Topic: CB550 Negative Tank Pressure

[DJPrototypikal]

Hi all

With a re-lined tank, new locking gas cap, and low fuel - I am experiencing negative pressure buildup  in the tank which stops fuel flow.

Does anyone know if the gas caps should handle negative pressure, or if there is a method of venting the tank that I missed?

P.S. After sitting in a garage for 15+ years, it passed its safety yesterday! This is the 2nd last issue before going on the road….

Thank you. Niall

[jlh3rd]

sounds like the gas cap vent isn't working. I'm unfamiliar with the "K" model caps....if that's what you have...
The "F" model cap I could show you the vent.....

[Nurse Julie]

Sounds like the vent in the cap is blocked. Take it apart and give it all a good clean.

[DJPrototypikal]

New gas cap from Common-Motors. I’m assuming new ones vent positive and negative pressure.

There is one screw on the underside. On pretty tight. I’ll see if it comes off.

Thanks.

[ekpent]

I'm sure you have cleaned the carbs well but they also have air vents also that need to be clear if the cap is good. Does everything work normally when the bike is running and the gas cap is left open a little ?  Fuel line routing good and are you doing any inline filters ?

[PeWe]

I had under pressure in a CB750 K2 tank. When warm tank with fuel cooled off after a ride, under pressure happened. Bubbling in carbs too.

The gasket blocked the vent hole.
I think it sat wrong, one lip is wider.

[Kenzo1979]

Low fuel?  Too low?  Petcock on reserve?  Petcock clean and working properly?  Pics of this “new locking gas cap”? 

[MRieck]

I had to drill a 1/16th hole through the F1 tank cap
 That was for a high flow Pingel petcock

[Stev-o]

The fuel system is gravity fed, need a vented cap!