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 10 2018, 02:03 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 |
My engine is running well at all speeds, except for an occasional small hesitation when accelerating vigorously from idle, after changing from 60's to 55's idle jets. This change has helped increase idle A/F's from 10.4 to 11 at 800rpm. My idle air adjustment screws are several turns open, and air/fuel mixture screws are turned out only 1/4 turn.
So I am continuing trials and researching options to; 1) eliminate the occasional small hesitation when accelerating from idle, and 2) increasing the idle A/F ratio into the 12 to 13 range. 1) I am waiting for a set of "Hatchet" carb acceleration fuel pump cams, and a set of 175 air correction jets to replace the 180's installed, to address acceleration hesitation. 2) I have a range of idle jet sizes and have discovered that the idle jets that are 55's, 60's, or 65's (maybe even smaller and larger sizes) all have the same diameter side air inlet holes, that introduce air into the idle jet metered fuel, with this emulsified fuel mixture descending to the air/fuel mixture screws that control the volume of this fuel mixture into the carb throats. Thus, I conclude that if I replace the 55's in my carbs with 50's and open up the air /fuel mixture screws to regain lean best idle, this may result in a higher A/F ratio at idle. Comments? |
72hardtop |
Oct 10 2018, 08:17 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 |
My engine is running well at all speeds, except for an occasional small hesitation when accelerating vigorously from idle, after changing from 60's to 55's idle jets. This change has helped increase idle A/F's from 10.4 to 11 at 800rpm. My idle air adjustment screws are several turns open, and air/fuel mixture screws are turned out only 1/4 turn. So I am continuing trials and researching options to; 1) eliminate the occasional small hesitation when accelerating from idle, and 2) increasing the idle A/F ratio into the 12 to 13 range. 1) I am waiting for a set of "Hatchet" carb acceleration fuel pump cams, and a set of 175 air correction jets to replace the 180's installed, to address acceleration hesitation. 2) I have a range of idle jet sizes and have discovered that the idle jets that are 55's, 60's, or 65's (maybe even smaller and larger sizes) all have the same diameter side air inlet holes, that introduce air into the idle jet metered fuel, with this emulsified fuel mixture descending to the air/fuel mixture screws that control the volume of this fuel mixture into the carb throats. Thus, I conclude that if I replace the 55's in my carbs with 50's and open up the air /fuel mixture screws to regain lean best idle, this may result in a higher A/F ratio at idle. Comments? Running Weber IDF's? IDF's start with combined with vacuum advance distributor. No lean tune cruising without one. 47.5 ilde jets 200 air corrector jets 120 main jets F11 tubes Float height 10mm - 11.5mm (ball not depressed gasket in place) Critical Float drop 28-32mm (less critical) Fuel pressure - no more than 3.5lbs (no dial regulator allowed) Critical Once the carbs are bench set you should start with no more than 1/2 in from contact with the idle speed screws. This is a starting point only. It will very likely run like a tractor initially. Opening to far and you will expose the 'progression circuit' Once it's running....LBI each screw to the smoothest highest idle speed with the air mixture screws. Once done...use a snail and check each carb (2 & 4). Note your idle speed (engine warm) bring high flow down to low flow carb. Now LBI each air mix screw again. Once done check carb sync. The same? If so now check idle speed get it to roughly 800 rom or so by bring each carb up slightly being sure to keep them synced until you reach idle speed. Now LBI each screw again. Once done check your carb sync again. If the same....Done. Now you can work on jetting. With regards to jetting start by driving around with only the idle jets (no main stacks installed) This allows you to see and feel when it falls on it's face. A bigger air corrector tips the main jet in sooner. A smaller air corrector delays the mains from coming in. |
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