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Bings on my R90/6 did not come with throttle slide springs and in the process of chasing some slow running/starting issues I decided to fit some.

All was well and I was under the impression that these springs helped to sort out the slow running issues. This was around Town.

I have just got back from an end of season trip to the south coast of Spain, I travel with a mate of mine who has a R1100R and we have an informal competition to see which bikes uses less fuel, to my dismay on the way down to Cartanega, my bike was using 25-30% more fuel than the R1100. So after some thinking about what might have been causing this, I removed the slide springs in Cartanega and my fuel consumption was back to normal with a days best of 63mpg (imp).

I'm not sure why the big differeance, maybe Bing reshaped the throttle slides when they introduced the springs.

Currently using 145 mains, standard needle jet and needle set on second groove from the top.

Seems to me that the springs will naturally resist the rise of the slides, thus holding them in a slighly lower position. With a smaller area, the higher velocity will create a greater venturi effect, drawing up more fuel. Could be as you say, the lower portion of the slide was modified to reduce this effect.

Kurt in S.A.

Charlse - have a '74 R90/6

Using the stock 150 main (stock carb), I added the springs a few years ago.

I was getting right around 43 mpg (AMERICAN) before and after installing the springs.

The later CVs that had springs also had different slides. I don't know how Bing's numbers work, but there will be a different shape to the venturi part of the slide. If you can find some later slides and then match the needles and needle jets you should see better results.

When I rebuilt the carburetors on my 74 I added the springs that Snobum mentioned on his site. I bought both sets he gave numbers to and ended up using the softer (most coils, closer together) of the ones I received. The thing I noticed is that when I reach a certain throttle point, it seems to come on strong with a bit more effort to turn it past that point. I really haven't been watching the mileage that close, but it's close to 40 based on around town activity.

In my case it was a really significant difference in MPG, traveling on similar roads at similar speeds, to put it another way the difference was between switching onto reserve at 180 miles or filled before reaching reserve at 230 miles. When I travel with Marcel, we always fill the bikes at the same time and brim the tanks to the same point, so we have a fairly good idea of how much fuel was used.

I'm fairly sure that it was the spring that was at fault, when I took out the springs I checked the needle positions and cleaned out the main jet stack, but there wer no problems there. I also double checked the choke adjustment ensuring that the chokes were fully closed.

Do they really help with power? That's much more interesting than mpg in my opinion..

chasbmw wrote:Bings on my R90/6 did not come with throttle slide springs and in the process of chasing some slow running/starting issues I decided to fit some.

All was well and I was under the impression that these springs helped to sort out the slow running issues. This was around Town.

When you did this did you change any settings, needle height or needle jet size?

The springs slow the speed that the slides rise. This lengthens the amount of time that the venturi velocity is increased giving an extended "accelerator pump" affect. At some point the vacuum will lift the slides to very close to the same level as without the springs.

This is a common performance mod to make CVs "feel" more like the crisp response that slide carbs offer.

To meet emission standards Bing used leaner midrange settings as most BMWs spend most of the time at cruising speeds. This made for an even worse CV bog. Slide springs reduced the bog. At the same time the slides changed part numbers, as most likely the venturi changed shape.

So, for fuel economy, a reduced needle jet size along with the springs would be needed. Of course all the re-jetting cautions apply.

I always heard the effect of the springs was a quicker return to idle. I put them on my bike some years ago. Have never thought they were much change at all but I still use them. Friends of mine say they notice a marked improvement.

Chas, if you are getting 63 mpg you dont really want to mess things up!

According to Mr Haynes your needle clip should be in the top groove, so that is one avenue to try with the springs fitted, but your Euro spec bike shouldnt have the weak settings the US bikes had.

I fitted the weak springs to a R75/7 that didnt have them,but at the same time as 900 barrels, a timing chain and a Boyer Microdigital , so the changes were lost in the general improvement to power, response and fuel consumption.

I subsequently fitted a unequal lengh 2 in 1, and a free flowing muffler and that knocked the jetting around a bit, but the latter stronger springs and dropping the needle one groove sorted it out.

Ideally you want as lean a needle /jet setting as possible, with the spring forming the "accelerator pump" so that you get lean running on a dead throttle and response when you need it, but if you are getting 63 mpg and acceptable throttle response you are probably just about as good as you can expect.