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jetting dcoe 40's...

Discussion in 'Modified Alpine' started by SIVAllan, Feb 15, 2010.

  1. SIVAllan

    SIVAllan Gold Level Sponsor

    Does anyone have a view on optimal jetting for 40DCOE's on a CLONE-H120 engine?

    In the WSM.149 (Rootes Hunter Range) I see for example that from the factory, for the Hunter GLS:

    Model carb:
    40DCOE 34 & 35 (Italian not Spanish)

    --choke tube (large venturi) 30 mm.
    --Auxillary (small venturi) 4.5 mm
    --Main jet 100
    --Idle jet 45 F4
    --Pump jet 35
    --Starting jet 60 F.S.
    --Emulsion tube 4.14
    --Air correction jet 140
    --Needle valve 1.75

    In contrast here are specs on a pair of new DCOE's offered by a UK vendor:

    ... very latest specification 40 DCOE Weber carburettor.

    Brand new stock from the re-tooled Spanish factory & back to Italian quality.

    Supplied with:
    30mm Chokes,
    4.5 A/vents
    115 mains
    200 airs
    45F9 idles
    40 pumps
    50 b/bleed
    F11 E/tubes
    175 N/valve


    40DCOE 34 & 35 vs. NEW DCOES

    --choke tube (large venturi) 30 mm. --SAME - 30
    --Auxillary (small venturi) 4.5 mm --SAME 4.5
    --Main jet 100 --115
    --Idle jet 45 F4 --45 F9
    --Pump jet 35 --40
    --Starting jet 60 F.S.
    --Emulsion tube F.14 --F.11
    --Air correction jet 140
    --Needle valve 1.75 --SAME--1.75

    I am just now beginning my search for a replacement pair of the webbers, and see that most currently available are jetted differently than the vintage 40DCOE 34/35's.

    Given my lack of webber knowledge it seems safer to seeK the same configuration as used by the factory, but the factory specs may be out-of-date compared to more recent experience.

    Sooner or later a pair of 34/35's will tun up, butg there's no sense in waiting if anyone knows of good reason to ignore the Hunter GLS H120 configuration...

    Thanks in advance,

  2. alpine_64

    alpine_64 Donation Time


    There is a very good reason to go for different jetting in alpines, whihc is most people using H120/GLS spec webers are not using a holbay head.. and thats the reason.. the holbay head is VERY different to a stock alpine head. The inlets are smaller, the combustion chamber shape is different the timing was different with a different distributor.

    Without a Holbay head you are likely to run VERY lean on the factory holbay specs.

    You will also find that most people run F16 E-tubes which are richer and solve (hide) some issues, 45f9 idle jets and in AU we use 115-120 mains.. USA seem to use 130.. not sure why.. and i think 175 airs.. we also use 30mm chokes.. though id love to try 28mm for faster air flow.. but im sure that would mean jetting it all again.
  3. SIVAllan

    SIVAllan Gold Level Sponsor


    Sorry - I should have stated that I'm using a Holbay head, and the motor is basically to be a clone H120.

  4. alpine_64

    alpine_64 Donation Time

    Allan if you have a holbay head.. flat tops.. you can recurve the dizzy as needed.. and if you dont enlarge the inlet ports and use the holbay manifold modified to run upright.. in theory the stock jetting should work.
  5. alpine_64

    alpine_64 Donation Time


    I know a guy in the USA with a spare set of the holbay DCOE's.. extended venturis and all.. i can put you ion contact with him.
  6. SIVAllan

    SIVAllan Gold Level Sponsor

    Thanks Michael.

    I do have flat tops and will use a Redline manifold to run upright--maybe I can get a vendor of new 40DCOEs to re-jet a pair to match the WSM specs...
  7. SIVAllan

    SIVAllan Gold Level Sponsor

    Ok, just saw the post.

    Please do put me in touch.


  8. Jim E

    Jim E Donation Time

    The F 16 e tubes seen to cure a lot of ills, idle jets seem to move problems around in the RPM band a bit. These carbs are reaL interesting to work on and I sugest you find someone who knows how to balance and adjust them to give you a lesson will make it a lot easier in the long run.

    When I was running a set of these on my SV was lucky enough to have a friend who knew how to tune on them and owns boxes of e tubes and jets. Spent a lot of time tinkering and trying different set ups and did who knows how many plug cuts. In the end was jetted near what 64 listed and it ran pretty good, after that mostly just tried different idle jets because they were there to try. Never got it absolutly right but then think I needed smaller chokes and the progression holes are just where they are..
  9. RootesRacer

    RootesRacer Platinum Level Sponsor

    Nah, you will never find the special aux venturies, and IIRC you arent running the actual holbay cam are you?

    The holbay jetting is odd becuase both the camming and head config is quite differrent compared to normal setups.
    If you dont have EXACTLY the holbay setup, the holbay jetting will not work

    I suspect you are going to have to jet this one out like the rest of us..
  10. H One-Twenty

    H One-Twenty Donation Time

    If you are going to use the Redline intake with the Holbay head you will have to open up the intake ports on the head, or somehow sleeve the intake to match the ports, otherwise airflow will be severely disturbed.
  11. SIVAllan

    SIVAllan Gold Level Sponsor

    Correct. I will not be using a Holbay grind due to it's power curve starting abt half way to the RPM redline.

