Commando exhaust

Hi Steve. Yes Iâve also seen this phenomenon. I also had a new 750 Commando and there was a slightly more blue / purple pipe on one side. (Canât remember which) The bluing did not go too far down the exhaust pipe though.

Blueing, as you know, is caused by the temperature that the pipe reaches, the higher the temperature the more blue to purple the metal turns.

So you have to work out which one of the following is the most likely and which one is not likely.

1) The steel of the pipe is thinner on the darker pipe than the other one.

2) The bluer exhaust pipe is more shielded from cooling air.

3) The darker blue side has a mixture that is more lean or weaker than the other one.

4) The darker blue side has an ignition timing that is more retarded than the other one.

5) Slight discrepancies on the porting or inlet tracts that cause a different swirl or flow in the combustion space.

6) A combination of any of the 4 above.

Logically, you can almost certainly rule out the first two.

Point 3 and 4 have a fair chance of being the culprit/s.

Weakness of the carb: Obvious checks of jet sizes, needle clip positions, pilot screw settings, and often overlooked is the fuel float level height. A spark plug check prior to doing the carb checks might give you a pointer to any weakness bios for the left and right. Additionally, the air filter might be offering slightly more air resistance to flow on one side to the other.

Make certain your ignition timing is identical for both sides. Check with a strobe up and down the rev range. You should with a little effort get them very similar if not identical.

After your pipes have blued, you may well find that even if you have found a problem and rectified it you are not going to get the darker blue one return to match the other, in effect the damage is done. You might be able to get some lightening by using Solvol Autosol to polish out some of the blue however.

Some Jap bikes actually jet the carbs slightly different from side to the other due to inherent air flow problems, so if you were to fit new pipes, I would monitor them closely and correct at the first sign of different amounts of bluing occurring, by changing the needle clip position on one carb to balance the other. The easiest way would be to take your bike to a workshop equipped with exhaust gas analyser (performance car garage perhaps) they will be able to see what exhaust gas differences you have and they would be able to advise you.

Les H