Need Carb Jet Guidance, Weber 40 IDFs Options

[bbrock]

Plans to Megasquirt my car got delayed over the winter so my Webers will stay on for another summer. So, I finally hooked up my wideband to see if any adjustments are needed to keep the engine happy and safe until I can do the conversion.

The only real "problem" area is idle and low speed driving on the progression circuit where it is running too rich. It idles smooth and steady at 700 rpm with AFR of 12.3. That with idle mix screws ~ 2 turns off their seats. That's the leanest I can adjust and keep the engine happy. Just off idle at 20-40 mph @3000 rpm it goes even more rich to around 11.0 +/- 0.5. I don't think that's alarmingly rich but wasteful and I do get a bit of annoying surging/bucking driving below 30 mph.

Current jetting is as follows:

Venturi - 28
Emulsion tubes - F11
Main jet - 115
Correction Jet - 200
Idle jet - 50

I'm thinking I should reduce the idle jet but not sure by how much. I can get .047, .045, or .040. Any suggestions?

I could also use some advice on mains and/or air correction jets. If I didn't plan to swap the carbs to FI, I'd try for a little leaner highway cruising mix for better economy and maybe flexibility. In limited testing running in good ranges for performance at the expense of economy - WOT ~12.5, Highway cruise ~13.5 but rarely goes above 14.0 even on downhill and decel. Weather is crappy here this weekend so I've only done a short drive in hilly conditions. I need to get it down in the valley for some long level runs to see where it settles. I think I can live with the numbers I'm seeing now, but do wonder what it will do when I attempt to cross a 10,000 ft. pass. I'd rather it not die of oxygen starvation if I attempt it. Any thoughts?

BTW, I live at 6,000 ft. elevation so I'm not surprised it is running on the rich side with out of the box jetting.

[nditiz1]

My earlier point about setting the bypass screws 1/2 open to start as Tomlinsons suggests is that it would reduce the butterfly opening needed to achieve the desired idle. In which case, less likely to expose the the progression ports. I can also see how it could cause the lag that John mentioned on that Shoptalk thread since I suppose you could end up with the butterflies so closed at idle there is a bit of lag between squeezing the accelerator and exposing the first transition port. I'm not saying I think Tomlinsons' procedure is the right or wrong way to do it, just stating what the effect is. I actually don't think my idle speed screw is anywhere close to a half turn out but will follow the aircooled.net procedure to make sure. That procedure makes the most sense to me because you wind up with throttle plates precisely where they should be.

I'm not sure I agree with this. The air bypass screw is not there to increase idle, but to balance flow. It is the same effect as drilling a hole in the butterfly, but not opening it. When you open the butterfly you expose more of the progression ports as rfinegan stated, which in turn is drawing more fuel and air into the throat.

Idle is the last thing you want to account for.
Balance
Best lean mix
Adjust idle
Maybe do best lean again
Finally adjust idle once more.

Each time you touch the idle screw you change the equation as to how much of the first progression port you expose. Which is why you will need to adjust the mixture screws again.