32mm to 40mm Carburetor (1982 R100)

Hi All,

I got my hands on some new Bing Carburetors to replace some old ones on my bike. The problem is that the originals had slightly larger 40mm. Would i be able to convert it with some kind of adapter? Currently I have the a clamp super tight around the rubber in order to make the connection to the engine. I think I'm going to try and find some outright replacements but wanted to see if and adapter would serve as a temporary fix.

I'm a little confused on what you're trying to do...putting 32mm carbs onto a setup meant for 40mm carbs...or vice versa. Possibly what you could check into is the spigot that attaches to the head on the intake side. Part 11 12 1338 361 is the spigot for the late 1980s GSPD 40mm carbs. Part 11 12 1 257 396 was used on the pre 1980 32mm carbs.

You may have to consider changes to the throttle cables and maybe even the handlebar throttle gears.

Kurt is correct, there are lots of differences. Throttle cables for sure.

But ... bikes with 40 mm carbs came with different cylinder heads than bikes with 32 mm carbs. The intake valves are larger, as are, I think the intake ports themselves. Whether the screw in adapters interchange I don’t know, but they are obviously different on the carb side. The plastic tubes to the air filter could differ as well.

Not so sure your bike would run properly with the smaller carbs, but the reduction in power could be problematic. Bikes with 40 mm carbs have a different rear drive ratio than bikes with 32 mm carbs ... could your bike pull the lower ratio with less power?

lkchris said:

Kurt is correct, there are lots of differences. Throttle cables for sure.

But ... bikes with 40 mm carbs came with different cylinder heads than bikes with 32 mm carbs. The intake valves are larger, as are, I think the intake ports themselves. Whether the screw in adapters interchange I don’t know, but they are obviously different on the carb side. The plastic tubes to the air filter could differ as well.

Not so sure your bike would run properly with the smaller carbs, but the reduction in power could be problematic. Bikes with 40 mm carbs have a different rear drive ratio than bikes with 32 mm carbs ... could your bike pull the lower ratio with less power?

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Noticed the biggest differences were the choke cable and the larger intake port. I did a test run and and did a manual choke for the time being. Bike runs but you can feel the loss of power for sure. To save on any headaches I’ll just replace outright with the 40mm.

Not Advisable

The intake spigots on the 1980 to 1984 r100's are indeed larger to accommodate the 40mm Bings. Even if you could change them to the 32mm version, the performance would suffer at higher rpm's, where these were designed to deliver the most torque. This era of the R100 has the large 44mm intake valve and the intake ports are also larger than on the small valve heads. It was a rather inefficient head design which BMW greatly improved on post 1984 when they modified the port shape and returned to the small intake vale. The later bikes had much improved torque in the lower RPM range but were fairly flat in the upper range.

The large valve, large port heads need the larger carburetor, large snorkel airbox and more fuel for optimum performance. On a side note, replacing the US spec 8.2 compression pistons with the Euro spec 9.5 pistons and going up in main jet size, similar to the original R100RS, makes quite a difference in performance on these bikes.

Jim

It will cost more to make them work!

To be clear: It will cost more $$$, adjustments, time and frustration to make the wrong carburetors work OK than it would to get the current and correct carbs to work well on your R 100. Unload the wrong carbs and use the money to get the correct ones refurbished and set up to work as they should.

Don't waste money trying to fix what isn't broken, fix what you have and enjoy the journey!

Zeff

So why the change?

So I must ask why do you feel you need to go to 40mm carbs? As the other respondents of this thread have said, it makes no sense unless you match them to the proper heads with the proper valve size.

Putting bigger carbs on alone won't do much good and in fact can make things worse because of the configuration of the heads you have.

I advise you fix your original carbs they will give you the best performance with your bike on the street. Installing bigger carbs doesn't mean an instant increase in power, things have to be balanced to do that.

If you feel the need for more oomph in passing power, there are sources for kits to install Delorto or Mikuni carbs on your bike. They have an accelerator pump which gives a shot of gas when the throttle is cranked open. The same rules apply however, they have to be set up and sized properly in order to work properly.

I have two bikes, a European specification 78 R100RS with 40mm carbs and exhaust plus higher compression. I also have a 84 R80RT, with the 32mm carbs and 38mm exhaust. While riding, the only time I can really notice a difference in power between the two bikes is when I am passing a car. There, the bigger engine and such makes a huge difference. Really however, it is not such a difference that I would go out to spend the money to rebuild the R80 to R100 specs. I have not even chosen to switch to the accelerator pump carbs. It is not worth the time nor money to me. St.

STEVENRANKIN said:

So I must ask why do you feel you need to go to 40mm carbs?

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I must have gotten lost someplace. I thought he wanted to replace the 40mm with 32mm carbs.

Header

Hey Paul, I am going by his header of his thread, 32mm to 40mm carbs, I may be wrong. Cheers, St.

STEVENRANKIN said:

Hey Paul, I am going by his header of his thread, 32mm to 40mm carbs, I may be wrong. Cheers, St.

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The first sentence in the original post: "I got my hands on some new Bing Carburetors to replace some old ones on my bike. The problem is that the originals had slightly larger 40mm. "

Then Kurt followed up in post #2: "I'm a little confused on what you're trying to do...putting 32mm carbs onto a setup meant for 40mm carbs...or vice versa. "

Then the original poster came back with: "To save on any headaches I’ll just replace outright with the 40mm. "

Okay

So I guess I have failed to read the post again. To be honest, I didn't think the 82 R100 had 40mm carbs but had the smaller ones.

So am I missing the reason he is changing carbs in the first place? Going from 40mm to 32, makes no sense if the bike was set up for them in the first place? Going from 32 to 40 makes no sense unless the heads are modified?

Having the original carbs rebuilt to solve problems with them makes the most sense unless you have a set like I did with extreme wear and an unrepairable vacuum leak.

This is one of the drawbacks to forums, it takes a few messages back and forth sometimes for everyone to understand each other. LOL. St.

jchussey said:

The intake spigots on the 1980 to 1984 r100's are indeed larger to accommodate the 40mm Bings. Even if you could change them to the 32mm version, the performance would suffer at higher rpm's, where these were designed to deliver the most torque. This era of the R100 has the large 44mm intake valve and the intake ports are also larger than on the small valve heads. It was a rather inefficient head design which BMW greatly improved on post 1984 when they modified the port shape and returned to the small intake vale. The later bikes had much improved torque in the lower RPM range but were fairly flat in the upper range.

The large valve, large port heads need the larger carburetor, large snorkel airbox and more fuel for optimum performance. On a side note, replacing the US spec 8.2 compression pistons with the Euro spec 9.5 pistons and going up in main jet size, similar to the original R100RS, makes quite a difference in performance on these bikes.

Jim

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Not so much inefficient as subject to warping ... too much material missing due to the larger valves. This was reported long ago by Oak. Also ... the oil cooler.

I looked it up in the Bing manual when I fitted 9.5 pistons to my ‘84 R100RS ... the only jetting change required was raising the needle jet one notch. Did that and had no problems.

Clearing things up - my r100 has 40mm carbs stock. I bought 32mm by mistake and tried to see if I can use them somehow. Decided to sell the 32mm and find the proper 40mm replacements .. installed them and it’s as good as new.

ginfante said:

Clearing things up - my r100 has 40mm carbs stock. I bought 32mm by mistake and tried to see if I can use them somehow. Decided to sell the 32mm and find the proper 40mm replacements .. installed them and it’s as good as new.

Click to expand...

Thanks for the update. Glad it worked out for you.