gas tank pressure?

Posted by David Grubbs On 2014/11/28 19:10:02

Most of the snow finally melted today, so I took the 39 120 out for a test drive, having messed with the timing. I usually time it by ear, advancing it until the engine hesitates a moment before starting, then turning it back a bit. Drove it for about 20 minutes and it ran nicely after the one stuck valve finally opened up and it quit back firing. Drove it to the gas station and removed the gas cap and heard a "pop" as the tank released pressure. The cap is a new one. Do I need to switch back to the vented version? My dumb assumption is that if the cap wasn't breathing, the tank would have a slight vacuum, not an overprice. Ideas?