When I bought my new 426 I put on an FMF PCIV s.a. on it. I live at 5000 feet, the dealer told me they had jetted it for this altitude #158 main, well it didn't run for s!!t with that jet, I actually was lean fouling plugs. It wouldn't start for jack either. I ended up with a #168 main (SEA LEVEL)!! Now it starts second kick every time and doesn't backfire! No other adjustments were made to the carb! Woo Hooo!