Need help sizing Weber main jets

[Alfajay]

I am running a pair of DCOE 40's on the Nord 2 liter engine in my Sprint Gt - 10.1 pistons, 10548 cams, Pertronix ignition, but otherwise, pretty stock.

Dave Andrews' paper and jetting.exe both recommend 130 mains for my application. The problem is that my simple mixture gauge, connected to a narrow band oxygen sensor shows that I'm running rich at freeway cruising speeds (yes, fully warmed up). I have tried reducing the mains from 130 - 125 - 120 - 115 and it has made NO difference. It's kind of weird - no flatspots, no reduction in power, and the indicator still shows rich - I'm thinking I could solder the holes in the main jets shut, and it wouldn't change things!

I have checked the float heights. Fuel comes from a Facet cylindrical-style low-pressure pump, going through the original filter/regulator. The mixture indicator shows lean when the engine is idling, so it isn't stuck at the rich setting. Also it displayed mid-range when I connected it to the O2 sensor on my 164, so I believe it is accurate.

Other jets & parts in the carbs are:

Chokes: 32
Air correctors: 175
Emulsion tubes: F9
Idle jets: 55F8

So my questions are:
- Is sizing the main jets the right way to adjust high speed mixture?
- Does it make sense that the car would still be rich with mains as small as 115, when Andrews' formula suggests 130?

[alfaparticle]

You could be running on the idle circuit if you have a small throttle opening. My carbs start to switch from idle to main at about 3500 when I am cruising. I am fully on the main circuit at around 4000. You could try one step leaner on the idle jets, say 50F11.

I am running 40DCOE116/117 which use different emulsion tubes and idle jets. I had a massive improvement in mixture control when I switched emulsion tubes from F41 to F34. You could try a set of leaner ET's if idle jets do not do the trick.

[Alfajay]

Ed:

Thanks for the interesting reply. When the engine is just idling - the tach shows about 1,000rpm - the mixture gauge shows lean. If I take my foot off the gas while cruising on the freeway, the mixture gauge will dance around in the "correct" zone. And, here in S. Calif. there's little in-between: you're either idling, or on the freeway.

But, you may be on to something. I could try leaner idle jets. I fooled with enrichening them, but encountered a flat spot with 60F9's.

[alfaparticle]

My first GTV had a similar engine setup to yours. It ran fine with 50F11 idle jets F16 emulsion tubes.

[101 Alfa Mike]

Leaner idles will help. Alternately, you might try larger air correctors to delay onset of the main circuit. FWIW, I'm running 32 mm chokes in the Spider with 125 mains, 200 air correctors, 50F11s and F-34 tubes.

Mike

[Alfajay]

Ed:

Well, the funny thing is that my car runs great with ALL of the jets that I've tried. From a performance perspective - just using the "seat of the pants" dyno - they're all the same. So, I'm just doing this for the challenge of getting a proper mixture shown by the oxygen sensor & gauge.

Mike:

Well, those values are close to what I'm running. That's about where I started out.

I've kind of decided to not worry about it - it's certainly not so rich that it backfires on decel or shut-off, nor does it put out black smoke. I guess if I owned one of every Weber jet available, I'd fool with it further. But at some point, ya gotta say "good enough" - and for now, I'm to that point.

Thanks for the help

[alfaparticle]

Jay:
You may notice a difference in gas consumption if that is of interest to you. Leaner idle jetting and ET's could get you an extra 1 or 2 mpg.

My take on all of this as follows:
Leaner mixtures are more difficult to ignite. Since it is hard to maintain strong sparks with the old points and condensor ignitions, a lot of Alfas were run with rich mixtures to avoid flat spots. Electronic ignitions can make consistently strong sparks and that allows for leaner mixtures, particularly on the idle circuit. My GTV was getting 18 mpg when I bought it and it improved to 23 after installing a Shankle Marelliplex and leaning out the jetting. The only other changes that I made were to replace the low-end headers with a factory 2 piece manifold and the ram pipes/K&N filters with a factory air box..

[1,6 HF]

Quote:

Originally Posted by Alfajay So my questions are: - Is sizing the main jets the right way to adjust high speed mixture?

[BlpltGTV]

Here is another thought. Narrow band O2 sensors are somewhere between worthless and useless. You need a wide band to really see what the O2 is doing. You might be closer than you think, but the o2 because of its inability to see the whole picture is telling you otherwise. What do your plugs look like. You might also try a colortune.

Will

[Alfajay]

Will:

Good point. I could be running very close to the ideal mixture, and the narrowband would just tell me that I was "off".

1,6 HF:

Another good point. OK, so the emulsion tubes are a better tool for adjusting the mixture than the mains. I didn't have any F11's in my box of Weber parts, but I did have some F16's. I installed them in place of the F9's that I had been running and....it ran exactly the same way. Hmmm.

Again, the car runs great. So, I should probably unplug the gauge and just be done with it. Still, it seems if my old '91 164 can put the narrowband sensor at midrange, that with enough fiddling, Webers should be able to come close to that. I guess the issue is defining "enough" !