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Join Date: Nov 2007
Location: Chandler, Arizona
From your description, your rear cylinder is definitely flooding out, which can only be caused by the carb. Be careful running it like that, because if the rear cylinder gets enough liquid gas in it, you could wind up with a condition known hydrostatic lock. Liquid will not compress, and when the piston tries to compress it, something is going to break, usually the piston or con rod. It can also crack the cylinder and/or head, and if enough force is transferred through the con rod, it can also destroy the crank and crank bearings. I don't like the thought of pulling the carbs on a Vulcan 750, but in your case, I don't see any choice, since there is no way to tell what is going on in there without taking them apart, or at least the rear one.
Considering you recent had the carbs rebuilt, and it ran fine, there are few possibilities. You can eliminate the float level being off, as it does not change by itself. Either the float is sticking, which is unlikely if the carbs were professionally rebuilt (seems a number of Vulcan 750s had defective floats from the factory, with a small piece of flashing on them that hit the side of the float bowl, causing the float to stick, I found this on the rear carb of my then new '93), the float sprung a leak and sunk, ot the float needle is leaking, either due to damage to the needle/seat itself, or something stuck in it.
One thing you might try before removing the carbs, is to drain the float bowl and leave the drain screw open, remove the fuel line to the carb, and shoot some compressed air in there. With the float bowl empty, the float should be all the way down, and the needle will be open, so it shouldn't hurt anything, and might clear out anything that might be causing the needle to not seal properly. It's a long shot, but easy to do.
I am a motorcyclist, NOT a biker.
1997 Vulcan 750, purchased about a week ago
2006 Sportster 1200 Low
2013 Royal Enfield Bullet 500, converted to carb
2001 Yamaha XT225, heavily modified
2004 Honda Rebel 250
1979 Vespa P200E
2002 Vulcan 750 parts bike
1994 Yamaha XT225 parts bike