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. |
914Toy |
Oct 11 2018, 12:43 PM
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#2
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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 |
After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?
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