Weber 40 IDA 3C technical | FerrariChat
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Weber 40 IDA 3C technical

Discussion in 'Technical Q&A' started by P400, Dec 14, 2006.

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  1. P400

    P400 Formula Junior
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    Mar 27, 2005
    535
    east coast
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    Craig
    #1 P400, Dec 14, 2006
    Last edited by a moderator: Sep 7, 2017
    I am looking to eliminate flat spots or hesitation with quick throttle application using 40IDA3C Webers. I think I see .50 pump jets and .50 inlet/ bleeds are the standard sizes? i realize i could consider many other things to resolve this, but i really only want help with inlet /bleed sizing in a Weber 3C. I went to PMO http://www.pmocarb.com/bulletins.htm
    (a replacement carb for 40 IDA 3C) and read the following:
    "4. Reducing Injection Quantity- We have found that race engines often need varying levels of injection quantity. Coming out of a turn, too much injection quantity can actually cause bogging. PMO has different float bowl check valves to adjust the injection quantity up or down. Rob King at S-Car-Go has cut the injection quantity down to almost nothing with excellent results. The normally installed float bowl check valves have a .5 mm side hole for bleed back. The larger the bleed back hole the less injection quantity. The valves come in increments of .1 mm up to 1 mm. We have 1.5 mm valves for people who want to almost eliminate injection quantity but still start the engine."

    So, based on this injection quantity control explanation(accelerator pump jet quantity) from PMO, Weber carb people are varying their inlet bleeds to get proper pump quantity out of an acceleration jet? I have never read this before. No doubt i must be out of touch. I have not used the little clear plastic vials for quantity check.

    Is this correct, when you use the little measure vials, find quantity off, you purchase and install smaller or larger bleeds?
    any help is great
    Craig
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  2. ferrarifixer

    ferrarifixer F1 Veteran
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    Jul 22, 2003
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    Phil Hughes
    #2 ferrarifixer, Dec 14, 2006
    All sound in theory... but you need to know what's happening to your engine to correct it's faults... it could either be too lean or rich...

    Take gas analysis readings from road test/dyno and re-act to what you find.

    First of all, triple check the ignition system is working correctly and timed as it should be..

    What car? Miura I assume?

    Be sure you're not getting fuel evaporation problems, and that air feed it all correct.. ie air boxes installed.
     
  3. snj5

    snj5 F1 World Champ

    Feb 22, 2003
    10,213
    San Antonio
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    Russ Turner
    #3 snj5, Dec 14, 2006
    Last edited by a moderator: Sep 7, 2017
    My experience is limited, but Weber accel pumps typically way over do it in richness, as the requirement for fuel supplementation varies widely (e.g. atarting, low rpm operation, flat out)and richness is a bit safer and easier to do. For Example, the standard DCNF pump cam is WAY rich for most of it's applications.

    As I understand it, the amount injected by the pump jet is a function of 3 things - stroke volume off the cam, bleed back and pump jet. The stroke of the pump determines the total volume pushed in the system, which then flows proportionately through the pump jet into the engine or the bleed-back back into the float bowl - with the amount proportional to the size ratio of the two jets. So, if the the bleed back is larger, more gas will be shunted away from the actual accel jet into the engine.

    There is a great on board Air-Fuel monitor, the LM-1, which I got from Pelican Parts which I would use to monitor and fine tune my jetting by recording info from actually driving. You can watch the pump jet fire when you put your foot on the throttle, then fine tune the response. Usually, as was said, the Webers tend to fire too much instead of too little. So, you can bring down the amount by increasing the bleed back or decreasing the actual accel jet size. Pierce Manifolds has a great little book which explains a lot on how to do this. (www.piercemanifolds.com).

    I've heard from Porsche guys that the PMO Weber replacement is VERY good.

    The first thing I would think about doing is verifying all of my pump and bleed backs are what they are supposed to be with no odd outs (this happens not uncommonly). Clean them with Carb cleaner solvent and compressed air (never a wire or metal brush!).

    Do you have any more info on where it bogs down in the power curve and what brings it on. Sometimes in hard corners, some Webers actually go lean and cause a miss due to fuel sloshing in the float bowl..

    If you look at the sample tracing below, the Air Fuel Ratio is on the left as a function of time, with richer being lower on the graph. I tried to tune my car where if I dump the throttle, even at low rpm (low air flow), the A/F does not fall below 11 or so. You can see when the throttle is applied, the richness increases (graph goes down) as the pump fires. Under hard accelleration, a richer mixture tends to ptotect the engine, so being a bit richer than the desired best power 12.8 - 13.2 is not all bad. Even EFI turn up the richness a bit under hard acelleration, just as the Webers do. You can also see under a HARD turn, there is a lean spot due to sloshing in the carb, and I corrected it with applying throttle and firing the accell jet.

    Hope this helps get you started - Good luck and let us know.
    We are all in this together.
    Russ
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