rising gas in tank at idle

[GS550joe]

I have a 78 gs550c while I was testing for a bad charging system I noticed a strange occurance with the gas tank... I had filled up the night before and had about 1/2 -1in of space left in tank.. I choked the motor and started it up.. and while it was choked the gas began to to overflow the tank.. I killed the motor and the it stopped.. thinking it might be the cap I opened the tank and left the cap off.. started the bike again and could see the gas rising.. when I dropped the choke off the gas returned to normal.. I am running on prime because I do have a bad petcock and the vacuum nipple was open.. I did this several times and the same thing happened.. outside temp was about 67 degrees.. has anyone else seen this??

[fgh]

One of the Wildest I've heard

I would guess yours is like mine in that there is a short sleeve under the gas cap. If you've got the bike on the center stand and the gas level is above the bottom of the sleeve then air escaping into the tank would collect above the gas outside the sleeve and push the gas up through it. With the choke on you are probably getting enough vacuum at the petcock to pull air into the fitting somehow that escapes up and into the tank. It would be interesting to know if it does it with the vacuum line removed from the petcock and plugged.

[Mr. Jiggles]

Has your tank been kreemed??

[GS550joe]

The vacuum line is disconnected and plugged/blocked at the carb side.. petcock vacuum side is open..

As far as I know the tank has not been kreemed but I would not put any money on my knowledge... I bought the Bike thru a bike shop and I know it has had at least 2 po before I got it..