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]

Okay, it might be time to eat crow. I managed to get some testing and tuning in yesterday afternoon and think I found the winning combo. I am surprised but shouldn't be because my jetting now matches what @NARP74 posted on the first page of this thread and @rfinegan and @r_towle also called it. Maybe some others too.

I took it for a drive with the .50 idles and 2.10 air jets and found the idle circuit running just as rich as before. There was a big dead hole transitioning to the mains and then the mixture lean out to mid 13s as before. I was thinking that if .50 was too rich and .45 was too lean, then .475 might be what I needed and still kind of think that, but I realized I hadn't tried the .55 idles with the larger air jets so pulled over to the side of the road and swapped them out and readjusted.

To my surprise, the idle leaned out. I don't understand how, but suspect sorcery (IMG:style_emoticons/default/smile.gif) Still a little rich IMO, but an improvement. It idles right at 13.0 - 13.2 which is exactly where ACN says to shoot for. Immediately off idle, it drops to about 11 and then climbs back up. I suspect that is from the accelerator pump rather than the jets. I had to crank the pump in enough to get the car to cold start and think I may have overdone it and also didn't get them even. However, once it gets to 30 mph, the mix leans to mid 12s to mid 13s and pretty much stays there except a dip into low 12s in the transition zone. Also, the mix leans to 15s to low 16s on decel/overrun which is more what I expected. WOT under load is in the mid 12 range. I drove 30 miles and was generally happy. Also, the oil temps were running a little warmer than usual which is actually a good thing as I think the engine has been running too cold (rarely gets more than a 1/4" off the bottom even under hard driving). It is still running cool, just a little warmer.

It isn't perfect though. Cruising at 80 mph I noticed a bit of roughness and the AFR fluctuated going rich and then leaning out again. Also, driving the gravel road back home @20mph, the AFR fluctuated a lot and would run in the 13s for awhile, then drop to low 11s. I suspected my nearly new plugs were fouled from the overly rich mix and sure enough, when I pulled them, they were covered in dry brown soot. I cleaned them the best I could but will buy a new set if the weather clears.

I consider all of this preliminary, but promising.

Something still puzzling is that Tomlinsons and ACN both suggest backing the idle mix screws out 1/2 turn beyond best lean idle. They claim it is more "stable" whatever that means. When I do that with this jetting, the idle mix is way too rich, but adjust just to best lean idle and no more lands me right about 13.0:1 which is where I have it set for now.

[Superhawk996]

To my surprise, the idle leaned out. I don't understand how, but suspect sorcery (IMG:style_emoticons/default/smile.gif)

Think of it as like a straw. You pull liquid up the straw by putting a partial vacuum above the fluid.

Small straw - very little air need to be evacuated from the straw to pull the fluid up say 1" in the straw. Now think 4" PVC pipe -- same amount of vacuum will provide same fluid lift but you'll need a good set of lungs to move enough air to establish that partial vacuum and to to lift fluid an inch in that PVC pipe that would be no problem in a small straw.

Now think of your jets. Bigger fuel jet and bigger air correction jet are more like the 4" PVC pipe. Your engine can only displace so much air per revolution -- so you get less lift of the fuel when flowing the same amount of air.

The "trick" here is that both jet and the air correction jet need to change (get larger) together for that analogy to hold. Otherwise, you're back to a singe variable and the way you're thinking of it (i.e larger fuel jet = richer & larger air correction jet = leaner). The two of them at the same time yields the 4" PVC analogy.

The main problem with carb tuning is that there are so many variables and they aren't all related in a linear manner and they don't always abide by the Superpostion Principle.

[bbrock]

To my surprise, the idle leaned out. I don't understand how, but suspect sorcery (IMG:style_emoticons/default/smile.gif)

Think of it as like a straw. You pull liquid up the straw by putting a partial vacuum above the fluid.

Small straw - very little air need to be evacuated from the straw to pull the fluid up say 1" in the straw. Now think 4" PVC pipe -- same amount of vacuum will provide same fluid lift but you'll need a good set of lungs to move enough air to establish that partial vacuum and to to lift fluid an inch in that PVC pipe that would be no problem in a small straw.

Now think of your jets. Bigger fuel jet and bigger air correction jet are more like the 4" PVC pipe. Your engine can only displace so much air per revolution -- so you get less lift of the fuel when flowing the same amount of air.

The "trick" here is that both jet and the air correction jet need to change (get larger) together for that analogy to hold. Otherwise, you're back to a singe variable and the way you're thinking of it (i.e larger fuel jet = richer & larger air correction jet = leaner). The two of them at the same time yields the 4" PVC analogy.

The main problem with carb tuning is that there are so many variables and they aren't all related in a linear manner and they don't always abide by the Superpostion Principle.

So in other words... it IS sorcery! (IMG:style_emoticons/default/av-943.gif)

[Superhawk996]

So in other words... it IS sorcery! (IMG:style_emoticons/default/av-943.gif)

Gotta agree to a degree (IMG:style_emoticons/default/laugh.gif)