Run the same timing with gas or alcohol in a flathead -- it's so low on compression that you're not giving up any power.
6* key may be more than what's needed for best power, or it may be just fine. Different coils retard different amounts, piston speed/velocity, crank keyway, flywheel keyway, etc also determine how much advance is required.
Typically a 6* key puts more timing in a flathead than I want (I generally don't use keys, I lap the flywheel to the crank.) 6* key would put you closer the 30* timing range on most flatheads I've seen.
You're probably close enough.
Watch your temperatures and don't overheat the engine.
You may need to back some timing out if that's the issue, but you won't see detonation or knock with a 6* key and stock rotating assembly due to such a low compression ratio.
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Brian Carlson
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