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Smile Tuning carbs.

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

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.
Old 10-05-2018, 08:54 AM
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great results! go carbs!!!
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Old 10-05-2018, 03:14 PM
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I like the dual o2 setup. Your numbers are about right except for idle. You should be able to get to 12-12,5 hot idle. 13 on a cold morning.

Iím running a msd ignition and that made it possible. Stock cdi box would not let me get that lean. 11 was the best I could do.

Oddly I had to use different idle jets to get the two carbs equal. 60 on the left and 62 on the right. Thatís what they wanted.

Running cleanly at 12.5 to mid 13s is good. Also explains why carbs were replaced with fuel injection.
Old 10-05-2018, 05:26 PM
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Thanks for your responses. Increasing my A/F at idle remains a challenge but not a priority. Testing 55’s for idle jets causes a hesitation in acceleration from idle and almost no impovement in th A/F. Perhaps larger air correction jets - 200’s?
Old 10-05-2018, 06:24 PM
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that's about where ive ended up and im thrilled. a bit of a lean spike when on throttle hard from low R's for a 1/2 second is my only kink currently. I can get freeway numbers mid 12 to low 13 depending on throttle input. it can be frustrating as hell at times and I bet you've looked into performance oriented by now?


my biggest improvements from a rich across all ranges to where I am now was to move from F3 ET to an F8 and moving away from a 2 in 1 out exhaust to a freer flowing 2 in 2 out.
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Last edited by juanbenae; 10-05-2018 at 08:03 PM..
Old 10-05-2018, 08:01 PM
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IMO you should be able to get the idle a little leaner, say around 12-13. I would shoot for 12 -13 at WOT. Otherwise your settings seem about right from my experience.
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Old 10-05-2018, 08:16 PM
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Two things that help the tip in hesitation.

Go to the ďhatchetĒ style accelerator pump lever. I run zeniths and made my own hatchet style. I think PMO or performance oriented might sell the hatchet.

Increase the acceleration jet size.

Tip in leanness means you have more air suddenly than fuel. Solution: add more fuel for that split second. The acceleration circuit does that.
Old 10-06-2018, 07:53 AM
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14.7 is theoretical but 12.7 is sweet spot for power. My jetting & mileage matched yours (2.3 liter, twin plug, S-cams & open exhaust) but my idle jets were 52s and idle air bleeds were 90s. Main air correction jets do not affect idle/progression fuel mixture. I had a very short "blip" in throttle response at 3000 RPM but that went away when I installed full-radius air horns.
Old 10-06-2018, 07:54 AM
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Sorry, but increasing the accelerator jet size does not affect output volume or timing. A larger jet will decrease the amount of squirt duration. The hatchet was OEM for 1969 through 1973 Webers and can help adjust for an earlier & more sudden squirt than the standard IDA cam-lever arm does.
Old 10-06-2018, 07:58 AM
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True. Volume is set by the pump displacement. Larger acceleration jet will shorten the duration and a corresponding flow rate increase.

My zeniths took 1.5 seconds to flow at 0.3mm. With the 0.5mm jets math would say ~0.5 seconds. For me, that seemed to do the trick to have the carbs fuel flow catch up to the air. Much smaller afr spike.

Iíd llisten to 1QuickS. Does it for a living.
Old 10-06-2018, 10:05 AM
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Quote:
Originally Posted by VFR750 View Post
Two things that help the tip in hesitation.

Go to the “hatchet” style accelerator pump lever. I run zeniths and made my own hatchet style. I think PMO or performance oriented might sell the hatchet.

Increase the acceleration jet size.

Tip in leanness means you have more air suddenly than fuel. Solution: add more fuel for that split second. The acceleration circuit does that.

I was made aware of the updated hatchet in Gordo's carb tuning thread and intend on moving on those in addition to installing fresh diaphragms in the pumps this winter. I checked the flow volume on the squirters and the best I could achieve with adjustment was .45cc when the range depending on season suggested .55 to .65cc per Porsche data.

my lean spike does not cause any sort of noticeable hesitation, but im sure that it can yet be approved on.



paul, do you have the updated hatchets available? getting in touch with PMO is hit and miss at best. if so please PM me as to how I can pick up a set from you. thanks in advance.
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Old 10-06-2018, 10:57 AM
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Trying the "hatchets" makes sense. What are they - pic?
Old 10-06-2018, 11:14 AM
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PMO i think has them for no cost, just fax Richard.
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Old 10-06-2018, 11:20 AM
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I looked and don't have any decent photos that show them clearly. look down where the linkage is for the accelerator pumps and you will see a lever that actuates the diaphragm rod, it looks just like a little GI Joe sized hatchet. that's it.
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07.5 2500HD GM Diesel 4x/93 tuff-gong 4.oL YJ Wrangler
08 DRZ400SM+FCR Mikuni
78SC PRC Spec911(sold 12/15) https://www.youtube.com/watch?v=f7I6HCCKrVQ
Old 10-06-2018, 11:23 AM
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Best way to describe them is the ramp on the bell crank is more abrupt. The pump goes full stroke with much less throttle opening. And that is more realistic for street driving. You rarely need to go WOT cruising about.

Richard Parr at pmo is very reasonable. He is a small shop and you canít expect him to stop what he is doing every time he gets a fax. Within a day he or someone (wife?) will call. And then you get what you need.
Old 10-06-2018, 02:30 PM
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If your injection squirt is below .65ml/stroke then you need to first address if the check valve in the bottom of the float bowl is sealing correctly. Remove top cover & stroke the throttle & look to see if there is a disturbance at the surface of the fuel, a disturbance means fuel is bypassing the ball in the check valve and you are losing squirt volume. Possibly you have check valves with bleed holes in the side, they will be marked for bleed hole size.

PMO supplies hatchets for PMO carbs for free but I doubt if he is interested in doing the same for Webers.

Photo shows the three variations of cam levers used on our Webers:
  • top left is earliest version, used with non-adjustable pump rods
  • top right is typical IDA version
  • bottom center is IDTP version (1969 911T & 914/6); the "Hatchet"

Old 10-06-2018, 04:47 PM
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Alfa1750 on eBay has a huge selection of parts for carburetors.

I found just about every part for my zeniths
Old 10-07-2018, 03:48 AM
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looking for the IDTP hatchet here>>> https://www.piercemanifolds.com/category_s/249.htm as you can see there appears to be two options for the part in the linked exploded view, item 34, weber part number 61287.002 & item 42 weber part number 61287.001. is either one of these two the desired IDTP unit?

the ebay alpha guy did not seem to have the part available in their store and in fact the IDTP seems to be very hard to find any specific parts for even at pierce as id expect many are consistent with the IDA.
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08 DRZ400SM+FCR Mikuni
78SC PRC Spec911(sold 12/15) https://www.youtube.com/watch?v=f7I6HCCKrVQ
Old 10-07-2018, 12:30 PM
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Those items are assembled from the same parts as left hand & right hand. I bet they are IDA cam levers since those were most prevalent item to triple throat Webers.
Old 10-07-2018, 01:41 PM
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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.

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Old 10-10-2018, 10:25 AM
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