Too lean ?

Dunno. It’s obviously stone cold. It would be strange to set a full on race bike to run well from stone cold wouldn’t it...?
 
It’s a racing 500 single with a megaphone exhaust. What do you expect!

The immediate cause of the misfiring may be a rich condition from exhaust and inlet reversion at lower rpm.

Also, if you manage to get megaphone sonic tuning to actually work at peak rpm, it does the opposite of working at some lower rpm.
 
Yup, racing cams and megaphone exhaust don’t help.

Neither does having a stone cold engine though, note how the guy literally grabs a handful of cylinder head fins (46-50 second in), right next to the exhaust port.

Probably got a sump full of treacle too (sorry, I mean cold Castrol R).
 
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Even if the motor is stone cold, it should not require fanging the throttle to keep it running. In fact, with my own bike, I let it idle with the choke on before I rev it. Even if that motor is fitted with a GP carb, there is a slide which richens the mixture. I don't like revving a motor which is cold inside - might damage a piston.
I've had a few friends who owned Manx Nortons. I've never heard a Manx which sounds like the one in the video. Lean is usually good, but too lean is bad. If my bike sounded like that, I would expect it to damage itself internally.
 
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