Might be not big deal actually
If your engine runs good down the track and the plugs appear ok then you likely have a over rich condition at idle, not a lean condition. Lean conditions appear hot on the plugs under full power not apparent at idle. If your header is hot at idle then you are likely rich and possibly late on timing a bit to boot. The late timing will allow more fuel to have to finish burning in the header. Rich mixtures burn longer looking for oxygen to finish burning often in the header.
Causes could be as simple as a blown or too high a number power valve. If you do not run one or them then a rich adjusted idle mixture screw or 4 will cause this too. Leaking in the idle circuit, high float level or leaking needle and seat are possibilities to check. The timing set too low at idle for your engine will enhance the header heat if an over rich condition exists at idle.
Food for thought
Bruce