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jetting dcoe 40's...

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

  1. sunbeamowner

    sunbeamowner Bronze Level Sponsor

    Well do you want the good news or the not so good news ??
    I will start off with the good news. I have just converted my 1725 running a hunter GT cam to twin 40mm dcoe (151) carbs from a single 45 mm dcoe. An increase of 20 bhp at the wheels was achieved by tuning on a dyno .
    The Alpine has never run so good. It is very drivable in all traffic hot or cold wet or dry.
    Now the not so good news. You get what you pay for $$$$. It can take an experienced operator hours on the Dyno to get the jetting right for your particular set up. Did anyone notice the '151' in my carb description ?
    Not all 45 mm dcoe webers are the same. Some are made for specific applications like Fiats Alfas etc etc. The '151' is a universal carb that has more scope (different internals) for finer tuning to suit your set up (so my dyno guy says). I originally tried 40mm DHLA Dellorto carbs but even the importer of Dellorto carbs could not supply the jets required to get the right numbers on the dyno.

    And don't get me started on 'velocity stacks' and air filters.

    Attached Files:

  2. SIVAllan

    SIVAllan Gold Level Sponsor

    I believe the Hunter GLS H120 engine used type 34 & 35, and the Rapier H120 used type 90 & 91, but am unclear on the differences one to the other or vs. type 151.

    That's the number after "40DCOE" ? As in 40DCOE151?

    Possibly a clue on best stack or "ram tube" length is provided by the factory's selection?

    20 bhp gain?? Outstanding :)

  3. H One-Twenty

    H One-Twenty Donation Time

    Speaking of air cleaners - My 73 H120 is completely factory stock in every way and runs very well, no flat spot, popping etc anywhere through the rev range. However, if the air box and cleaner are removed the motor will not pull under load at all, at any RPM. It runs way too lean. I experimented with a simple 'free flow' air filter element and it really upset the running. I did not want to re-jet the carbs so I put the stock one back in. Even a slight leak around the air filter box to carb gasket will create a flat spot at low RPM.

    If anyone is having trouble getting the 'stock' Holbay Weber set up to run well, make sure and have your air filter(s) fitted when jeting the carbs.
  4. RootesRacer

    RootesRacer Platinum Level Sponsor

    I am a weber insider as I design equipment sold by the US's largest supplier of weber carbs.

    The difference between DCOE numbers (models) is two things:
    1) Number, size and positions of the progression holes.
    2) The specific linkages that are provided (why there is a front and rear carb typically).

    All other internal details are replaceable tuning components.

    The type 151 (40DCOE) is the ONLY 40 DCOE still in manufacture.
    The type 152 (45DCOE) is the equivalent in the 45mm form factor.

    When weber went out of business (before Magnetti Marelli resurrected the name) there was an unforeseen market of high performance carbs needed for the non OEM market (weber had previously done very well in the OEM market).

    The lack of replacement carbs was a big problem for the folks selling webers, and Marelli finally decided not to ignore the market.

    They setup shop in Spain to be able to produce a low cost carb (and low quality one for a while too) as the Italian labor rates were too high.
    Since this was a reduced quantity market, Marelli had to decide what made sense to build, and what did not.
    With all the crazy models of DCOE that were produced over the years, it was unclear what made the most sense, to they developed the "universal" DCOEs
    so they could produce only one flavor of each DCOE.

    What these carbs did do make them universal was this:
    1) They selected the most common and usable progression configuration.
    2) They added throttle bypass screws for each butterfly.
    3) They added a single compromised "starter" assembly to allow for rudimentary cold weather enrichment (as DCOEs dont have stranglers).

    With the addition of the throttle bypasses, this allows a clever tuner to be able to effectively move the positions of the progression holes by allowing the idle speed (and relative throttle plate positions) to be moved, which adds another tuning dimension to the already vast number of tuning parameters.

    Other than the above items, which only augment the tuning, all other attributes are exactly the same as the traditional weber DCOE carb and respond identically to tuning procedures.
  5. djnippa

    djnippa Donation Time

    Standard 1725 fast-road to Holbay H120 specs.

    I have just had my standard 1725 upgraded to fast-road Holbay specs. There are still a few creases to iron out. I'm hoping that a few people on here may know the answers.

    Holbay Head
    Flat-Top pistons
    Cam cut to Holbay Specs at (www.kentcams.com)
    Twin Webers 40DC0E 90/91's
    Distributor with electronic ignition and no vacuum advance.
    Lightened fly-wheel
    Full balance.
    All new shells, springs, etc.
    Crankshaft - Micro polished (didn't need grinding)
    Custom made -7 degree plate to compensate for the angle of the inlet manifold.

