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Weber 40s and The Magic Combination of Jets & Tubes

September 28, 2010 | Troubleshooting & Repair
By Jeff Stevens

 

I have been messing with these carbs on my car for about 2 years now. I probably would have been happy if I were most other people, but being an engineer, I had to fool with them to get better results. I have tried many combinations of venturis (28 and 32), jets (115-145), idle jets (50-65) emulsion tubes (F11, F7, F3) and air correctors (165-225).

My car is a 62 S90 with a 86A webcam (290 duration), Crane HI-6 ignition and pertronix ignit0r in an .050 dist. The following are some of my observations.

Dump the F11 tubes. No matter how many times I went back to them or with the combinations I tried them with, the car always felt like the brakes were partially on. The F3's are identical to the F11's except the body diameter is smaller. I believe they are just not a good choice for the application. They also tended to shift the mixture very rich. The F7's are the way to go. As one other list member pointed out, they will increase the richness of the lower end of the main circuit.

I WANTED 32 mm venturis. Why have a S90 with 28's?

My biggest struggle was still getting the flatspot out which is a lean condition when the butterflies just start to open off of a cruise speed. This is caused by a lean "transition" flow which is really controlled by the idle jets. I'm not sure, but if you tried hard enough, you may get rid of the flat spot with an emulsion tube that allowed the main circuit to come on a little earlier. Only when I went to the bigger idle jets could I get rid of the flat spot.

Then I had another problem. When I ran plug checks at 3500 rpm, the plugs looked good. When I parked the car It smelled rich. Anyway, I had such a hard time, I bought a air/fuel gage and welded the O2 sensor in my exhaust to figure out which
way to go. I ended up with:

  • 32 mm Chokes
  • F7 tubes
  • .64 mm Idle jets (I have access to small drill sizes and moved in .02 mm sizes until happy)
  • 135 Main jets
  • 225 air correctors (yes 225)

This combination eliminates the flat spot, runs stronger than any other of the combinations I tried and does not lean out at the top end like you might expect with the big air correctors. I tried to keep the mixture on the rich side of 14.7 throughout the range, but not rich enough to foul plugs. I also do not feel the car is "boggy" on the low end from the 290 cam and the 32 venturis. Your results will vary, but from this and previous posts, you should have a better idea where to start.

Well, I hope this helps. Good books to read to help you understand Webers are Weber Carburetors by Pat Braden, Weber Tech Manual by Bob Tomlinson and Weber Carburetors by John Passini. I liked Passini's book best.

This is part of our module on how to tune up your 356. Components include:

2 Comments

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Jim Kellogg
December 25, 2010 at 1:30 PM
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Profile missing thumb
Jim Kellogg
December 25, 2010 at 1:30 PM
SOLD