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Chain fence eating turbo
 
Join Date: Dec 2008
Location: Austin, TX
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I can tune my 911 to stoch (14.7) with fully programmable EFI, but it doesn't like it at all.

13.5-14.0 is about as lean as the engine likes.

Get down in the 12's, and it really wakes up torque wise.
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Old 10-10-2018, 10:35 AM
  Pelican Parts Catalog | Tech Articles | Promos & Specials    Reply With Quote #21 (permalink)
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Quote:
Originally Posted by Porsche 2 View Post
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?
Weird you are rich at idle with 55 idles. That combined with you only having the idle mixture screws out 1/4 turn makes me thing something is wrong. Perhaps the floats are too high? Also I have read you should start with the bypass screws all the way in.
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Old 10-10-2018, 12:22 PM
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I have recently reset the float levels to the middle of the gauge level marks - probably not my issue, but thanks for your response.
I see that I missed clarifying my discovery and conclusion on the idle jet hole diameters. I am suggesting that given all three jet sizes have the same side air entry hole diameters, I am concluding that the smaller jets with smaller fuel entry hole diameters result in a leaner emulsified mix of air and fuel delivered to the air fuel adjustment screws, leading to a leaner (higher) A/F reading(?).
Old 10-10-2018, 12:44 PM
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Quote:
Originally Posted by Porsche 2 View Post
I see that I missed clarifying my discovery and conclusion on the idle jet hole diameters. I am suggesting that given all three jet sizes have the same side air entry hole diameters, I am concluding that the smaller jets with smaller fuel entry hole diameters result in a leaner emulsified mix of air and fuel delivered to the air fuel adjustment screws, leading to a leaner (higher) A/F reading(?).
True.
But I still can't believe you only have your mixture screws out 1/4 turn. My thought is somehow extra fuel is getting through to lower your AF ratio.
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Old 10-10-2018, 01:12 PM
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If the idle screws were tightened too much, the tip of the screw can get little trough cut into it. Effectively ruining the ability to meter the flow. They become very non-linear. They are closed, then, with a little bit of rotation, they flow a large amount.

Your idle air screws should only be used to balance the three throats when you use the airflow meter.

If you are very rich with 55s or 60s then the idle air correction jets might be too small.

I have zeniths, but I run 60 left/62 right idle jets with 135 idle air correction jets.
Old 10-10-2018, 05:55 PM
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VFR750, thanks for your comments - yes it seems logical to me 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. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?
Old 10-11-2018, 10:39 AM
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Apparently you have IDTP Webers and not IDAs (IDAP, IDT or IDS). IDTPs have 1.40mm bleed hole diameter while all of the others have 1.10mm bleed hole diameter.

IDTP Webers partially expose first progression hole at idle which adds fuel to what the mixture screw provides.

IDTP Webers have mixture screws with a needle tip vs. a constantly tapering needle; an IDTP screw is the mixture screw to the right in the picture while IDA Webers use the middle one. The left needle has a steeper taper than IDA screws which would provide a more rapid change in mixture than the IDA needle.



All of the triple throat Webers use 1.0mm dia transfer holes in the throttle housing to meter fuel through.

If you have a brass mixture screw (by PMO) then it is quite possible to have a ridge in it from over-tightening but the Weber screws are of steel and the grooving is not quite as likely.

The sealing cup washers can effectively prevent the mixture screws from sealing the transfer hole which is due to tolerance stack-up issues. If you cannot definitely stop fuel flow by closing the mixture screw and you have the cup washer with o-ring I recommend removal.

Depending upon venturi size in your Webers the air bleed will rather dramatically lean transition mixture so I recommend caution in this approach of drilling to larger size. Venturis larger than 30mm are susceptible to this issue.

Air flow balance should be somewhere close to 5 using STE synchrometer. I recommend using this tool since other tools only provide air flow parity which means you could have idle air balance at higher air flow which would indicate rich idle mixture due to excessive idle air flow requiring more fuel to support combustion.

If the needle tip of the IDTP screw is broken off or if you have mixture screws with tips that are around 1.0mm or larger then a minute adjustment of the mixture screws will completely open the 1.0mm dia transfer port.

Last edited by 1QuickS; 10-11-2018 at 11:37 AM..
Old 10-11-2018, 11:32 AM
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I measured the orifice of the idle jet with my old vernier scale micrometer - not good enough, i believe they are actually stock 1.1mm. And I checked again, my carbs are 40IDA's. I am now in email contact with Paul Abbott, and have decided to pass on drilling these air jets. I am going to find some "hatchet" accelerator pump cams and check the check valves at he bottom of the float chambers to reduce/eliminate the slight acceleration hesitation when accelerating from low rpm's. Will also stay with the 55 idle's.
Thanks for your input.
Old 10-11-2018, 01:03 PM
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Update.

Installing "hatchet" shaped accelerator pump cams has solved the hesitation in acceleration issue

I have today replaced the 55's with 50's idle jets and this has increased the idle A/F's from 10.3 to 11.5 at 800 rpm's - a good improvement. A test drive and seat-of-the-pants judgement, there is no noticable change in power in the midrange mode with no change in WOT mode. Will stay with this set up and see what miles per gallon I get.
Old 10-15-2018, 01:51 PM
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Quote:
Originally Posted by Porsche 2 View Post
Update.

Installing "hatchet" shaped accelerator pump cams has solved the hesitation in acceleration issue
did you get them from PMO or else where?
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Old 10-15-2018, 02:11 PM
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From Paul Abbott.
Old 10-16-2018, 06:32 AM
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Quote:
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From Paul Abbott.
not all heroes wear capes.




(though I've never met Paul in person so he might wear capes around who knows)
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Old 10-16-2018, 06:55 AM
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Paul Abbott is an awesome fellow. He helped me fine tune my webers and is a wealth of knowledge.. My west coast weber expert. He's now in Oregon.
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Old 10-16-2018, 08:21 AM
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Aw, shucks...Thank you to all!
Old 10-16-2018, 10:14 AM
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