85 cb700sc Nighthawk carburator air tubes

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85 cb700sc Nighthawk carburator air tubes

SoCalMotoClassics
Greetings fellow rider's . . .an update on my 85 cb700 Nighthawk . . . Thanks to Shinyribs recommendation, I went with a new set of Shinkos, and Tools help put me on the right track with my clutch issues, new brakes, fluids,  corrosion control, lube, oil and cleaned the carbs.  Some idiot used silicone sealer on the bowls.  Had goo stuck to everything.  The bike had California smog, so there were multiple vac ports were capped, but two hoses in between carbs 1&2 and 3&4 had some L Shaped open lines connected to what the service manual refers to as "air tubes".  I assumed it was more Cali smog crap, decided to plug them so the carbs wouldn't suck crap through the open tubes.

After assembling bike, would only run with choke full on, suspected fuel issue, but had replaced fuel line and filter, tank is very clean with fresh gas.  Decided that since it ran with open tubes it could be the caps I put on the air tubes, took them off and the bike started right up on first crank with new NGK iridium plugs (thanks to the forum tip).

Big question, what's the function of the air tubes? And, if they need to run open, should there be some form of filtration?  Perhaps it was part of fuel vapor recovery?  Anyway, the bike runs and rides so much better. The Shinkos ride just fine and I was able to get them mounted and balanced for 47 bucks.  

Thanks in advance
1973 CL350, 1975 CB750K, 1980 Yamaha DT175E . . . 30 yrs later . . . Picking up where I left off . . . Now riding a 1985 CB700SC Nighthawk S (Fair-weather rider by geography . . . sunny Southern California) . . . Ride to live . . . Live to ride!
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Re: 85 cb700sc Nighthawk carburator air tubes

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I assume they were the carb bowl vents. You could put a fuel filter on the open end of them, but most bowl vents just hange down open.
TOOLS
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1976 CB 750-A X 2
1977 CB 750-A X 4
1977 CB 750-K
1976 CB 750 F
1981 CB 750
1966 Kawasaki SG 250
1981 KZ 750 LTD
1973 CB 350
1979 CM 185 Twinstar
1982 Honda XL 80
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Re: 85 cb700sc Nighthawk carburator air tubes

SoCalMotoClassics
The service manual made little reference to them, but did see an exploded view of engine assembly showing hoses on the air tubes dressed back towards air box connected to nothing.  I'll just do the same and call it done.  Thanks for the support.  Now for some cosmetics . . .
1973 CL350, 1975 CB750K, 1980 Yamaha DT175E . . . 30 yrs later . . . Picking up where I left off . . . Now riding a 1985 CB700SC Nighthawk S (Fair-weather rider by geography . . . sunny Southern California) . . . Ride to live . . . Live to ride!