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.

[rfinegan]

You need to get off your transition slots it they are open. This may be why your see a rich signal on the O2 on idle
No more than 1/2 turn from full close.
http://cbperformance.net/pdf/WeberIDFCarbs.pdf


Here is a tuning guide for Weber IDF and Wide band O2:
https://www.aircooled.net/how-to-use-a-wide...-flat-4-engine/

[bbrock]

You need to get off your transition slots it they are open. This may be why your see a rich signal on the O2 on idle
No more than 1/2 turn from full close.
http://cbperformance.net/pdf/WeberIDFCarbs.pdf


Here is a tuning guide for Weber IDF and Wide band O2:
https://www.aircooled.net/how-to-use-a-wide...-flat-4-engine/

Thanks for this! This gets really interesting though. In the aircooled.net article you linked written by John Connolly, He writes,

"Synchronize the second carburetor.

From now on, DO NOT TOUCH THE IDLE SPEED CONTROL SCREWS.
carbs_g.jpg

From now on, any change in the idle speed setting will now be done with idle bypass screws or ignition timing."

What's funny is that in this thread on another forum he wrote,

if you want your engine to hesitate, by all means open up the bypass screws.

You need to have them closed all the way, then when you are syncing them, check the bbl/bbl readings on the same carb, and use the bypass screw on ONE bbl to even them up.

I have no idea where the rumor came from that says they should be 1/2 turn or more out, this is WRONG.

Later in that same thread he says he doesn't like the recommendations on jetting that CB gives.

So... It seems that Tomlinsons book is wrong, CB Performance is wrong, and according to John, John is wrong! (IMG:style_emoticons/default/av-943.gif)

Obvious absurdity of contradicting information aside, those links you provided are going to be super helpful.

Thank you! (IMG:style_emoticons/default/beerchug.gif)