Weber 48IDA Basic Education & Help Needed!

[Ragtime Kid]

Hi all,

GT40 guy new to this forum, but previously built and owned FFR Cobra!

Just completed a 3 year build of an RCR GT40 MkI. Power is a 363ci small black Ford (Dart block), 10:1 compression, with 4 x 48IDA Webers and MSD 6AL ignition. Carbs, intake manifold, and cam were all spec'ed by Jim Inglese and shipped directly to Craft Performance Engines who put the motor together last year. According to Craft's dyno, it is producing 467HP and it starts, idles, and runs smooth and cool following initial issues with fuel percolation preventing re-start attempts that were solved with phenolic spacers.

Carb specs are:

Idle Jet: 65
Idle Jet Holder: 100 (F10)
Main Jet: 155
Air Corrector: 200
Emulsion Tube: F11

The question I have is: Why is this car getting 4.1mpg?? The jetting was all specified by Jim Inglese and re-confirmed with him to be correct for this motor. Running 2.5psi fuel pressure and shows no visible smoke Produces great power. I have been driving it fairly aggressively (revving to 5,000+ rpm consistently for each shift) but only because I notice the engine is smoothest and happiest there. Very twitch/jerky when rolling along in 2nd or 3rd gear at 2,000 or so RPM.

Here are some specific questions and concerns I am hoping those with Weber IDA experience can help me address:

1. Is 4mpg fuel economy insane or about right for a 6L motor like this? Wouldn't I notice if it were crazy rich? I can run my finger inside the tail pipe and it comes out black and somewhat greasy -- is that normal?

2. I have a crazy amount of backfiring through the bundle-of-snakes exhaust on deceleration following hard acceleration. In other words, when I gun it, then let off the gas, there is a (glorious) cacophony of snapping and crackling through the pipes complete with flames shooting out. No amount of fiddling with the idle mixture screws changes this.

3. The carbs sat for a year after the engine was built and there was varnish that caused the needles to not seat and fuel to flood through the accelerator pump jets. The carb has been disassembled and cleaned since then and all pump jets function as they should.

4. The idle speed is steady around 900-1000 rpm but occasionally after a spirited run, the idle speed will creep up to 1300-1400 for a while then work it's way back to 1,000 a few minutes later. What could be causing this?

My biggest concern is if the carb is running crazy rich and washing the lubrication off the cylinder walls and hence the 4 mpg. I also suspect a slightly dragging parking brake which I am going to address this weekend but can't imagine that is having a massive effect on fuel economy. What is a reasonable MPG I should expect from a powerplant like this?

Thanks very much in advance!