I wrote to another fellow 2old4this,but after seeing that you were the expert I had to contact you.Bike is a 1978 CB400T store for past three years.Son wished to resurrect an dtodate has spent approx 400.00 on bike.It simply after 3 different Bike shops and abot 4 carb cleaning and re-seting bike performs fotr abot 20 minutes then stalls.Has to almost walk it home.There was some black smoke.It acts as if either gas or fire is not happening.tank has been cleaned and interior coated.All the so-called experts after charging him 100.00 say all is a-ok. He is a young fellow and does not want to return to these shops.Can you suggest anything??? All advice would be extremely appreciated.