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: 718 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 9 2018, 09:10 AM
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#2
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Senior Member Group: Members Posts: 1,246 Joined: 19-March 13 From: Pittsburgh PA Member No.: 15,674 Region Association: North East States |
Spent some time with the Samba thread - very informative. you can get much of the benefit from that thread in the first 4 pages or so.
Yes it appears that the webers do produce mixtures over a much larger range than I thought. Nailing idle jets is still very important and difficult. Although my car runs pretty well, this reminds me that it could be better - alas this will be a project for another day. In the mean time I am confident that I am running with good AFR's in the critical 2/3's and above load conditions. Starts well, idles rock steady but probably richer than optimum in some regimes. Probably similar case to Toy and many others. The "published" jetting charts are pretty good, but fine tuning is probably necessary to be optimal for each motor, depending on ignition timing, cams, compression, piston type, fuel etc. Lots of good info here but is just a basis for further work. Thanks |
72hardtop |
Oct 9 2018, 11:48 PM
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#3
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Member Group: Members Posts: 120 Joined: 11-September 13 From: Seattle/HB Ca./Fujieda-Japan Member No.: 16,378 Region Association: Pacific Northwest |
Spent some time with the Samba thread - very informative. you can get much of the benefit from that thread in the first 4 pages or so. Yes it appears that the webers do produce mixtures over a much larger range than I thought. Nailing idle jets is still very important and difficult. Although my car runs pretty well, this reminds me that it could be better - alas this will be a project for another day. In the mean time I am confident that I am running with good AFR's in the critical 2/3's and above load conditions. Starts well, idles rock steady but probably richer than optimum in some regimes. Probably similar case to Toy and many others. The "published" jetting charts are pretty good, but fine tuning is probably necessary to be optimal for each motor, depending on ignition timing, cams, compression, piston type, fuel etc. Lots of good info here but is just a basis for further work. Thanks Easy enough to do.... Pull the main stacks out entirely....everything and run on the idle jets alone. Note on your tach when it falls on it's face. |
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