If that atmospheric vent to the float bowls is restricted the fuel flow dynamic is upset. It's the steady atmospheric pressure that pushes the fuel up through the jets as the pressure is lowered in the venturi, very much like a wing gets lift. Rob the bowls of that pressure and there's not enough fuel introduced into the intakes. You should get enough discoloration ont the plugs to see a difference, it's just way more subtle without lead.
You are correct, very very lean builds up a ton of heat and that's the enemy of air cooled motors. But a short ride on clean plugs should be okay and give you time to check it out.
