Phil,I had a similar problem.It turned out to be a perforated diaphragm and the right carb vacuum could not lift the piston to lift the needle needed to increase fuel into the intake system.There were two invisible holes the size of match heads.Holding the diaphragms up to light to look for holes (as the books say )is useless as the elasticity of the diaphgrams keeps the holes closed until under vacuum.Gently stretch the diaphragms, with your fingers ,working your way around.,to expose any holes.I installed new diaphragms in L and R carbs and the problem was solved.
Best Regards, Eric. South Africa
