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Hi, I’ve a ‘72 CB450 and after a long ride I got back and noticed air bubbles bubbling back up to the tank on the left fuel line, with the engine off. Anything to be worried about? Also the idle is weird sometimes too high and sometimes it stalls?
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Air being sucked back into the tank through the fuel lines usually means a vacuum is forming in the tank. Check the vent in the cap for blockages.
 

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Depending on the seasonal blend and the additives used, gasoline can begin to boil anywhere from 90 to 400 degrees Fahrenheit. Sitting there, after a long ride in hot weather it's likely those carbs are hot enough to boil the fuel and send a few bubbles into the lines. Ditto the idle settings, that's an air cooled engine, you can't expect it to idle for very long in hot weather without things changing.
 

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I guess it depends on how long after stopping you noticed the bubbles. Carbs are cooled by the vaporization process, so are initially cooler than air temps, so if the bubbles occurred right away, you have a tank vent issue. After sitting several minutes, however, they will obviously heat up, perhaps enough for the gas in the carbs to bubble, but I would expect this to exit the carb vents, not up the fuel lines, as the float valves should be closed, and the vents should prevent a pressure build up. Even if the gas boils off enough to open the float valves, however, there would have to be a fair amount of pressure built up in the carbs to push against the weight of the fuel. I have never observed bubbles like this in a properly vented carb and fuel tank.
 
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