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]

While I cannot disagree with Phil as his knowledge on all things far surpasses mine. Got more years on me Phil, but im still learning. Anyhow, I can't understand how the air corrector has any effect on the idle circuit. You can drive around with the main stacks gone. There would be no air corrector.

[Superhawk996]

While I cannot disagree with Phil as his knowledge on all things far surpasses mine. Got more years on me Phil, but im still learning. Anyhow, I can't understand how the air corrector has any effect on the idle circuit. You can drive around with the main stacks gone. There would be no air corrector.

I don't want to claim any sort of mastery over carb tuning. Nothing is 100% clear in any of the theory of operation that I've looked at over the years. Everyone seems to do things a little bit differently.

So here goes:

At first glance the air correction jets (on top of emulsion tubes) don't seem to be related in any way.

But look at the fuel flow diagram:



You can see the idle circuit actually pulls fuel from the base of the emulsion tube.

Printed Weber theory of operation also states this. Fuel drawn though the main jet and though emulsion tube wells on its way to the idle jet.



From there on, it shows idle jet and the calibrated idle orifice (fixed - pressed into carb body) as controlling the idle mixture (along with needles).

My analogy is that by increasing the size of the main air correction jet (even at idle) you're affecting how much (or how easily) gas is lifted to the idle circuit for a given vacuum input. I'm just thinking of the pressure balance above and below the fuel level in the emulsion tube well at idle.

Who's to say I'm not smoking dope. (IMG:style_emoticons/default/smoke.gif) No one ever says that the main air correction is related to the idle circuit per se. I'm just thinking that at idle, if I completely sealed the main air correction closed, it would be easier for the idle circuit to pick up the fuel to a point higher than the emulsion tube well. As the air correction gets bigger, and more open to atmosphere, it gets slightly harder to make that lift (IMG:style_emoticons/default/confused24.gif)

The only thing I've really learned over the years with carbs is that there really are no independent variables and that all things like idle jets, main jets, air correction jets, and even the emulsion tube hole sizes and placements are all inter related. We all know how hard it can be to get a smooth transition from one circuit to another. What governs the mixture in that gray area where you're not quite off the idle circuit but not fully onto the mains. Only God knows for sure.

I don't think there is a perfect theory of operation that ever explains it all. I'm not convinced anyone knows other than what has been learned by 100+ years of experimentation.

[bbrock]

The only thing I've really learned over the years with carbs is that there really are no independent variables and that all things like idle jets, main jets, air correction jets, and even the emulsion tube hole sizes and placements are all inter related. We all know how hard it can be to get a smooth transition from one circuit to another. What governs the mixture in that gray area where you're not quite off the idle circuit but not fully onto the mains. Only God knows for sure.

I don't think there is a perfect theory of operation that ever explains it all. I'm not convinced anyone knows other than what has been learned by 100+ years of experimentation.

Which is why I don't quite follow the "carbs are simple and FI is complicated" logic. Sure, FI has a seemingly complicated array of sensors, but each is doing its discreet small part to measure factors influencing combustion and feeding them to a programmable processor that for the most part just opens and closes a collection of valves at the right time. Carbs have this strange alchemy of fluid dynamics of both liquid and gases swirling through a Rube Goldberg contraption for trying to meter a spray that is close enough to some ideal.