Holley Street Avenger on 408 / 416

Gonna need a lot more info than that. When is the engine stumbling? Idle, acceleration, WOT? What are the specs on the engine- compression, cam, intake, timing, heads/flow? An 870 could be a lot of carb for a mild 408, all the way up to not enough carb for a hot 408. Auto or manual transmission? Was the engine running right before this carb?

I tried both a 670 and 770 street avenger on the 340 in my Duster. Couldn't get rid of an cruise-to-hard acceleration stumble with either one, and I played around with jetting, pump nozzles and cams, secondary springs, the works. All tuning was done using a wideband A/F gauge, I can see exactly what my A/F ratios look like. Finally went to a 750 DP with mechanical secondaries, no problems after some minor jetting changes. The vacuum secondaries on the Street Avengers weren't fast enough for the transitions I was getting with my 4 speed. But my 340 is pretty far from stock- .060" over, KB 243 hyper pistons, Lunati 60404 cam, duration 276/ 284, Lift .513/ .533. Compression ~9.8:1, 308 heads ported and flowing 264cfm @.500, Eddy Air Gap, Doug's headers, Holley 750 Ultra DP. It runs at 20* advanced at idle, 34* all in, and pulls 9-10" of vacuum at idle.