In your photo it's on the far left. On the underside of the carb, located on the port going into the cylinder head are 2 cylindrical bosses. One tall one and one short one. L and R carbs are identical.

The short boss has an external straight slot screw in it, which when removed will allow a vacuum hose to be forced over the boss. Don't forget to put the screw back in to block the port when you finish !!

The tall boss also has an internal straight slot screw, but that is the idle mixture. The idle mixture is typically set at 1-1/2 turns out from seated. Careful to use only 2 fingers on the screwdriver to turn the idle mixture screw. They can be fairly delicate.

Thanks once more

I got back to looking at my carburetors. Apparently, the former owner ran rubber tubing between the two vacuum ports. I'm guessing he lost the screws. I can't think of any other reason he would have done that. I'm hoping Bing will have the screws.

I just looked at some carbs on Ebay. There is a pair from 1979 with screws on the vacuum port. Seems later models didn't have the screws, so, I'm guessing the hose between the carbs is correct on the '81. Tell me if I'm wrong.

I got back to looking at my carburetors. Apparently, the former owner ran rubber tubing between the two vacuum ports. I'm guessing he lost the screws. I can't think of any other reason he would have done that.

The hose on your bike probably won't make a big difference either way. Could be a California EPA thing, if your bike was originally sold in CA. It's done all the time on other carb bikes to help them idle smoother.

I've never seen it done on a BMW carb bike, but the fuel injected BMW models do it as a way to burn off excess volatiles collected by the fuel tank breather system. But then the tank is under vacuum on those models.

I'm hoping Bing will have the screws.

Lowes and Home Depot will have those screws. I believe they're simply 2mm screws.

I replaced this fuel line with smaller outside diameter, blue plastic hose, after I gave up on the idea of pulling an OEM; fabric covered version through there. Had I more experience then, I might have removed the airbox, which is somewhat tedious.

Except, my R65 has only one petcock. It's on the left, so when I run completely out of gas, the right lobe of the tank still has a quart left. I didn't know this for the first 25 years I owned the bike and once limped to the next gas station on Coleman gas. Lay the bike down to get some of that last quart, or take the tank off to get the last drop,

I got back to looking at my carburetors. Apparently, the former owner ran rubber tubing between the two vacuum ports. I'm guessing he lost the screws. I can't think of any other reason he would have done that. I'm hoping Bing will have the screws.

I just looked at some carbs on Ebay. There is a pair from 1979 with screws on the vacuum port. Seems later models didn't have the screws, so, I'm guessing the hose between the carbs is correct on the '81. Tell me if I'm wrong.

I must revise my previous answer to you in great shame. The rubber hose connecting the 2 vacuum ports is standard on later year models. My 1979 has the blocking screws; my newly acquired 1988 has the hose. Exactly what year the hose appeared I don't know, but obviously someone thought it was needed.

So the bottom line is... if the hose is present then make sure it has no cracks or holes, and reinstall it. All the examples of this hose that I've seen were very generous in their length. 99% of the time the only problem areas (if they develop) will be the extreme ends that slide over the carb. 40 years on is a good time to snip off the last 1/2 inch and slide a new section of hose up onto the carb.

If the hose is present, then be sure and replace it on the vacuum ports after the vacuum gauges are removed.

Hope this helps.

The 1988 R100RS that I used to own already had the pulse air and other emission related parts removed by a previous owner. It had the vacuum ports capped with rubbers caps that looked a whole lot like little bitty condoms. That would be 1 way to close off the vacuum ports on the underside of the carb body. The 1985 R80RT that I recently picked up has the emission components in place including a vacuum line between the carbs routed thru the starter cover near the air filter box. That vacuum line is connected to the carbs on the port that Wobbly pointed out in the picture above. Capping those ports is high up on the to do list for that bike.