Jetting Webers on 2.7 911 engine., Tuning. |
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Jetting Webers on 2.7 911 engine., Tuning. |
914Toy |
Oct 5 2018, 10:52 AM
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#1
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Senior Member Group: Members Posts: 717 Joined: 12-November 17 From: Laguna beach Member No.: 21,596 Region Association: Southern California |
I recently installed a "double" Innovative air/fuel (A/F) gauge with sensors in both headers of my 1977 2.7 911 engine, which is stock except for: ignition, Weber carbs, reground cams (modified SC) for spirited street performance with the carbs, and exhaust headers (MB911). The A/F gauge has assisted final tuning, including fine balancing of the carbs (well worth it).
Ignition is Clewett crank fire with ignition timing set at: 10 deg. idle 800 rpm, 29 deg at 3000 rpm, and 33 deg at 6000rpm. Carbs are Weber 40IDA's with 34 main chokes and tall secondary chokes. Jets are: 145 main's, 180 air correction's, F3 emulsion tubes, and 60 idle's. Engine is running very smoothly at all rpm's, no carb "spitting" or exhaust popping, no hesitation under any acceleration, and instant accelerator response with plenty of power. In other words, running great. However, A/F's are not perfect with 10.5 at idle, 12.5 to 13 at cruising, and high 13's up hill WOT. On a recent uninterrupted 180 miles run on 101 at 3400rpm (78mph), fuel consumption was 24miles per gallon (IMG:style_emoticons/default/smile.gif) I have discussed the A/F ratio issue with several carb "experts" - mostly Old School guys. Consensus is my numbers are as good as it gets vs. near perfect 14.7 A/F one can expect from EFI. Any comments will be welcome. |
gereed75 |
Oct 8 2018, 08:20 AM
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#2
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Senior Member Group: Members Posts: 1,241 Joined: 19-March 13 From: Pittsburgh PA Member No.: 15,674 Region Association: North East States |
Hardtop,
that is a very good graph and if you understand it you are a long way towards understanding mixture control. A couple of problems though when trying to apply it to this automotive carbureted applications= With Weber’s there are only so many adjustments available to control mixture. Within the limits of these adjustments I see know way you could achieve lean of peak operations under part throttle/ light load conditions and then dependably transition back to rich of peak under load, no matter what kind of distributor you are running. We got no mixture control knob. With EFI you could program for lean of peak, but this ain’t EFI. That’s why you have to settle for 12.5 - 13.5 or so. As you know those AFR’s are all Rich of peak. I think we are ranging well beyond the scope of the original question |
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