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]

I'm not understanding how increasing idle jet will decrease fuel in the mixture. Does not compute...



I was looking back through my notes about the air correctors, but then remembered the
air correctors are in the main stack which would only affect the main circuit. You can actually remove the air corrector jet which will be the same as adding a 300 air corrector. This will bring in the mains sooner, but it is only a test to see if that will clear up the transition issue.

I did however stumble across this exact problem in a discussion with John ACN. I was getting 11s in the just off idle light throttle. I was running 50s at the time and dropped them down to 47.5 and it brought me into 12 to 13.

[bbrock]

I'm not understanding how increasing idle jet will decrease fuel in the mixture. Does not compute...



I was looking back through my notes about the air correctors, but then remembered the
air correctors are in the main stack which would only affect the main circuit. You can actually remove the air corrector jet which will be the same as adding a 300 air corrector. This will bring in the mains sooner, but it is only a test to see if that will clear up the transition issue.

I did however stumble across this exact problem in a discussion with John ACN. I was getting 11s in the just off idle light throttle. I was running 50s at the time and dropped them down to 47.5 and it brought me into 12 to 13.

Very interesting... It will be interesting to see how this plays out on my car. Right now I don't feel any transition issues but assume that might be because of all the surplus fuel swirling around at transition. It is easy to see when it switches from pig rich at 11 to 12-13s watching the AFR but it feels like it's pulling smooth and steady through the whole range.

The only way I could see larger idle jets would lean out the mix is if the extra fuel lets you screw in the mixture screws to be really stingy with the now even richer mix. I still haven't been able to grok how those work together. The ACN article does mention that a .50 jet can produce leaner idle than .80 but doesn't explain the mechanics.

The .45 and .55 idles should be here Saturday so am prepared to go either way. CB didn't offer a .475 so if I have to go smaller and the .45s are too small, I'll have to come back up a step (or is that a half step?).