If the valve seats wear, possibly due to no lead, the valve clearance reduces. It can get to the point where the valve edge burns away. That allows unburnt fuel into the exhaust system and can cause back fires. The higher the revs the more fuel by passes the exhaust valve. A head overhaul may be on the cards. Getting another head converted with hardened seats and new valves allows a quick change over. Check the cam with a dial gauge.