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Hi everyone i am currently undergoing a starter clutch, head gasket, possible valve job, and carb rebuild i am waiting on the parts and gaskets ordered everything today 350$ for all the parts......anyway. The petcock that come on the bike i believe is aftermarket not stock. The bike is a 1977 KZ750B (twin) the petcock only has one fuel outlet on it and runs lean. There is a T in the fuel line between the carbs. when i am cruising 70+ the bike wants to putter out, i pull over and idle or just pull in the clutch for about 15 seconds and get back into it its fine for about 2 mins. City driving is fine but highway driving is fine as long as i don't exceed 55mph for too long. could this be a venting problem because the one fuel outlet off the petcock? I see they make a double outlet petcock for that bike i will invest in once i do the carb rebuild. thanks for the knowledge and help.

Did you check if the vent in the cap is clear? Try opening the gas cap and see if it clears. Could be the petcock is too small.
Steve

My 750 twin was doing the same thing, turned out it was too small of float valve. I had 2.0 in there, I switched to 2.5 and it fixed it. The BS38 carbs use the same float valves as the 29 smooth bores so you should have no trouble finding some