Carb difference

[6659]

I have been looking at the different cv carbs and I see that they mostly are 64/32/XXX. I have wondered what those last 3 numbers mean. Also if the first 2 numbers are the same, wouldn't a set from a 1991 R100 fit and work on a 1974 R90?

[srb]

I think the last three digits help identify if it is the right hand side or left hand side carb.

[20774]

The first two numbers represent the type of carb, the next two are the diameter for the carb (either 32mm or 40mm), and the last three are the specific model usually associated with a specific year/model bike. Over the years, internal changes or settings were made to adjust for performanc and emissions. As long as the diameter is correct, I would think the carbs could be swapped but there may need to be internal changes to get the right performance.

Kurt in S.A.

[rinty]

There's another factor to consider in addition to whether they fit, and that is whether you prefer mid range power or top end power. The 32's will deliver more mid, the 40's more top.
Just my opinion, but I always thought the 40's were just a bit too much diameter for even an R 100, let alone an R 90. I think they are more of an autobahn carb.
You might want to talk to an experienced airhead tuner before going to the trouble of doing the conversion.



Rinty

[Bill Burke]

Robert Fleischer (Snowbum) gets into this question in some detail here:

http://pweb.jps.net/~snowbum/earlybingR75CV.htm

[lkchris]

I think the last three digits help identify if it is the right hand side or left hand side carb.


They do.

Plus the jetting.

Since probably the early '80s the carbs are the same castings--just the jetting differs.

Mounting for needle jets changed in late '80s; don't know if this required a casting change, but it would only be the top cover in any event.

[PGlaves]

I have been looking at the different cv carbs and I see that they mostly are 64/32/XXX. I have wondered what those last 3 numbers mean. Also if the first 2 numbers are the same, wouldn't a set from a 1991 R100 fit and work on a 1974 R90?
As noted the 64 is carb type, the 32 is the bore size, and the last three numbers denote the setup of the carb internally. Specifically, those last three numbers identify the combination of main jet size, needle jet size, idle jet size, and needle size and taper used for that specific carburetor.

If you have a good Bing manual you will find each of those specifications for each carb model number. This helps in ordering replacement parts of course - but it also allows you to set a 64-32-xxx carb up just like it were a 64-32-yyy carb provided you install all of the yyy parts.

We once bought an R80 which had a somewhat noisy aftermarket exhaust. It came with the stock exhaust in a box. I re-installed the stock exhaust and the bike ran a bit poorly. Carb adjustments wouldn't cure it. I finally used the Bing book to discover that the main jets, needle jets, and needles had been changed (presumably to match the aftermarket exhaust). Installing new jets and needles as specified for the bike cured the running problems.