backfire... too-rich or too-lean?

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Jun 1, 2003
I am trying to tune the carbs on my newly acquired Commando 73 750. It keeps backfiring (when the throttle twist is released after revving up).
I checked the exhaust and muffler nuts, they're OK. I tried tuning the carbs but all settings to give the same results, not even sure if it's too rich or too lean...
The bike was running relatively Ok when I got it, the only thing I did is add gasoline (96 octane).

any idea?


My bike started backfiring badly the other day. I found that the balance tube between the inlet manifolds had fallen off.

It is worth checking to see if the tube has perished - heat, age and of course vibrtion get to the rubber tube in the end.
Lean or Rich

It's very easy to tell if you're too lean or too rich. Take out your plugs and examine them. If one or both are black and sooty, you're running too rich. If grey or white, your're too lean. To solve this first make sure you have the right set of plugs. I know for a '72 Commando I use NGK BPR7ES. You must also make sure the gap is correct. For mine it's about .25. Once this is done you may have to make individual adjustments with the Pilot Screw located on the side of your carbs. There are two such screws on each carb but the one you want faces directly at you, not the one pointing downwards. You should start with the screw 1 1/2 turns from the fully in position. From this point if you wish to make the mixture richer you turn the screw inwards and vice versa for leaness. This a trial and error procedure that will take time to correct. Remember though that it is always better to be too rich than too lean. If you are the latter you could damage your pistons. The colour you are looking for is a light rust colour.
You probably solved the problem by now, since you posted back in June, but for anybody else that comes in here, here's something simple to check.

I had problems with my 71 Norton backfiring and running rough (wasn't too hard to start though), that ended up (after replacing condensors, etc) being rotted O-rings on the amals set screws (jet and throttle). Air was getting by them. I thought for sure it was flaky electrical at first, cuz it was doing at any speed/rpm

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