Jetting Webers on 2.7 911 engine., Tuning. Options

[914Toy]

I recently installed a "double" Innovative air/fuel (A/F) gauge with sensors in both headers of my 1977 2.7 911 engine, which is stock except for: ignition, Weber carbs, reground cams (modified SC) for spirited street performance with the carbs, and exhaust headers (MB911). The A/F gauge has assisted final tuning, including fine balancing of the carbs (well worth it).

Ignition is Clewett crank fire with ignition timing set at: 10 deg. idle 800 rpm, 29 deg at 3000 rpm, and 33 deg at 6000rpm.

Carbs are Weber 40IDA's with 34 main chokes and tall secondary chokes. Jets are: 145 main's, 180 air correction's, F3 emulsion tubes, and 60 idle's.

Engine is running very smoothly at all rpm's, no carb "spitting" or exhaust popping, no hesitation under any acceleration, and instant accelerator response with plenty of power. In other words, running great. However, A/F's are not perfect with 10.5 at idle, 12.5 to 13 at cruising, and high 13's up hill WOT. On a recent uninterrupted 180 miles run on 101 at 3400rpm (78mph), fuel consumption was 24miles per gallon (IMG:style_emoticons/default/smile.gif)

I have discussed the A/F ratio issue with several carb "experts" - mostly Old School guys. Consensus is my numbers are as good as it gets vs. near perfect 14.7 A/F one can expect from EFI.

Any comments will be welcome.

[914Toy]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

[72hardtop]

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

The graphs you posted to are a bit incorrect.

The first graph say fuel enters main jet into idle circuit. That is incorrect. It enters the idle jet circuit. The main jet does not meter fuel for the idle jet or circuit.

On IDF's the air entering the idle circuit flows in from the idle air bleed ports not idle air correction jets as the graph refers to.

The main jet on IDF's is at the end of the emulsion tube.