One carb, One side lean

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Sep 12, 2008
I am working my way through another issue with my newly acquired 74 Commando. I wrote about my carb set up (one 32mm Amal) in the topic,"Is one Amal Enough" on Oct 5.
I have experienced one pipe bluing so I did a plug check after running the bike. The left was a nice tan color and the right ( side with the blue pipe) was white with a white haze on the side electrode.
My first thought was a lean mixture. I richened the carb by raising the needle and screwing in the air/fuel screw to see if that made any difference. It didn't.
I switched plugs and the same result occurred. Left-tan, right-white.

I'm running a Boyer which, if I understand clearly, wouldn't cause one side to advance over the other.

I also checked the tightening of the carb and manifold for leaks. I can't reason a lean one side from one carb, but I'm new to these bikes.
Thanks in advance for any help.
Gotta be an air leak, unless somehow you put a hotter plug in one cylinder than the other....even so, that should mostly show up in the ground strap, as opposed to a "lean" reading plug....good luck
with the engine hot & idling try spraying `crc' or similar flamable aerosole lubricant around the manifold to head joint and see if the revs change, indicating an air leak. The fuel distribution can`t be that bad,although it may not be perfect.
Another possibility would be induction bias, caused by unequal porting of the single carb manifold or inlet tracts?
I believe some single carb BSA twins suffered from this problem, and a tapered "bias gasket" or plate was used to angle the carb slightly in the direction of the weak cylinder.
Worn valve guides can provide a air nice leak too. But start with the easy fixes first.
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