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]

Thanks all. The engine is stock 2L (1971cc) with european pistons getting 8.2 compression. The carbs are balanced for air flow (4 throats). The car is running great except the slight bucking at low speeds when running on the progression circuit.

@Superhawk996 I'm not chasing stoic at all. In fact, I'm completely ignoring stoic. AFR 12.5 at WOT and 13.5 accelerating or hill climbing seems good and I'm happy with that. What surprises me is that it doesn't lean out when the throttles close on decel or downhill. I would also expect a leaner mix at steady highway cruise although I need to collect more data on that. That makes me wonder if it is starving for air which is what I would expect at my elevation. Interesting that @NARP74 is running a larger correction jet than mine. That said, I'm happy with these numbers so long as the car doesn't choke climbing Beartooth Pass. I'm skeptical though. I remember the days of trying to drive from Denver to the Eisenhower tunnel in a car jetted for low altitude (not fun). And not being able to start a VW bus parked at 11,000 ft. at a remote trailhead after a week of backpacking. Call me paranoid (IMG:style_emoticons/default/smile.gif)

The real issue that I would like to address is the rich mix a low speed. @rfinegan , if I tune the idle mix by ear, I'm about 2-1/2 turns out on the screws which is right where Bob Tomlinsons' Weber Manual says is typical. However, the AFR at that setting is in the low 10s which seems way too rich for idle so I turned them back in a half turn where I still get smooth idle but a little more reasonable 12.3 AFR which is certainly not lean. Seems like a larger idle jet would be in the wrong direction. Also, there is the bucking I'm getting when driving at low speed on the idle/progression circuit. Tomlinsons says that is usually from too large of an idle jet so I'm a bit confused.

[Superhawk996]

What surprises me is that it doesn't lean out when the throttles close on decel or downhill.

It won't, and you shouldn't be too surprised. @bbrock

At decel (especially down grade) you have high vacuum below the throttle plate and you're pulling plenty of fuel via the idle circuit yet the throttle plates are fully closed and the only air is via cylinder equalization bypass and what little bleeds past the throttle plate edges.

No decel fuel shutoff like modern fuel injection.

As noted by Nditiz1 -- you can pull an awful lot of fuel via the idle circuit. Most people don't realize that they are running on the transition ports more often than the mains unless accelerating hard or under high load.

Don't know if you've ever stumbled across Performance Oriented website but wealth of information on carbs, jetting, etc. Primarily aimed at sixes but the Weber theory of operation is largely the same for IDF's.

http://www.performanceoriented.com/performance-tuning-2