Weber 40DCOE x 2 Air Vac Sync and lean best idle
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Thread: Weber 40DCOE x 2 Air Vac Sync and lean best idle

  1. #1
    1000+ Posts Shoji's Avatar
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    Default Weber 40DCOE x 2 Air Vac Sync and lean best idle

    I have been busy with trying to get my New 40 DCOE 151s running right.
    I also have a new 123Tune USB coming from Germany. It should make some differnce compared with the 40+ year old dizzy.
    I bought a German synchrometer for balancing these 40 DCOE 151.
    After some thorough searching of the net and forums I found out that these models of webers have Air Bypass Screws and I have learnt about idle balancing of each choke. I also need to sort the correct jetting as she is running very rich at idle fouling the plugs

    This Air Bypass Screw allows air supplied from the front of the throttle valve to the back of the valve, which can only mean its for air volume balancing adjustment
    I fully understand the progressive circuits of the Weber, Idle with valves shut or just cracked and no more, the Progression with fuel/air coming from the same idle jet and then the main circuit which kicks in just as the progression is about to finish. My valves at closed position are exactly at the first progression hole which there is 3.




    • It is written on a few sites that this linkage system between both carbs is where you adjust the synchronization levels but if Im not mistaken it not the case as you can easily break the idle to go into the progression circuit on the following carb
    • Both carbs must be not cracked no more than half a turn at idle or the progression holes start supplying fuel
    • The only amount of adjustment here would be to a turn and this may bring the following carb to close sync to the lead carb
    • I removed the throttle linkage and screwed out the link screw of both carbs so they are running independent at idle
    • Running the engine (warmed) and all the Air Bypass Screws wound in closed I have checked 1 cylinder choke with the meter on the lead carb. It runs at a value of 4 of vaccum on the meter.
    • Choke 2 runs at 3 and with a bit of winding out I can add more air behind the throttle valve and achieve a value of 3 on the meter
    • Next the following carb I checked 3 & 4 chokes and they were much lower at 1-2 on the meter.
    • I opened up the Air Bypass Screws on both
    • 3 was easy and was brought up to 3 on the meter
    • 4 not so easy, wound it out so much I could hear the air rushing into this hole inside the choke
    • But I got it to the value of 3
    • On the directions for the meter it said if values are within 1 of each other that this is ok
    • So running at idle all are within 3-4 on the meter
    • I think that the No1 choke is letting too much air in so that valve cannot be closed exactly.
    • I then adjusted idle mixture screws to about 2 turns each
    • I cant go in any further or it starts to run poor.
    • It does seem to be running rich and smells bad even when driving. It just comes in my side window.
    • After doing this I reinstalled the link screw, checked operation and added the throttle linkage
    • It is written on a few sites that this linkage system between both carbs is where you adjust the synchronization levels but if Im not mistaken it is not the case as you can easily break the at idle to go into the progression circuit meaning one carb is not in sync with the other
    • Both carbs must be not cracked no more than half a turn at idle or the progression holes start supplying fuel
    • It makes sense that when the throttle valves are shut the air levels at idle must be similar before idle mixture is adjusted.




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    https://www.lotus7.club/forum/techta...t-idle-mixture


    Study from other set ups
    Factory 40 Dcoes 152 Currently in the R17 Andrews set up -1600 Pinto Mod Cortina 1600 mild cam 40 DCOEs Renault 12 G (45 Dcoes) ???
    Venturi chokes 30
    30 34 32 34
    Venturi Aux 4.5
    4.5 4.5 4.5 4.5
    Emulsion tubes F11 F11 F16 F16 F9
    Main jets 115 115 140 120 135
    Air correctors 200 180 190 180 200
    Idle jets 45 45 F9 40 F9 50 F9 55 F8
    Pump jets 40 40 40 40 45
    Pump spill 50 50 50 50 60
    Float needle valve 175
    150 175 ? 150


    Yesterday I removed the webers to do a check on the mounts and the manifolds gaskets. All seemed very good


    I also checked the valves for the lead carb that was sucking more air than the following carb and adjusted the valves seating
    Anyone with more insight is most welcome

  2. #2
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    I can offer no direct assistance but am puzzled as to why you wouldn't hire a DC0E savvy dyno jockey to sort things with feedback from the car via the rollers.

    cheers! Peter

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    1000+ Posts schlitzaugen's Avatar
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    I am curious if such a setup/person exists here in the wild west (dyno & guy with carby know how). It would be quite exxy if it does too. Most places (and there's not that many) remap and chip Japanese turbo engines.

