Forum bmwbike.com

Even with new later type carb tops my 69S left hand carb leaks at the drop of a hat-even parking the bike on the centre stand wont stop a pool of petrol on the ground unless you turn off the fuel some time before coming to a halt.
I saw a Dutch enthusiast BMW rider who had somehow modified his carb idle jet outlet with a plastic tube positioned upwards so the excess fuel just stayed in this tube, and then went back in the carb when next the bike was run-anyone have a suggestion to solve this highly irritating problem?

I always close the petcock about a minute before stopping the engine. This is to allow the running engine to consume most of the gas in the float bowls.
If this is not done, the gas in the float bowls will rise and cause the carbs to overflow because:
1. Gas remaining in the bowls will become hot from engine heat and expand.
2. Gas remaining in the fuel lines above the carb will gradually flow down into the float bowls.

It is not clear from your description where the plastic overlfow tube was attached to the carb. It sounds like the screw covering the hole for the idle jet was removed, and the plastic tube was inserted in place of the screw. I wonder how the tube was sealed to the carb.

Point taken Allan re: closing petcock before stopping-expansion of hot gas etc. but other bikes with semi downdraught carbs dont suffer from this problem-the /2 BMW is the first bke I've had which has a leaky carb when you stop. The modification I mentioned looked, and I didnt have a chance to closely examine it, as if the tube had been stuck over the idle screw jet and its cover, so that the hole in the end of the jet for overflow fuel was within the tube. The tube was routed upwards and had a plug in the other end-to stop vapour escaping I suppose, so that the whole assembly acted a bit like those aircraft sight gauges for fuel tanks.