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 7 2018, 07:43 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 |
i Am not a “Weber guru”. I have worked pretty diligently to get mine set up and tuned pretty well and have studied a lot. Paul Abbot really is a guru and has made a huge amount of info available on his site
In my experience, it is very hard to nail the idle jet size. Although my car runs at numbers very close to OP’s (rich at idle, slightly rich throughout range) on 60 idle jets, it would not run at all on the next smaller idle jet (55) In talking to and reading Paul Abbot, he found it necessary to ream idle jets to “between” sizes on his personal (perfect I am sure) car.. Again, that is why I said that you might get slightly better AFR’s, but it will take lots of work and will only improve slightly. I think you will find that the leaner 14:1 ratios seen with EFI are achieved at part throttle, steady state, low load cruise conditions. That is where EFI can shine in getting better mpg. As soon as load increases or throttle is opened, the EFI’s will start to richen back towards 12-13:1. Unless you want to be a guru, relax, have fun, Run it!! |
porschetub |
Oct 7 2018, 04:16 PM
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#3
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Advanced Member Group: Members Posts: 4,698 Joined: 25-July 15 From: New Zealand Member No.: 18,995 Region Association: None |
i Am not a “Weber guru”. I have worked pretty diligently to get mine set up and tuned pretty well and have studied a lot. Paul Abbot really is a guru and has made a huge amount of info available on his site In my experience, it is very hard to nail the idle jet size. Although my car runs at numbers very close to OP’s (rich at idle, slightly rich throughout range) on 60 idle jets, it would not run at all on the next smaller idle jet (55) In talking to and reading Paul Abbot, he found it necessary to ream idle jets to “between” sizes on his personal (perfect I am sure) car.. Again, that is why I said that you might get slightly better AFR’s, but it will take lots of work and will only improve slightly. I think you will find that the leaner 14:1 ratios seen with EFI are achieved at part throttle, steady state, low load cruise conditions. That is where EFI can shine in getting better mpg. As soon as load increases or throttle is opened, the EFI’s will start to richen back towards 12-13:1. Unless you want to be a guru, relax, have fun, Run it!! Good points there,I didn't fully understand the changes necessary when I went to larger vents (30mm) and have a better understanding now,and no my idle jets weren't easy to get right,went for the common #55's as a starter,engine didn't respond well on the idle screws and was very finicky balancing on the air bleeds,I dropped to #52' and was amazed what that small change made,went back to stock idles as an experiment which worked out rather lean with lots of spitback even with a warm engine. You learn as you go along,one thing that was obvious was when the idle jetting is wrong which ever way you get poor response when tuning and balancing ,my high speed jetting was somewhat easier went 115 to 130 then back to 125,fuel economy certainly improved. My testing has been on Zenith carbs but the principals are basically the same,any tuning is a waste of time unless valve setting, timing and ignition are totally up to scratch. |
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