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: 716 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, 09:54 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 |
Hmmm. Interesting. Did not think that the Weber’s could do that.
Can’t say that I ever collected any data for sustained light load part throttle ops. I was only concerned with safe mixtures under load. Following your thought process, I would think that if your Weber’s are tuned to produce 13 AFR under load that they will intrinsically yield leaner mixtures under light loads part throttle. By there design, I Don’t see how you could have one without the other unless you were way off on jetting. Looks like good info in the samba thread- all 97 pages!! That will have to wait for another time. In the mean time - drive! |
72hardtop |
Oct 8 2018, 10:37 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 |
Hmmm. Interesting. Did not think that the Weber’s could do that. Can’t say that I ever collected any data for sustained light load part throttle ops. I was only concerned with safe mixtures under load. Following your thought process, I would think that if your Weber’s are tuned to produce 13 AFR under load that they will intrinsically yield leaner mixtures under light loads part throttle. By there design, I Don’t see how you could have one without the other unless you were way off on jetting. Looks like good info in the samba thread- all 97 pages!! That will have to wait for another time. In the mean time - drive! What's to be avoided is 14:0 - 15.5:1 AFR WOT condition. Under 3/4 throttle positions there is not enough heat combined with load to be an issue. Even though the AFR is hotter than 13:1 16-17AFR is where you want to be during part throttle cruise (cracked throttle) for MPG's (vacuum advance distributor). And cooler running temps. Your foot dictates load. Even with a 009....which will run lean (jetted right) at ~14-15AFR no more (w/light throttle). With a cracked throttle (low load cond.) you will not be on the main circuit (even at 60+mph) You'll be on the idle/progression phase. |
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