system but haven’t completely ruled out individual filters.

I’ve been working with Bill at Rocky Point Cycle and he has been very good to work with the little coordination required to this point. Much more direction with proper set up will come next. I am mainly after easier cold starts and the Mikunis may do that. Time will tell.

I have adaptor drawings if anyone wants to use them.

This is fun.

Charlie
 
Much more direction with proper set up will come next. I am mainly after easier cold starts and the Mikunis may do that. ..

I had always wondered whether the accelerator pumps would ease cold weather starting on my Airhead; just give the throttle a few twists before cranking.

Doing the spring start on my 82 RS could be a bit of a challenge.
 
Drawings

View attachment 77263

After searching for a long while, I went ahead and designed an Adaptor and had a pair fabricated to match a Mikuni VM 26 8074 Carburetor.

There were a few pics as to the fabricated model and the design but nothing serious. I couldn’t visualize the strength of a 3mm wall thickness and now realize I could have had a little more of a Venturi effect from the 35mm carb throat to the 24mm head intake. I also could have shortened the center section maybe 5mm + - as the original mounting studs have plenty of room to spare.

This is just the first step and the Air Intakes need to be solved. I would rather pipe the Intakes from the stock system but haven’t completely ruled out individual filters.

I’ve been working with Bill at Rocky Point Cycle and he has been very good to work with the little coordination required to this point. Much more direction with proper set up will come next. I am mainly after easier cold starts and the Mikunis may do that. Time will tell.

I have adaptor drawings if anyone wants to use them.

This is fun.

Charlie

Hi Charlie,

Great job on the manifolds.

I'm looking at putting a set of Mikuni's on a '68 R50 and this is exactly what I have been looking for. You mentioned you would provide the drawings and I was hoping I could get a copy. Would you email them to me? I would gladly pay for them.

Thank you,

Paddy
 
Not BMW-specific, but a few of my old Japanese bikes had Mikunis on them; they were ultra-reliable and easy to swap the jets. I also put a Mikuni on my old Sportster (with a Jerry Branch manifold) - best thing I ever did to it, it started more easily and idled better. But on the other hand, it replaced a Tillitson, so maybe even an Amal would have been better...
 
Not BMW-specific, but a few of my old Japanese bikes had Mikunis on them; they were ultra-reliable and easy to swap the jets. I also put a Mikuni on my old Sportster (with a Jerry Branch manifold) - best thing I ever did to it, it started more easily and idled better. But on the other hand, it replaced a Tillitson, so maybe even an Amal would have been better...

The Tillotson’s main problem on the Sportster was that big, fat, juicy mixture adjustment knob atop the carburetor, just inside the rider’s right knee and right where every Sporty owner could see it and (unfortunately) not resist tinkering with it...

But, I digress. Accelerator pumps were mentioned in this thread. The Mikunis the OP selected don’t mention a pump and I’m wondering—did Mikuni ever make a round slide carb with an accelerator pump? The only slide carbs I remember with pumps were the Dellortos used on the R90S.

DeVern
 
Paddy, sorry I haven’t responded in a timely fashion.

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 at the mounting studs should have been shorter in length. 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 or use the foot controls. 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 @ 10 plus degree<was sufficient.
View attachment 80033

I made 18° 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.

C43B7591-22B2-4CF9-AC7D-29A2AA92E372.jpeg

12248112-4516-4F4B-A33B-2AE419D86440.jpg

29FA4DD2-8F03-4B16-9508-3A2CB9562008.jpg

4C036E73-EC42-48C0-A55F-82478B51D988.jpg

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 is 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

Charlie
 

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Thank you

Hi Charlie,

Thank you for getting back to me with more photos and the drawing. I haven't decided what to do but the information you have provided will help in a big way if I go with the Mikuni carbs.

Take care,

Paddy
 
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