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Motorcycle Repair/1981 honda cb900c


on the carbs between#2 and #3 there is a vacuum tube that hooks up somewhere it goes to the barrel slides in the carbs but I cant find where the vacuum comes from the vacuum tube from #2 runs to the inline fuel diaphragm but cannot find where the other vacuum is going to get to the slides


The only vacuum line you have to worry about is the #2 line to the fuel control valve. DO check it to ensure that the diaphragm that it is attached to is not leaking. If the diaphragm fails, you get a vacuum leak at #2 intake runner and a reduction or stoppage of fuel delivered to the carburetor rack.

There are three interconnecting lines between the carburetor bodies...
Vent lines...
Fuel lines...
Accelerator pump feed lines...

The carburetor slides get their vacuum from passageways inside the carburetor bodies, fed by the intake manifold vacuum signals.

The vent lines just vent to atmosphere, so don't attach to anything.

Bill Silver

Motorcycle Repair

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Bill Silver


Need help with vintage Hondas from the 1960s? I am an expert with 250-305cc bikes in particular and most all of the other pre-91 models, in general. I do NOT claim to have a great deal of experience on Gold Wings, Cruisers, ATC/ATVs and dirt bikes.


I have owned/ridden/maintained Honda motorcycles for 35 years. I have written five books on Honda repairs and collecting. I was a service manager for two Honda shops back in the 1980s.

VJMC (Vintage Japanese Motorcycle Club) of North America

VJMC newsletter, as editor for two years and as contributing editor currently.

3 years auto shop in high school, teacher's aide in Automotive Technology in Jr. College, Diesel mechanic course in college, self-taught mechanic and automotive writer.

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