Slippery GRM+ Memberand PowerDork
7/7/22 9:34 a.m.

I am rebuilding the carbs on my brother's '75 BMW R90/6 and have a question you guys will sure have the answer to. The motorcycle has not been ridden in a couple of years, but before that it did run fine. Carbs are factory Bing units, one on each side as its a flat twin.

I have only messed with the Mikuni carbs on my Yamaha R1, but when I disassembled the Bing carbs I realized these do not have a spring pushing on the throttle slide. I did some digging as at first I thought the parts were missing from a previous owner not putting back, but realized early Bing carbs did not have them. I also see that some people retrofit springs from later motorcycles on them. 

Does anyone know what the pros/cons of having the spring there are? Should I go ahead an order them or leave it alone as it was fine before? I am wondering if they help the slides go down faster for less rev hang or something.

Some pictures as everyone likes those:

The spring I am referring to is #22 on the picture below


gearheadE30 Dork
7/7/22 11:20 a.m.

For those CV style carbs, there is  both the slide and the butterfly that the throttle itself is connected to. When you chop the throttle, it is going to close off the intakes anyway with the throttle plate. The spring might cause the vaccum slides to drop faster, but the only real difference would be that you might have a bit less of a rich spike on throttle chop. I doubt there would be much difference in rider feel in that regard, especially because those are not quick revving engines just from an inertia/compression/performance perspective. They are nothing like the ultra-low inertia engines that are popular today.

I have not found slide bounce to be a huge issue on those bikes unless you're going to ride them off road, and CV carbs are never good for that. In that case, you really need more slide damping (or a heavier slide) rather than a spring on the slide. On the flipside, a lighter slide typically gives better throttle response on tip in because a lighter slide will react more quickly to the change in vacuum.

I would expect that the spring could be a good "safety" or consistency addition in that if the slide binds or hangs at all, you have some positive force there to make sure it still closes. I would also expect that the slide itself is different, or if not, the jetting (especially the needle) is different between sprung and floating versions. All else being equal, adding a spring would cause the slide to sit at a lower position for a given airflow/throttle position which would increase velocity and would increase vacuum signal to the jets. CV slide springs are usually a very light rate with a ton of preload, so I don't think it would impact the linearity of slide opening too much if these follow that trend.

Personally, I wouldn't bother changing it unless there is a specific problem you were trying to fix.

Slippery GRM+ Memberand PowerDork
7/7/22 11:35 a.m.

In reply to gearheadE30 :

That was a great explanation. 

I hadn't thought about the slide sitting lower with the spring in place. 

Given that the motorcycle run fine before, coupled with your explanation, I will reassemble as is. 


Slippery GRM+ Memberand PowerDork
7/7/22 11:41 a.m.

Another question. 

Any drawback in using avgas on this motorcycle? There is a small airport by the house and since he only rides it a few times a year it might be a good way to keep the carbs clean. 

Or would it be better to use ethanol free that's a lower octane?

He is just cruising to the beach and around the neighborhood and it probably has never reached anywhere close to redline in the last 10 years he has owned it. 

Carl Heideman
Carl Heideman
7/8/22 8:50 a.m.

In reply to Slippery :

I doubt he'd get any advantage from AV gas except that it's ethanol free and it smells so much better.  I'd suggest good old ethanol-free if it's easy to find.  

914Driver GRM+ Memberand MegaDork
7/8/22 10:50 a.m.
Slippery said:

Any drawback in using avgas on this motorcycle? 

Or would it be better to use ethanol free that's a lower octane?

I have an R-90 also.  The drawback of Avgas is the expense.  Not much more power and the exhaust still smells like exhaust, only advantage is you can leave it in all year without stabilizer.  I burn 91 ethanol free in mine with flat slide Mikunis.

I would also look into an air blow gun with the small black rubber tip for blowing out the tiny holes.

MiniDave Reader
7/8/22 12:21 p.m.

I thought the point of the spring was mixture control, to slow the rise of the piston when the throttle is first opened to pull more gas past the needle?

I build engines for Minis which use the similar SU carbs....getting the mixture right under a variety of conditions is a real dance between needle size and profile, spring tension and the damper oil.

I would clean them up and put them back exactly as they are as well.

TurnerX19 UltraDork
7/8/22 12:56 p.m.

The spring makes the mixture richer for any given jet position and throttle opening. This is the same for any brand CV carb, as the spring controls the size of the venturi. The stronger the spring the faster the air passes over the jet. 

Cactus HalfDork
7/8/22 1:27 p.m.

I run 90 aki ethanol free in my '80 R65. I don't ride the snot out of it the way I do my r1150gsa. I've thought about getting a set of stupidly high compression pistons and running race/av gas, but it's just not worth it. I'd spend a bunch of money to get a small amount of horsepower that I wouldn't even use. 

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