Connecting carb vacuum ports together

[Manfred]

While synchronizing my carburetors, an older, wiser airhead suggested it might be good idea to tie the vacuum ports together, rather than merely cap them off - this helps balance the carbs. I had done this on a Yamaha V-twin and thought it helped.

Any thoughts as to whether this is good, bad, or indifferent on an airhead?

[carockwell]

In theory, this is supposed to help even the vacuum pulses seen across the carbs as each intake valve opens and closes. It seems to have a slight effect on off idle throttle response.

[88bmwJeff]

I've heard some people say it's a good idea, and others say it's not worth the time. The carbs were only linked together after the smog stuff was added. The older pre-smog bikes never had their carbs linked.

I ran my bike (88 R100 RT) for a while with the carbs linked together because I was too lazy to finish removing the smog stuff. Once removed, there was no noticeable difference in the way the bike ran/runs.

[Manfred]

I haven't removed the smog gear - thinking about it.

I ran the bike about 350 miles with the vacuum ports closed off with a short piece of hose with a bolt screwed into it.

When I got home, I took a piece of fuel line, connected to the vacuum port on each carb, and ran it through a hole in the top engine case, just forward of the air cleaner. Haven't put more than a few miles on it with this set up.

[sumran]

I have run mine both ways and don't notice any difference. I think the line was there to provide a balanced source of vacuum to the emissions system, not to help the carbs.

Theoretically, since vacuum should be drawing fuel into the system at just the right time, it seem that connecting the sides would be a disadvantage rather than an improvement. I don't think it has any effect in real life. However, the chance that I don't understand what is really going is fairly high.

[BubbaZanetti]

tried it on the vacuum ports on the throttle bodies on my oilhead, no noticeable difference and i kept dropping the small hose on the hot headers when i did tb synchs, i ended up just capping them instead.

[BMWRich58]

I haven't removed the smog gear - thinking about it.

I ran the bike about 350 miles with the vacuum ports closed off with a short piece of hose with a bolt screwed into it.

When I got home, I took a piece of fuel line, connected to the vacuum port on each carb, and ran it through a hole in the top engine case, just forward of the air cleaner. Haven't put more than a few miles on it with this set up.


I used that hole on top of the motor to pass a fuel line through. Connected each carb with "T" fittings *pet cock to T-T to carb-T to cross over line* to prevent either carb from starving for fuel during leans in turns or when the fuel level is low in the tank,one side might have better flow than the other when levels are low.

[Manfred]

I used that hole on top of the motor to pass a fuel line through. Connected each carb with "T" fittings *pet cock to T-T to carb-T to cross over line* to prevent either carb from starving for fuel during leans in turns or when the fuel level is low in the tank,one side might have better flow than the other when levels are low.

That's a neat idea! I'll keep that in mind.

[Guenther]

I heard another terrific advice and I can't believe the BMW engineers didn't think about it.

Connect your tailpipe with a hose to the filler of your tank. Make sure both ends of the hose make a tight connection. Gives you a hell of an ignition. :eat

/Guenther
(...sorry, couldn't resist)

[Manfred]

I used that hole on top of the motor to pass a fuel line through. Connected each carb with "T" fittings *pet cock to T-T to carb-T to cross over line* to prevent either carb from starving for fuel during leans in turns or when the fuel level is low in the tank,one side might have better flow than the other when levels are low.

I have the best of both worlds - my bike already has the fuel lines tied together, with a hose that runs right in front of the air box; about 2 inches below my "balancing" hose.

[jforgo]

None of my Japanese 2, 3, or 4 cylinders ever had this. It seems like a "just because" for the smog bikes, to me (?)

[Manfred]

None of my Japanese 2, 3, or 4 cylinders ever had this. It seems like a "just because" for the smog bikes, to me (?)

None of your Jap bikes had what? I've owned Yamahas with similar smog devices as the boxers have.

[lkchris]

The carbs were only linked together after the smog stuff was added. The older pre-smog bikes never had their carbs linked.


Nor were they linked together in markets not requiring smog equipment AFTER the implementation for USA market.

BMW engineers, then, obviously had the opportunity to study it, and ... .

Only thing the linking accomplishes is better operation of the air injestion system.