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]

Nothing new to add but I did some reading (yes, dangerous) about the air bypass screws that led to some philosophizing. I'm a curious guy and need to know how things work. I was curious whether the bypass screws could be used to lean out the idle mix (as opposed to the mixture screw which only adjusts the volume of the idle mix introduced at idle). The quick answer is No, but it took me into a controversy over the bypass screws. The CB (Tomlinsons) Weber manual suggests turning the bypass screws 1/2 turn out in pretune settings which is what I followed. Most people say the bypasses should be closed except as needed to balance the idle air flow of barrels within a carb. Some predict dire effects if they are opened except to balance a barrel. Well, maybe, but probably not.

Thinking through it, the idle adjustment screws simply open or close the butterflies to get a desired idle. Open a bypass screw will raise the idle without changing the position of the butterfly. So the effect of starting with bypass screws 1/2 turn open is that the desired idle will be set with the butterflies more closed than if the bypass screws are closed. In theory, opening the bypasses could drop the butterflies at idle farther below the progression ports than if they were closed. That could create hesitation on acceleration. I don't know if the opposite could be true where having bypasses closed could result in butterflies engaging progression ports at idle which would be bad.

I'm not noticing hesitation on acceleration, but will make sure the bypasses are closed to see if it changes idle AFR at all. Unlikely since if anything, having them open a little should ensure I'm not drawing mix through progression ports at idle.

This all led to a bit of an epiphany about the Tomlinsons manual that so many of us depend on to learn how to set up carbs. It was written prior to the days of O2 sensors. Tuning was by ear and by feel. Maybe occasionally a dyno was used but even then I'll bet tuning for max power was emphasized and economy was mostly an afterthought. As a result, recommendations in that book tend to create rich conditions that will make an engine run well, but not necessarily efficiently. We could probably use an updated manual that walks through tuning carbs with a wide band. Then again, I've come to the conclusion that other than for nostalgia or originality, going FI is the better, and ironically IMO, the less complicated path. Still, I'm glad I tried the Webers. It has been an interesting journey.

[Superhawk996]

In theory, opening the bypasses could drop the butterflies at idle farther below the progression ports than if they were closed.

During bench setup, you really should have the throttle butterfly set such that there is only one hole (the lowest - idle hole) open. All the other progression holes arranged vertically should be either blocked by the closed butterfly or above the butterfly.

Part of the problem with opening the bypass as you propose is that you will end up weaking the "draw" signal on the idle circuit by lowering the vacuum below the butterfly when it is closed. Likewise it will also decrease the transition signal to some degree as the throttle begins to open.

Honestly, I have never tried what you've proposed. Give it a try. I suspect you're going to find some negative aspects to idle stability and/or transition as the throttle opens.

@bbrock

[bbrock]

Part of the problem with opening the bypass as you propose is that you will end up weaking the "draw" signal on the idle circuit by lowering the vacuum below the butterfly when it is closed. Likewise it will also decrease the transition signal to some degree as the throttle begins to open.

I think I'm going to disagree with this. What you say would be true if we weren't adjusting for a desired idle. The air bypass simply allows air to flow around the butterfly so it has the exact same effect as increasing the butterfly opening. If you screw in the idle speed adjustment to open the butterfly, idle speed increases. Likewise, if you open the air bypass screw, idle speed increases. Either is allowing more air to pass through the throat of the carb into the combustion chamber. So a change in one requires a compensating change in the other to maintain the same idle speed. I don't see how it weakens the draw since that would be determined by the total amount of air allowed through the throat (air going around butterfly + air through bypass). In other words, opening the air bypass is the equivalent of screwing in the idle speed screw except that the bypass only operates on one barrel where the speed screw adjusts both simultaneously.