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'81 RS - dual plugged, 38mm exhaust, 40mm carbs, Uni foam filter, Luftmeister dual exhaust.

I'm going through a fresh set of 40mm carbs off a '79 RS. They have 2,66 jets with 170 main jets.
Looking at the carbs of the '81 it has 2,66s and 160 mains.

MAXBMW shows 2.64's for (01/78 to 09/79) and 2,66s (from 09/79)

I've been recommended that I go the 2,64s and 150 mains that reportedly helps 'livin' it up'.

Any thoughts, recommendations, thoughts, input on what's the best combo for my bike?
Not particularly concerned with mileage, elevation (I live at 2000 feet) just looking for the crispest performance.

Going to one size bigger needle jet usually improves throttle response, but at the penelty of fuel economy.

Another way to "crispen" up your throttle is to change up to the later damper (piston) springs that where introduced in the mid to late 80s. When you crack the throttle the carb pistons don't jump up imeadiatly, this causes a momentary increase in air velosity trough the venturi, sucking up a slightly richer fuel mixture, sorta like an accelerator pump.

By useing stiffer springs this is exagerated slightly giveing a more responsive "feel" but once settled into a stable cruise the standard mixture and fuel economy returns. The stiffer springs where introduced as BMW/Bing tried to meet tighter emissions standards by leaning the fuel,mixture but still maitaining driveability. It has now become a simple way to "Hop Up" your carbs.

...really...I must have those in the newer '81 carbs then...

is there springs available for earlier carbs as well? say 32 Bing CVs on a /6?

...truthfully, I think at least on these two carb sets the older carb spring feels heftier if at all.
On topic, what about the diaphrams? I've got some Jag/Volkswagen CV diaphrams I got from Moss Motors that feel heftier as well.
I suppose a heavier diaphram would work the other way around, slower response...yes?

hal wrote:is there springs available for earlier carbs as well? say 32 Bing CVs on a /6?

You can use the springs from a /7 32mm carb in the early non spring carbs. You may have to try a different needle position or jet size after to get it perfect.

Rapid Dog wrote:...truthfully, I think at least on these two carb sets the older carb spring feels heftier if at all.
On topic, what about the diaphrams? I've got some Jag/Volkswagen CV diaphrams I got from Moss Motors that feel heftier as well.
I suppose a heavier diaphram would work the other way around, slower response...yes?

I suppose yes sorta maybe in a non linear sorta way.

My 11/1981 build date R100RS had 160 mains when I got it. Specs say 170.
I put a set of Jim Cray mufflers http://www.youtube.com/watch?v=tnWMpZ4cf0I on it.

The JC's are a wider straight thru design, and the designer suggests the only change to be 170 main jets. The other jets are stock, and I still have some "popping". It's much less though. The plugs are a light tan colour, so I'm happy enough. Grant81rs gets better fuel economy than me, by a couple of litres per tank. I'm unaware of the jets Grant uses.

The performance is crisp, especially from mid range to higher rpm. I also use the later springs.

FH was a proponent of smaller mains, so if you're going to try them, I'd be interested to hear the results.

The big valve, dual plugged, 9.2:1, 336 cam, lightly modified '81 that I ran choked on (Bing 40s &) 170s ...
but jumped and ran on 160s.
Careful off-idle tuning gave the biggest bang for the buck.

Myth #1: Bigger jets = better performance.

... Listen to your engine, learn it's language,
no one here can know what it's trying to tell you.

...any input on the other jets, 2,64 or 2,66 ...?