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I only ever use plug readings for finding the correct main jet.  So I run the motor on full throttle for a substantial distence.  If I had one plug which looked as rich as that I would suspect either a leaking valve (low compression) or a carburetor which is affected by vibration (fuel frothing).  I set both of my carbs to be identical.  When I set the idle, and when I set the needle position, I do both in the same way - I induce the cough by leaning off, then enrich slightly. My main jets are usually slightly too rich, so  when I give the bike a real blast, I do not burn valves or pistons.

If you read LAB's document on Amal carbs, it says most of our riding is done using less than one-third throttle. If you burned a valve while doing that, you motor would have been coughing and spluttering for miles.


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