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Tech Corner

Technical Q and A for classic motorcycle maintenance and repair.

Norton Commando Roadster Idling Problem

Motorcycle Classics tech expert Keith Fellenstein

Editor’s note: If you’re having trouble with that old Suzuki, BSA or BMW, Keith Fellenstein is your guy. From motorcycle tuning tips to detailed motorcycle engine repair, he can draw from a wealth of experience to help guide you to success. Send questions to: Keith’s Garage, Motorcycle Classics, 1503 SW 42nd St., Topeka, KS 66609, or send an e-mail with “Keith’s Garage” as the subject.

Norton Commando idling problem

Q: About two years ago I bought a 1974 Norton Commando Roadster that was  in excellent original condition, with one exception: The left cylinder doesn’t fire regularly at idle. I had the original ignition upgraded to a Pazon electronic ignition, but the problem remained. I replaced the original Amals with new ethanol-resistant Amal carburetors, and still there was no improvement. I also tried reversing the coil wires to see if the problem changed sides, and the left cylinder remained anemic at idle. I recently replaced both coils, plug wires and plugs, but the problem remains. The old plugs indicate rich running on the left, which I think may be due to no fire at idle. The bike runs great otherwise, and is a joy to ride. I’ve compensated by using the idle mixture set screws to maintain a slightly higher idle, but the right cylinder is carrying the load at idle. Do you have any ideas why this is happening after installing new Amals and the Pazon ignition? Emmett Fox/via email

A: Having chased this problem through all new equipment with no resolution must be maddening. It also makes it hard to diagnose, as you have already taken all of the easy answers out of this equation.

The first check is compression: If that cylinder is low, that could be the issue. The only other easy fix that I can think of would be to run a separate ground wire from the cylinder head (under a head steady bolt perhaps), directly back to the battery. I’ve read of poor grounding resulting in one cylinder firing in a wasted spark ignition where you’d think they would both have to fire due to the nature of the system. Adding a direct ground fixed that problem. Now the other possibility is that for some reason the left side isn’t getting the right mixture at idle to fire properly. Does the left side kick in if you pull the slide up on that carburetor only? If so, then you have a problem with fuel, not fire. I’ve also read enough about the new Amals to know that you can’t take it for granted that they are free of machining swarf as delivered from the factory. It’s worth your time to make sure the idle passages are clear and working. MC