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.

[bbrock]

I would be curious if either was using any sort of A/F measuring device? or their ears or some other magic

Tomlinsons definitely was not. The book was published in 1993 and I don't think there is mention of wide band in the book. The ACN advice comes from their instructions from How To Use a Wideband To Tune Your Carburetor(s) On The VW Flat-4 Engine linked earlier so he definitely was. Here is what he says about it:

Note we do NOT want to set idle mixture for highest RPMs. Chemically the highest RPMs will be at “stoic”, but this is also unstable. 1/2 turn richer will be stable and still pretty lean. Setting idle mixture to stoic (highest RPMs) will result in an unstable engine, the engine idle speed can drop if you just turn the headlights on, or the idle speed will change as the temperature does. It will drive you crazy. On an engine with no choke, if you simply tune to 1/2 turn richer than “stoic” (fastest idle) the engine will be much happier and more predictable. A properly set idle mixture will likely pop and fart a little bit when dead cold, this goes away after 30 seconds or so of running. We want to get a steady idle speed in all conditions (hot / cold, dry / wet) and to achieve this, we simply adjust the idle mixture a little richer than “ideal”.

Sounds like you are making progress. You might want to try different emulsion tubes. I don’t have my Weber book, so I can’t make suggestions. I know I’ve used F3, 7, and 11. FWIW the different numbers are relational to nothing, that’s why you have to look at the chart.

I thought about that, but since I will hopefully be ditching the carbs for EFI over the winter, I think I'll call it good if I can get the mix in the 12-14 range. If I were going to keep the carbs permanently, I'd for sure be chasing that rabbit.