I agree, 20° BTDC is too much for an engine with mild cam timing. This will kick back and fight the starter, and "pop", and (unless the advance curve is slow) ping on hard acceleration at low speed.
Typically, between 5-10° is closer to what the engine wants.
With significantly longer cam overlap more advance will help idle quality and tip-in response. Overlap does not completely "flush" the chamber at low speed, and the remaining exhaust gas slows down combustion.
