[TonyLoz]

I have been using the wealth of information from this site for the last 4 years. Finally my ride is running and on the road. Building this car was a gas... So here I am looking for some help from some of the weber guys. I built a Ford 408 (stroked 351), please see engine specs below. I topped the Ford off with Webers from Carbs Unlimited. The engine runs OK but is running really rich, plugs are black and it has a stumble around 2000RPM. Once past that point it pulls pretty well. It starts good and runs pretty good (with an occasional pop or fart) but seems to go down hill after it warms up. Once warmed up I am getting a loud backfire from the exhaust when backing off the gas and sometimes while at idle. The side pipes are flanged as well as the headers so I doubt that I have an exhaust leak. One odd thing, I installed the small air filters (one on each stack) and it ran awful, almost undriveable. Removed the filters and it went right back to running OK.
I have not yet changed anything in the carbs,
Main Jet 135
Air correction Jet 120
Emulsion Tube F7
Choke 37
Idle Jet F10/70

Engine specs.
Ford 408 (stroked 351W)
10.5-1 C/R
Alu Heads
Hyd Roller cam: Gross valve lift .603 (Intake) .608 (Ex)
Druation @ .050 230 (Intake) 236 (Ex)
Lobe separation 114.0
Timing set @ 16 deg at Idle with an 18 deg bushing for total advance 34 deg. Full advance at 3200 RPM.

Any and all suggestions are welcome, I really think that the issue is in the jetting.
Thanks