Hi joea,
I'm in the process of dumping my Demon 850 vac sec from my 521
It is being replaced with an MPFI mostly from Holley. Ask me again next month if I like the MPFI
The carb gave me the most enviable idle - great noise and vibration - verrry intimidating to anyone around the car
Alas, it had a hesitation I lost patience with. Probably some carb Xpert could have made it perfect, but mine ran out
Also, there were several threads stripped during the dis- and re- assembly, in the air horn and bowl that required heli-coils - altogether frustrating
Don't know if this helps but there it is
Tom