For your Spitfire Hornet, Lund's ought to work fine. I've used them twice for Concentrics, and they worked great. I've never had them do Monoblocs. The erratic running, whatever that means, maybe jetting/mixture adjustment. Lund's work is more for sticking slides and fast idling

1967 BSA Wasp
1967 BSA Hornet (West Coast Model)
1967 BSA Hornet (East Coast Model)
1968 BSA Firebird Scrambler
1968 BSA Spitfire Mark IV
1965 Cyclone Competition where are you?