Many thanks for sharing your experience which is very much appreciated. I checked all of them on my engine but found everything okay (as far I can judge). On the week-end I started playing with the jets of the secondary and found that the misfiring rpm is depending on the jet size of the secondary. I got following: Jet size 74 provides heavy misfiring @ 4700 rpm, 78 misfiring @ 5200 rpm and 81 light misfiring @5500 rpm (I probably need to go up to 85 to shift misfiring to 6000 rpm). Seems that the mixture is leaning out with increasing rpm. The jet size for the prim is 70 and there is no misfiring at the full range of rpm. Was reading that leaning out at high rpm can be corrected by the air bleeds (smaller size provides richer mixture at increasing rpm) but on my carburetor those air bleeds are pressed in and cannot be changed. It is still difficult to understand why the required jet size between prim and secondary of my carburetor are so significant in size (Holley advises that the maximum jet size difference should be in the order of 4)
Brgds
SIG