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]

Getting dialed in a bit more. Swapped out the 200 air jets for 210s this morning and took it out to warm up and test. Way too lean on the idle circuit. Was running high 14s to high 15 most of the time below 3000. Transition to mains wasn't smooth but once I was in the main circuit, got the reasonable mid 12s to mid 13s as before. Brought it home and put the original .50 idle jets in but leaving the larger air jet. I have it idling at stop at 13.0 AFR which is almost dead on with the target ACN suggests. One difference is that I started adjusting the idle mix screws to best lean idle and no more. ACN suggests adding a half turn richer from best lean idle, but that seems to be contributing to the overly rich idle condition. Haven't actually driven it yet as I'm helping the wife with some chores, but will do so shortly.

Where are your mixture screws with the 45's, 50's, 55's? From what I have read/seen the mixture screws should be in the 1 - 1 1/2 turn our from lightly seated.

As expected, the 55s were the least (around a turn out) and 45s were the most (not quite 2 turns including ACN's half turn richer). CB says more than 2 turns indicates the jets are too lean and I've seen the 1-1/2 turn rule too. In the end, the wideband is what tells the story and it says 55 is too rich and 45 is too lean. I think either the 50 I already had or possibly .475 is the right number. Still more testing to do.

I don't understand why you're trying to achieve a 800RPM idle. Shoot for the same idle as a FIed engine. How ancient is the carb linkage? Open or close the bypass. I start at closed. Their purpose is to fine tune the flow to a common reading on all 4 barrels. The actual number value is somewhat insignificant as long as they read the same.

The carbs are perfectly balanced. That is not a problem. As I mentioned before, there is disagreement among the experts on how to use the bypass screws, but I went back and closed all of them and then opened up screws only as much as was needed to balance the two throats. They are dead on.

The linkage is a new (well, 3,000 miles on it) CSP linkage. No issues there. In fact, for those who don't know, the whole car is basically new. Just finished with a full bare metal restoration and every nut and bolt has been replaced or rebuilt. Only thing I haven't touched is the transmission.

Not necessarily chasing 800 rpm idle but ACN says this in their tuning guide:

"In order to properly adjust your engine, set the ignition timing to idle around 8 ° before top dead center. Then set the idle speed to 800 RPM, synchronize the two carburetors, adjust the idle speed (refer to the paragraph on adjusting the idle richness)."

And Tomlinsons says the following, "Don't settle for an idle speed of over 900 RPM, even if you're running a 290 degree cam. Most street engines can be tamed to idle at around 750 to 850 RPM once you show those dual carbs who wields the uni-syn [yeah, that dates it]."

The stock FI spec is 900 +/- 50 RPM. IMO, the best idle is the lowest speed the engine will idle smooth and not stutter on acceleration. I don't see much purpose in a higher idle. Just more noise and gas. This engine does that just fine at ~750 RPM - or at least it did on the pig rich tuning I was running on. We'll see where it ends up, but I'm guess in the ballpark of 800.