I have been running dual, Carter W-1s for a couple years now. For the most part I have been really happy with the performance and mostly with the fuel mileage. Over my other set-ups I have run before, this one is by far the best on mileage. I have gained 4-5 miles per gallon over the stock single Mono-jet.
What I didn't like was the lag from cruise to part throttle, such as passing or long up hill runs. The engine felt weak and made a lot of noise for little power. Full throttle was all right, but weak. I lived with it since the mileage was so much better. What I didn't know was weather it was rich or lean.
About a month and half ago I ordered a stand-a-lone Wide band air fuel gauge. I had to install an oxygen sensor in to the head pipe, but the rest of the gauge was a simple install with only a couple of wires to connect.
It didn't take long to see where the problems were with metering. At part throttle it was going lean and full throttle was on the high side of the what is considered the power ratio.
At 65 mph, fairly level ground, I was seeing 14.5 to 14.9 A/F, part throttle was 16.7 and higher, full throttle was in the 13.5 range. I had a makeshift throttle positioning gauge just to see what was happening. Throttle positioning gauge was a simple card board gauge with a spring pull pointer tied to the foot pedal by a long string. I marked idle and full throttle, then divided in half for 50% opening. All my driving around town was at 20% to 25% throttle. Highway was 18% to 20% throttle.
Vacuum readings were at 9 to 10 @ 65 mph, 20 @ idle, 18 @ idle in gear. I was happy with that.
I poured over all the W-1 papers I had looking for metering rods. I found what I was looking for in the Carter #420 W-1, 1939 Chevrolet , I am running dual #574's, the 1946 models. I had been running metering rod #75-485 which has three steps, .067" - .062" - .046". The new rod is #75-342, also three steps, .069" - .059" - .043. So its two steps leaner, three steps richer at mid point, and three richer at full throttle.
This single change made a huge difference in over all performance. Throttle is crisp, idles better, mid range is great, and full power is where it should be. From a dead stop to full throttle, the bog is gone, and power comes on steady and strong. A/F ratio's changed greatly, cruise is now in the 16.2 to 16.8 A/F ratio ( ideal fuel mileage range ), mid throttle dropped back to 13.5 to 14.2, and full throttle dropped to 12.7 to 13.0. I have read a lot lately on A/F ratio's, there are two schools out there, racing ratio's and mileage ratio's. I am leaning towards the mileage side since a stock 250 is not a race engine.
For me, the expense of the gauge has all ready paid off. With one metering rod change, I am really close on ideal tuning. I went back and adjusted the rod hight using a dial indicator just to see how the engine reacted. With a .040" hight change, I changed the ratio's one full point on the wide band. I raised them to move the mid throttle power curve in sooner.
I need some more drive time and a long trip to dial it in further, but for now I am really happy with it. Since it's running better, the awful smells of fuel are gone after shut down, so I can park in the garage without letting it cool off out side. Wife is happy about that!
Here is a link to the gauge I am using, I picked a color close to the '37s gauges. With more money involved, they would add a face to match just about any gauge you can think of.
http://www.speedhut.com/gauge/GL258-AFWB-09/7/Standalone-Air-Fuel-Wideband-O2-gauge-8-5-18Joe
