Hi - years ago I had a commando that would idle very slowly sounding beautiful. I now have an identical machine (MkII 1974) but I cannot get it to idle evenly. On one cylinder - no problem - nice slow idle. Bring in the second cylinder and the idle becomes erratic. It sounds as if they take it in turns to misfire.
The only change I have made is to siamese the inlet and use just one of the carbs. with a larger main jet courtesy of Les Emery. I get a lot of disapproving looks but to be honest (apart from the rough idling - which might not be anything to do with it) it works great and saves alot of setting up agro.
Any advice on the idling - I would love to be able to relax when I'm waiting at the lights....
Cheers - John
First you need to find out…
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You are passing twice the…
You are passing twice the volume of air through the carb so the pressure across the pilot jet will be completely different. That combined with the slide cutaway will control your tickover. The standard concentric has a pilot jet fitted inside the body you can remove it by drilling it out and fit a screw in pilot jet to the underside of the carb body. To qote the American tech digest, " A single concentric will NOT flow enough fuel for a Commando" 34mm Mk2 Amal or Mikuni is the way to go for single carb but you will need a stub mounting manifold to fit these.
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First you need to find out which cylinder is lazy, or which one is good, easy to do with seperate silencers, just feel the pulses at the back.Try swapping over plugs/leads to see if the problem moves,if it does you have a faulty plug or lead/cap. If the problem stays put then check the tappets and use a compressoin gauge to see if alls well .Could also be ignitonsystem or coil and i would take the plugs out --earth on the head and compare sparks,could also be a bad cam --worn on one lobe or bad made. Possible to check lift and timing same on both sides.Check plugs for oil fouling on one side.Also possible to have mixture bias due to manifold shape but most unlikely to affect tickover.