    Your information is repeated a couple of times. You need to start from the beginning and look at your idle jets. A bad choice here will not make things easier.

    From your tests I would say you need to tear down those carbies and rebuild them properly. Check the needle seats and passages for crap from machining and such, clean and so on. Something seems funny. Checked the floats?

    Also I think you managed to confuse yourself with all that info on the web. Look at those tables and see what idle jets are used on cars with similar power to yours and same carbs.

    Before anything else though, I would read the plugs. Easy to do at idle.

    And before that, I would check the ignition advance is correctly set and stable. Your dizzy does a good enough job. A programmable ignition is going to confuse you even further.
    Last edited by schlitzaugen; 10th April 2019 at 04:50 AM.
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    dvr
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    Get yourself one or better two of these https://www.ebay.com.au/itm/Gunson-C...-/130758004248

    In combination with a quality flow meter (not a chinese copy or the bubble-tube) they have never failed me for getting a smooth and balanced idle. Which with the right jetting transitions well into low speed driving.

    https://www.google.com/search?q=carb...w=1313&bih=640

    EDIT: I'd also suggest you don't want any of the progression holes exposed at idle settings. Use the idle bypass to adjust that. In the old days I'd drill holes in the throttle plates to make sure the progression holes weren't exposed. Generally I'd find around 1 1/2 to 2 turns of the idle mixture screws would be the setting.
    Last edited by dvr; 10th April 2019 at 09:08 AM.

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    PM sent....
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    I am unusually blessed in Launceston re a Weber-savvy dyno guy then (works on lots of Targa Tasmania cars).

  7. #7
    1000+ Posts schlitzaugen's Avatar
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    Quote Originally Posted by 4cvg View Post
    I am unusually blessed in Launceston re a Weber-savvy dyno guy then (works on lots of Targa Tasmania cars).

    That explains it then. We only have the nouveau-bogan japcrap and their blow off valves.

    Quick question. Ever seen a Toyota Sports 800 in action around there? There's a couple I have seen on the web coming down from Japan (I think) for the event.
    ACHTUNG ALLES LOOKENPEEPERS

    Das computermachine is nicht fur gefingerpoken und mittengrabben. Ist easy schnappen der springenwerk, blowenfusen und poppencorken mit spitssparken. Ist nicht fur gewerken bei das dummkopfen. Das rubbernecken sightseeren keepen hands in das pockets-relaxen und watch das blinkenlights.

  8. #8
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    Quote Originally Posted by 4cvg View Post
    I am unusually blessed in Launceston re a Weber-savvy dyno guy then (works on lots of Targa Tasmania cars).
    Yes Brad does a terrific job on the dyno.
    Regards Col

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    Shoji I decided it was all confusing as well, I needed another reference to measure against.

    I've got a wideband to install so I can measure against that.
    https://www.prospeedracing.com.au/pr...gauge-kit.html

    The old racing people have also told me not to bother with reading plugs with modern unleaded fuel, the sulfer or something in it makes old school tricks difficult unless you are running AVGAS, or leaded fuel types.

    In my case, I'm going to copy the main jets and air correctors from hot Datsun A12's and A13's, then just look at the wideband and go from there. This is of course before touching the ignition curve.

    Also be careful with idle jet screws. I've discovered that as they get older, the point on the end wears, and the good ol, "no more then 2 turns out" doesn't exactly hold true as a measurement any-more. If you are confident they are fresh and sharp sweet. if not, don't kill yourself getting it within 2 turns.

    Re that O2 sensor, I'm going to weld it into the end of the collector and measure the engine "Generally" since I'm only using one DGV. You might want to tap a port on each exhaust runner for cylinder reading. You could dump a small bolt back in there after you have done.
    Bustamif likes this.

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    dvr
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    [QUOTE=dvr;1609256]Get yourself one or better two of these https://www.ebay.com.au/itm/Gunson-C...-/130758004248

    Regarding my earlier post go to 3.30 in the video to see how this gadget works:

    https://www.youtube.com/watch?v=tPfelrWMfD8

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    Quote Originally Posted by schlitzaugen View Post
    That explains it then. We only have the nouveau-bogan japcrap and their blow off valves.

    Quick question. Ever seen a Toyota Sports 800 in action around there? There's a couple I have seen on the web coming down from Japan (I think) for the event.
    nope; thanks for the info

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