Motorcycle Repair: 1985 Yamaha xj700, butterfly valves, fuel tank

Question
I recently bought this bike which had been sitting for about 18 months. I changed all the fluid,cleaned the fuel tank and lines,and cleaned the carbs. Now it starts right up and runs great above 1500 rpm. However, after riding a few minutes, the engine doesn't want to idle down. It's like the throttle is sticking ,but it's not.I can hold the brake and ease out the clutch,and it will idle down. Any ideas on this? Thanks

Answer
I have had this same problem on other older bikes.The problem I found was vaccum leakes around the shafts through the body that the butterfly valves pivot on. Sometimes they cause the linkage to bind up not letting the rpm,s to drop off quickly,its usually creating a vaccum leak.Rarely one can back out the mixture screw 2-3 turns and this will help but not cure the problem.