Carb substitution.

So, I'm struggling to get this 1974 r90/6 into some semblance of order.

the carbs that were on it were so bad I don't know if they're fixable.

Also, they were for an r100/7, not an r90/6

I have a bunch of carbs, but not the exact match for an r90.

my question is this,

Which would be a better choice. A set of r80 carbs, a set of r75 carbs, or a set of r100/7 carbs?

Hang the best set and install new spark plugs. With respect to CV carbs, the jetting is the difference.

I put an R80/7 top end on a tired R100/7 and used the R100 carbs on the R80 top end. I bought a set of jets to retro- fit the carbs, but the thing runs great as it is. I left it alone, and have no worries.

The spark plugs, by their color, are showing a good range of air/fuel mixture. Not too rich, not too lean.

There is a line of thought that says Bing CV carbs are able to correct for small deviations in jetting.

I'd go with the R100 carbs if it were my project. However, I consider the Bings with 4 screws on the domes to be the best.

Get yourself a Bing Manual, it will tell you everything you need to know!

DARRYL CAINEY said:

Get yourself a Bing Manual, it will tell you everything you need to know!

Click to expand...

sure... -which one-? and I can tell you exactly what the manual is going to say. It's going to say 'use the factory installed carbs.' I"m psychic like that.

No, the manual (more of a pamphlet) lists carb numbers for various models. For each carb number, it lists the standard settings, jets, needles, etc.

huh. learn something new everyday.

Sorry if I came across sharp, I'm tired, dirty, bleeding, I've got carb cleaner in my eye, and texas fire ants in my shoes and underpants.



But the bike started. ran too. I used a set of 64/32/3-4 which are normally used on a r100/7.

If the guts of the previous carbs are recoverable, I may re-jet to /6 specs. Right now I'm just happy it runs.

Second time that Bing manual would have saved you some time.

Remember this one?

http://forums.bmwmoa.org/showthread.php?66384-Bings-Bings-Bings&highlight=

Some stuff isn't worth it and some stuff is. I think if you expect to be messing more with the Bings, this will be $11 you will be happy to have spent. Its basically an exploded parts manual for all the different versions and includes listings of the stock jet numbers, needles, needle positions, and all the other pieces. Its a great reference if you are putting one back together, or if you have a new-to-you carb that you aren't sure if a previous owner made "tuning" mods to, or as you describe, if you have a box a carb parts and are trying to match parts with bodies.

Barron

Well that's good enough for me. I called Fay at Bing today and the manual is on it's way. I think the carbs on both my airheads are fine but sooner or later they all need some TLC.

I should probably think about Oak's manual too. His web page has to be the most sparse site I've ever seen. Well other than 404 page not found that is.

Happy Wanderer said:

I should probably think about Oak's manual too. His web page has to be the most sparse site I've ever seen.

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I didn't think Oak had a web page at all...he's not that "on-line". He emails and posts on the Airheads emailing list, but there's never been a mention of a web page.

Roguetek said:

I used a set of 64/32/3-4 which are normally used on a r100/7.

Click to expand...

Just understand that the number simply indicates the basic size of the carb and then the jet sizes factory installed.

The number is no longer valid if different jet sizes are fitted, but it's quite ok to do so.

Just look at the Bing book and match the jet sizes/positions to the specifications for a particular motor.

Going from say, a 32 to a 40 Bing helps nothing unless you've got an R100 with the big-valve heads. A larger carburetor does not make your engine "request" more fuel and you can take to the bank BMW's carb specifications do not "strangle" your engine.

Note finally, that changing carbs can on occasion create problems with throttle cables. They're different at least between the 32s and the 40s.

For example, when I installed 9.5 pistons in my '84 R100RS I checked the Bing book and noted the only carb difference between the USA/8.2 compression bikes and the rest of world 9.5 compression bikes was one notch higher for the needle jet. This obviously made things a bit richer and I had no problems ... even though the numbers stamped into my carbs were no longer accurate.

One quick question, not railroading this thread!!!

So has I'm reading all this (thank you Roguetek for asking), I have come to the understanding then that 64---means type 32---means the size in MM, and the last two or three numbers are model applicable.....However I could technically interchange any 64/32/XXX to put on my airhead as long as I rejet, correct? So is there a difference in the flat top 64/32 Bings? Could I buy a pair of flat top Bing 64/32/ bings rejet them and put them on my R75/6?
mmmmmmm ya got me thinking!!

20774 said:

I didn't think Oak had a web page at all...he's not that "on-line". He emails and posts on the Airheads emailing list, but there's never been a mention of a web page.

Click to expand...

I think this is it, sort of ? : http://askoak.org/

I did some checking and it does appear to belong to Oak, but the registrant address is San Diego. I also found Scott Lydiard associated with it. He's a pretty savvy web guy and has his own website with pretty good articles on Airheads. I'm wondering if Scott was offering to set something up and that's as far as they got. I searched the backlog of Airheads emails that I have and there's no recent mention of this particular website. Oak certainly hasn't mentioned it in any of the Airmail publications from Airheads.org.