I just finished re-jetting my 2.3 S-cammed engine and replaced my F3 emulsion tubes with F26s with good results.
The engine is a twin-plugged hotrod with essentially open exhaust and 2.4 heads with 37mm & 36mm ports. My Webers are 43mm over-bored IDSs with 35mm main venturis and are generally "very trick" in many aspects but these do not really affect my current observations.
I sold my car about three years ago and thought the jetting was pretty darn good at that time and that was based upon multiple jetting configurations using AFR meter and "seat of pants" testing. Being in the business of servicing Webers I had complete availability of Weber parts to draw upon for my tuning efforts. Since re-acquiring my 911 I investigated its jetting with the conclusion that F3 emulsion tubes created a rich region of operation just past transition.
History of the F26 vs. F3 emulsion tubes:
The F26 E-tubes were used on the Normal 2.0 911 of 1966, through 1968 and also on the first batch of 911S engines, 500 engine numbers were generated for this first batch. These 911S engines used 40IDA3C Webers which were the same as used for the Normal 911 engines but were jetted differently to accommodate the higher engine speed of the S-engine. The Normal 911 used 30mm main venturis and 125 main fuel jets and the 911S used 32mm main venturis with 135 main fuel jets.
The angle between intake and exhaust valves in these 2.0 engines and the domed pistons did not work well in the 911S since flame propagation at high RPM and high power demand caused detonation and resulted in piston failure. This is when the 40IDS3C Weber was developed. These Webers were unique to the 911S in that they had an additional "High Speed Enrichment Jet" which would deliver fuel to the throttle bore at high engine RPM. These tubes would result in an unnecessarily rich fuel mixture if the 135 main fuel jets were retained so 125 main fuel jets were specified. The next piece of the main circuit needing attention was the emulsion tubes. F3 emulsion tubes were developed to replace the F26 E-tubes, the primary difference between these two types is that the first set of holes in the F3 emulsion tubes are located below the fuel level in the emulsion tube well while the F26 E-tubes have their first hole above the fuel level.
The significance of this difference is the F3 E-tubes will initiate fuel delivery for the main circuit somewhat earlier than the F26 E-tubes.
In 1970, the angle between the intake and exhaust valves decreased and the piston diameter also increased which decreased the flame propagation issue of the earlier 911 cylinder heads. Also helping is that MFI injection provided a mapped fuel delivery for the 911E and more importantly the 911S engines.
So, when I initially set up my Webers for my personal hot rod engine I used F3 emulsion tubes (911S E-tubes are better for performance than F26s, obviously) and I also used tall auxiliary (booster or secondary) venturis which were used on the 46IDA3C Webers for race engines (also better than for high performance, right?). The tall aux venturis advance main circuit initiation which is typically useful for small displacement, race engines with huge venturis and cams.
OK, history lesson is over so what did I find in my current jetting endeavors? The F3s and tall aux venturis created a region of rich fuel delivery in the 3000 to 4000 RPM range. I opined that perhaps the original F26 E-tube selection for the first batch of 911S engines was a reasonable choice so I installed them and after several iterations of main fuel jet, main air correction jet, idle fuel jet and idle air correction jet changes I stopped testing with the following jetting arrangement:
- 35mm main venturis
- 140 main fuel jets
- F26 emulsion tubes
- 170 main air correction jets
- 60 idle fuel jets
- 110 idle air correction jets
- short (OEM) auxiliary venturis
While driving I see flashes under medium power to 14 but during WOT operation the fuel AFR is typically from 12.5 to 13. Slow speed driving is richer (11 to 11.5) but a leaner mixture makes the engine a little less happy for smoothness.
My conclusion is that F26 E-tubes for performance engines with 2.2 or later heads and engines with larger piston diameters than 80mm should be considered for 40IDA3C and 46IDA3C Weber carbs.
Of course my findings are my own and I expect rebuttal from those with dyno runs having flat AFR curves for the main circuit.
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