    I believe the Coltec (UK) E78 grind is an equivalent to the Holbay grind. Coltex may even be current day Holbay - not sure.

    I plan to use the Delta KB grind.
  12. SIVAllan

    SIVAllan Gold Level Sponsor

    Thanks--I will ask the vendor about this. I don't think I want to disturb the intake ports.

    How disturbed is "severly" disturbed?
  13. serIIalpine

    serIIalpine Donation Time

    Fastroadcars.com has the "goose neck" intake that was not available for the longest time. This will help you with clearance issues around the brake M/C and the fender brace etc.

    If you have your heart set on the straight intake make me an offer for mine and I'll get the one from Fastroadcars.

    Mine has been relieved to accomodate the stainless header and mounting hardware.
  14. H One-Twenty

    H One-Twenty Donation Time

    As the ports on the Holbay head are smaller than the stock Alpine the Redline manifold will not 'match' the Holbay ports. This step down in size where the manifold meets the head will interrupt the airflow and reduce performance.

    I opened up the ports on my Holbay head to 'port match' them with the manifold. I had to remove quite a bit of material (increased radius 2.5mm+) to achieve a smooth transition from manifold to head. As the idea of polishing and porting a head is to maximize performance by improving the airflow, I felt that the large lip or step between the manifold and head was too much to ignore. I'm sure the engine would have run just fine without doing this, but I wanted to extract as much power and torque as possible from my set up.

    I believe the Holbay ports were made smaller to improve port velocity and boost torque. I only opened up the outer portion of the ports to try and maintain the good characteristics of the Holbay set up.

    I have some before and after pictures somewhere, I can try to post them when I get home if you like?
  15. H One-Twenty

    H One-Twenty Donation Time

    A stock 1725 manifold gasket on a Holbay head. The intake port on the right has been opened up to match the Redline manifold.

    Attached Files:

  16. RootesRacer

    RootesRacer Platinum Level Sponsor

    Just an FYI and since the thread is "jetting dcoe 40's" it seems in place here.

    I just went out and ran my alpine for the better part of an hour trying to get the jetting better, now with the benefit of the wideband I've been telling you you all to buy and use for years (doctor heal thyself).

    Originally I tuned this bugger by feel, but fouling plugs told me I wanst where I though I was.
    Old jets: 55F8 idles, 115 mains, 31mm modified chokes, 180 airs, F11 emulsion tubes

    Set to idle at best smooth "lean" idle, which was in the mid 13s (13.5 AFR).

    Wideband tells me that the idle jets are too large becuase AFRs drop to around 11 AFR in the progression portion of operation.

    Wideband also told me (while driving) that the AFRs in the mains portion of operation was running in the low 14 AFR range.

    Changed jets to: 45F9 idles, 125 mains and left the chokes, airs and emulsions alone.

    Start and reset idle and mixtures for best smooth "lean" idle.
    Wideband seems to think I cured the progression richness as now it runs in the 13 to 14 AFR range as it should. But when I get into the mains portion, it got back into the high 11s, so 125s were just too much, dropped them back to 120s.

    Wideband likes this all the way round, engine can cruise into the 14s like it should, and power is now in the low 13s as it should be.
    Engine occasionally "pops" on transision from progression into mains, I suspect that an earlier emulsion tube could be helpful, perhaps an F16.
    I have to ignore it based on the meter since before I would have (and did) just fatten the progression mixture, which was obviously NOT the right thing to do.

    After shutting the motor down, the gas started to boil in the carbs.
    A heat shield would probably help this.
    I also noticed the idle mixture setting was very particular to the under hood temp, and therfore should probably be quickly adjusted after opening the hood while at temp.

    This info is all pretty much for trivia since the carbs are coming off real soon and its going back to EFI. I though some of you might be interested in the jets that the meter thought were good as opposed to my butt dyno.

    God I cant wait to adjust all this from inside the car, and to be finished with the gassy fumes that boil out of the hot carbs.
  17. alpine_64

    alpine_64 Donation Time


    its interesting that your sensor has lead you to pretty much the standard jet settings we use here:

    30mm choke (not that much option on that)
    115-120 mains
    170-75 airs

    must be close on to what alpines like.
  18. RootesRacer

    RootesRacer Platinum Level Sponsor

    Actually if you go back in time to my earlier postings, its where I started.

    But the spitting back I experienced convinced me that these are not quite the right combos.

    The wideband sensor says otherwise.
  19. thill35803

    thill35803 Donation Time

    Alternate Manifold to Head Port Matching

    I am using the Redline intake with the Holbay head. I am using a NOS Holbay head and couldn't bring myself to grind the ports. I had the Redline intake welded up and massaged to match the Holbay ports.

  20. H One-Twenty

    H One-Twenty Donation Time

    I wouldn't have modified a NOS Holbay head either. That's a lucky find!

    Cracked or corroded ones on the other hand are pretty easy to pick up in the U.K. I had no worries modifying a head that I'd 'saved' (water jacket corrosion).

    I'd thought about modifying the intake, but I reckon it's worth more than the head!

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