Generally, blue pipes can indicate weak mixture, retarded ignition and / or poor quality chrome. One problem is that lots of our bikes are non-standard, the obvious example is that lots of us run without air filters which makes a big difference to the main jet you should use. Also, lots of us are running round with worn carbs, or concentrics which naturally run richer so need smaller main jets. An additional issue is, of course, today's fuel - otherwise known as P155, which requires the ignition to be retarded slightly, but overdoing it will cause the pipes to blue. One slightly 'left field' suggestion is, have you checked your points gap recently? If the gap has closed this will retard your timing and cause the engine to run hotter. In my experience blue pipes are caused by retarded ignition more often than weak mixture. I'd be interested to know what the spec is of your engine, i.e. which, cam, pistons and cylinder head?