    The garage who did it for me, said that from set-off the engine would shake violently. It was so aggressive that they thought it was going to rip off the engine mounts. They called everyone, but nobody knew the answer. In the end they decided to block up two of the three progression holes in the carb, and it seemed to solve the problem.

    I was obviously concerned about this, and contacted Webcon. They said they had never heard of anyone blocking progression holes, unless someone had drilled extra holes. They said the standard for the Sunbeam Rapier H120 40DCOE 90/91'a was three.
    He then said he doesn't think it's the progression holes, and that it might not be jetted exactly to my set up, as the cam has been changed. He suggested getting it on a rolling road, as soon as it had been broken in (500 miles).

    Has anyone else ever heard or had this problem?

    He was also kind enough to email me the original jetting / calibration from my carbs.
    Choke 30
    Auxilliary Venturi 3.5
    Main Jet 115
    Air Corrector Jet 200
    Emulsion Tube F34
    Idle Jet F18/50
    Calibrated port 110 1.1mm
    Pump Jet 35
    Valve at bottom of float chamber 100 1mm
    Jet that's in cold start F5/140
    Needle Valve 150 (Part No. 79503)
    Progression Hole T1 1.3mm
    Progression Hole T2 1.1mm
    Progression Hole T3 1.6mm
    Float level 7.5mm (Top of float and gasket when needle valve is closed, without ball in the needle being depressed.)
    No more than 5 degrees tilt recommended.

    Could I ask what's a REDLINE Manifold?

    I am still yet to drive the car, so cannot give any feedback, as to whether it was worth it or not.
    Last edited: Jul 31, 2014
  6. RootesRacer

    RootesRacer Platinum Level Sponsor

    Holy necro-posting batman!

    If your tuner cant figure out how to tune without blocking off progression holes, best be rid of them soon.

    You say you have a "holbay" cam ground? Which one?
    I true "H120" cam has about 8 degrees of advance on the cam, a stock alpine cam CANNOT be ground to H120 specs and install straight up.
    A H120 cam probably wont idle well installed straight up either since it has a lot of overlap.

    If you have an E128 cam, it is installed straight up and has less overlap.

    Under what condition is it shaking?

    If its at idle, fix that first (remove the main and progression from the carb).

    Why are you running F18 idle jets and not an F8 or F9?
    .45 or so would be the normal orifice.
  7. djnippa

    djnippa Donation Time

    Sounds like you really know your stuff.
    The guy working on it, is not a tuner, he's just the engineer who's working on it, as he does a lot of Holbay spec'd vehicles.

    It's not a Holbay cam. I asked for fast-road like an E128, or as Kentcam.com have put on the invoice P06 which is:-
    8mm of left
    Duration 283 degrees
    Running clearance 16 thousand on inlet and exhaust
    108 lobe separation

    The mechanic/engineer told me the car would shake under load when setting off from a junction or traffic lights.

    Since he blocked some of the progression holes, he said it has fixed the problem. He was really pleased about it, as it's been a problem he could never resolve before.
    He said none of the other suggestions the 'Weber Specialists' mentioned worked. Probably why he was so content with his solution.

    The F18 idle jets are already in the 90/91 carbs.
    Here's a download link to the PDF that the Webcon expert Andy Grey sent me.
    Do you think these may need changing?

    When I purchased the carbs, they were jetted to a Hillman Hunter GLS Holbay.
    Do you think it could be the jetting, distributor, cam or something else?

    Sorry, I don't know what '.45 or so would be the normal orifice" means.

    Last edited: Aug 15, 2014
  8. RootesRacer

    RootesRacer Platinum Level Sponsor

    The H120 jetting is very particular to that cam and engine configuration, a narrower lobe center will need an entirely different arrangement.

    The holbay specific DCOEs have an odd aux choke that extends into the velocity stacks. This will be problematic for a more conventional cam and its requisite jetting.

    The F8 and F9 (F2 also) are about the only idle jets made these days, they function in most any application, whereas oddball jets like the F18 are application specific to cure some sort of issue the factory tuners had for that app. The number designation for the idle jets includes an emulsion configuration that allows early vs late signal strength on the emulsion to correlate with the progression positions (which influences the vacuum strength acting on the jets orifice and progression holes).

    Idle/pilot jets should not be selected based on quality of idle, instead proper function of progression will determine the proper jetting.
    When the progression functions properly, the idle mixture screw range should be withing .5 turns of closed to 2 turns open. If its not, you need a different jet type "F" number and good luck with that...

    IIRC standard DCOEs come with a 4.5 aux choke that has a shorter annular section. The 3.5mm is to get more main signal with larger main chokes.
    This is an H120 specific attribute that hopefully you wont need.

