carb vacumn line

[remastersjr]

Ok. I've had an R100 (78) years ago. Now, I've acquired an 83 R89/RT after 20+ years of not riding. I've been reading about syncing carbs which was easier before we went to electronic ignition. Manual says remove plug from carb vacumne port. Only thing I see is a vacume line going to a port. So, do you use that port to hook up a gauge? I guess you must because I can't see any other port to use. Any help appreciated.

[jmerlino]

Yes. Disconnect the line that goes into the airbox, and connect your gauge to that nipple.

[20774]

I think the port you want is one on the carb, just downstream of the carb itself. On my /7, the port is a small tube with a very small slotted screw capping it off. The port sticks out to each side. I think I've heard that some ports might be on the bottom side. Some carbs don't even have ports but they can be added...I think Bing could do that.

I'm not familiar with the '83 carbs...I believe this machine has the external emissions tubing. These tubes serve a different purpose and I've not heard that you use them to synch the carbs.

Kurt in S.A.

[flash412]

There is a faster, easier, cheaper, better way to synch carbs. Lock the throttle at the RPM at which you generally cruise. The only tool necessary is an open end wrench to loosen the locknut on a cable adjuster. Bend one throttle cable between two fingers and your thumb. Bend the other one. If the bike speeds up when you bend one but not the other, adjust accordingly. If you do this, your bike will run the smoothest where you ride it the most.

[pedrocasper]

Welcome back to the Airhead world. There are several great tech articles on synch. carbs on the Airhead webpage:

http://www.airheads.org/old/index.php?Technical%20Tips

Including an article on building and using a $5.00 manometer, which i just used to build my own manometer for approximately $8.00. It worked too!

[pedrocasper]

Drawing of manometer.

[jmerlino]

I bought a Twinmax, just because it's less messy, and you don't have to worry about keeping it straight up and down.

On my carbs, the vacuum takeoff ports are on the bottom, just next to the idle mixture screws.

The problem with Flash's method is that I think you need to be somewhat experienced with it. I tried to do it, and I ended up making things worse. I'm sure Flash can probably do it in his sleep, but it didn't work out for me. With the Twinmax, I was able to get 'em synced in a couple of minutes, and it made the bike much smoother. The other nice thing about the Twinmax, is that you can watch the needle as you open the throttle and see that it stays centered as the RPMs go up.

[20774]

It's best to ensure that the engine is up to nominal temp before doing the synch. I always put a box fan in front of the bike because I can't do the synching that quickly and don't like to idle the engine that long without some air flow. I heard someone mention leaving the TwinMax attached and going for a ride to really get a sense for how the each side responds under load. I've never tried it but it might be kind of interesting. Of course, it's best to keep you eyes on the road ahead and not looking down at this thing strapped to the tank, but still... Duane Ausherman also suggested leaving the left side lock nut undone and while running down the road, reach down and adjust the left cable to get smooth running. Once it's running smooth, pull over and lock the setting down.

Kurt in S.A.

[puccini]

I've always wondered if retrofitting the cross vacuum line to the older twins with CV carbs would improve anything.

I've got a 77 R75/7

Whadda ya tink?

Gerry

[20774]

This has been discussed off and on on the Airheads list. I think you'll find people who think it's a good thing and those that don't. Personally, I wouldn't. Each carb-cylinder is a set with its own efficiencies and pulling its own vacuum based upon a variety of factors. It doesn't make sense to me to change that relationship by introducing something from the other cylinder. I think it's better to make each side pull its share of the load in a balanced sense rather than trying to degrade one to help the other by bleeding vacuum between them.

Kurt in S.A.

[lkchris]

The vacuum was taken off to drive the air injestion system.

In countries not requiring this the carbs weren't hooked together and in fact the engine top cover didn't have the notch for the lines.

You can rest assured BMW's engineers didn't find an advantage and we can rest assured some motorcycle guy in the USA isn't going to discover anything they didn't already test.

[flash412]

we can rest assured some motorcycle guy in the USA isn't going to discover anything they didn't already test.Like dual-plugging, for example?

[lkchris]

Like dual-plugging, for example?

Dual plugging is something not required--it's a shadetree solution looking for a problem.

[flash412]

Dual plugging is something not required--it's a shadetree solution looking for a problem.How does that explain why BMW used it on the carbureted F650, removed it and suffered surging and stalling on the fuel injected F650 (just like the oilheads) and then suddenly "invented" DualSpark and added it to all the bikes in their line? Were they shadetree engineers looking for a problem in 1994 or are they shadetree engineers solving a problem that doesn't exist now?

http://tinyurl.com/bx4j6