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]

Slight bucking at low speeds? How low and @ what RPM? What do your plugs look like? Drop down a gear or speed up and stay on the main circuit. It will hurt your MPG a bit but so what? That's the hand 100+ year old technology deals you.

I've said bucking but I think a slight surging is a better description. As long as I drive a steady 20-30 mph, you can feel the power hunt maybe a second or two between oscillations. I try not to lug the engine and keep it above 3000 rpm. Standard residential speed limit in our town is 25 mph and I'll go as low as 2500 rpm cruising those, but no lower. MPG is more important to me than HP. Fuel economy on this engine has been less than ideal, especially when I do much city driving. Of course, the ultimate solution will be to ditch the carbs. The more I use them, the more I hate them. I know some people love their carbs. I'm not one of those people.

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.

Okay, that makes sense. It does bring me back to the idle jets though. Larger jets would let it suck more fuel (wasted) on decel - no? I assume most city driving is happening on idle and progression ports and that's where I'm running rich. I feel like a better number for those slow speeds running on idle jets and progression would be 13s or even low 14s since the load is light.

[Superhawk996]

I assume most city driving is happening on idle and progression ports and that's where I'm running rich. I feel like a better number for those slow speeds running on idle jets and progression would be 13s or even low 14s since the load is light.

Yes, would agree under light load city driving, you're definitely on the transition ports.

I might be inclined to try a larger air correction jet.

Stealing directly from Performance Oriented w.r.t. air correction jet:

Larger orifice diameters shorten the duration and leans the fuel mixture delivered through the idle jet.

Looking at how NARP is calibrated at altitude, he's larger on fuel jet and larger on air correction. It very well be that both need to change to get what the engine wants.

But, if it were me I'd stay where you are on the idle jet and go larger on air correction trying to lean out the idle circuit a little and maybe move you to the main jet a little sooner. However, I think the problem you'll have is that if the air correction leans it out too much - you're going to end up backing out the idle needles more. The more you move them out the less effect they have as you get the needle taper further and further out of the needle bore orifice. You're already two turns out so I'm not sure you want to go much more. That may very well be why NARP ended up with a larger fuel jet AND a larger air correction jet.

Having said all that . . . you're not going to be happy until you have FI given your OCD nature ( (IMG:style_emoticons/default/grouphug.gif) ) and what you're trying to achieve (Fuel Economy). Per my 1st post, you're using a highly sensitive modern tool (Wideband AFR) to try to tweak an ancient technology (carbs) for Fuel Economy. You're bound to be left wanting what you can't have some where in the operating range.

@bbrock