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.

[Jake Raby]

Most carb issues are really ignition issues. If 50 idles are too rich and 45s are too lean, try 50s with more ignition advance. All my engines have wanted more timing at elevation, and modern fuel exacerbates this.
ALL the old carb jetting guidelines are superseded today, you can't use anything that's been written in stone for the last 50 years. Today's fuel likes timing, and more jetting.

[bbrock]

Most carb issues are really ignition issues. If 50 idles are too rich and 45s are too lean, try 50s with more ignition advance. All my engines have wanted more timing at elevation, and modern fuel exacerbates this.
ALL the old carb jetting guidelines are superseded today, you can't use anything that's been written in stone for the last 50 years. Today's fuel likes timing, and more jetting.

Thanks for the tip! That would be wonderful it if turned out to be a timing issue. I'm chicken to advance the timing much beyond stock spec without guidance from someone in the know.

It also occurs to me that the readings I took with the 45s would have been when the old plugs were beginning to foul. Wondering if it would be worth throwing them back in to see if they still read lean.

[Jake Raby]

Most carb issues are really ignition issues. If 50 idles are too rich and 45s are too lean, try 50s with more ignition advance. All my engines have wanted more timing at elevation, and modern fuel exacerbates this.
ALL the old carb jetting guidelines are superseded today, you can't use anything that's been written in stone for the last 50 years. Today's fuel likes timing, and more jetting.

Thanks for the tip! That would be wonderful it if turned out to be a timing issue. I'm chicken to advance the timing much beyond stock spec without guidance from someone in the know.

It also occurs to me that the readings I took with the 45s would have been when the old plugs were beginning to foul. Wondering if it would be worth throwing them back in to see if they still read lean.

You have to make the engine happy. That means throwing everything at it that you can, and note the results of each. Nothing runs worse than a rich engine that's running retarded. This will also run hotter, since the combustion process is occurring late, and the mix is still burning once the exhaust valve opens.

In other words you have to try everything. You have to try the stuff that you don't believe is the issue, or what you don't correlate to being a player in the issue.

Mr. Mistake is my teacher, and he has been for 3 decades. He's always right.