    You will probably want 28 or 29mm main chokes for a better signal and start with 115 main jets.
    Mains should ONLY be selected based on mid to high RPM function at wide open throttle. All non wide open throttle issues should NOT cause you to adjust the main jet for any reason.

    The reason your tuner blocked some of the progression holes is probably becuase he doesnt understand how the progression is supposed to work.
    The idle/pilot jet contains the emulsifier for the idle/progression circuit.
    If the emulsion is wrong, it will seem like the progression is too strong.

    To get this sorted, remove the main jet and emulsion tube and get the engine to idle and rev cleanly to 3500 RPM. After you get it to successfully function in the progression region of the throttle, then you can try it on the road or rolling road if finances allow.
    Dont install the mains till you have sorted the pilot jetting and progression for light load. After the throttle begins to pass the final hole, the engine will fall on is face so dont confuse yourself tuning for this sudden lean condition, its to be expected.

    With a mechanical only distributor set the timing at high RPM to the max advance figure. Dont set timing at idle, only the max timing matters.
    Do make sure though that the idle to max timing figure follows the timing number stamped on the weights times 2. This will be 9 to 12 (18 to 24 degrees max timing).
    If for instance you have a 11 stamp and set your max timing to 32 degrees, the idle timing should be around 10 degrees BTDC (32-(11*2)=10), if not determine why.

    All jet numbers on webers are specified in hundredths of a mm.
    So a 115 main will be 1.15mm orifice.
    a 45 idle/pilot jet will have a .45mm orifice.

    Hope this helps a bit.
  9. djnippa

    djnippa Donation Time

    OK. It's official, you know more than anyone else I've spoken to.
    To be honest a lot of it went over my head, but what I picked from it was that I need to get this on a rolling road, surrounded by a team of experts to do the fettling.

    I think the engineer, just wanted to get it running, but I think he knows deep down that it's not right.

    Huge thanks for giving me an in depth step by step process on how to fix the problem.
    I think you could be right about it all.

    I'll keep you informed of the progress, and hopeful resolution.
  10. alpine_64

    alpine_64 Donation Time

    the extended auxillary venturis on the the 34/35 and 90/91 holbeay spec webers are to solve the pulsing issue the holbay motor had.. it would spit fuel vapor back down the inlets and the extended venturis allow the carbs to draw fresh air instead of the fuel rich vapor that would cause a rich condition.
    The holbay setup also ran a large air box and filter setup. They also vent through the special crowns on the top covers as opposed to the face as per normal DCOE.

    As Jarid said they run a very specific lean jetting setup... i posted ages ago a bunch of generic hjetting that seems toget you in the ball park on a sunbeam.. the main size in the USA seems larger than we use in Australia.. not sure why... but anyway..

    130 mains (states) in AU we seem to use 115-120
    F16 e-tubes
    175 air correctors

    thats what i recall off the top of my head.. will try and dig the info up later.. also how have you set your linkage.. do you have a centre rod that activates between the 2 carbs with an adjustable yoke to allow you to get correct sync on the carbs? What inlet are you running?
  11. Dodgealpine

    Dodgealpine Donation Time

  12. djnippa

    djnippa Donation Time

    OK, I had it on a rolling road yesterday, with a guy who has over 40 years experience. When he first tested it, it only had a pathetic 77bhp.
    He then put in different jets, and after removing the four 'custom plugs' from the progression holes, he balanced the carbs,
    It still ran like a dog with one leg but he did manage to get it up to a slightly better 114bhp.

    The top end was good, the idle was horrific. To get to the top end would require flooring the accelerator, then waiting for it to smooth itself out, after a huge engine vibration. The gearstick was wobbling like crazy.
    He tried everything for 3 hours, and still no joy. So he took it for a drive, and when he got back he said "I've driven a lot of these in my time, but that has to be the roughest. I would suggest getting a new set of carbs, or have these ones looked at, as something is definitely wrong with them".

    So he put everything back as it was except the 115 Main Jets and I drove the car home. It was the hardest drive i've ever had to do. It was worse than when I drove there. It stalled 16 times, and the engine violently vibrated all the way home. I could've even damaged the engine.
    So I'm none the wiser.

    Somebody said perhaps the timing gear has been put on one tooth out, or the cam-shaft has been ground to the wrong specs by mistake. Or the carb has been restored incorrectly.
    Does anyone have any ideas, as everyone else hasn't got a clue.
    Last edited: Sep 19, 2014
  13. sunbby

    sunbby Past SAOCA President Donation Time

    What exactly does this mean? Are you talking about the actual progression hole plugs that screw in or something else?

    If he removed the progression hole plugs I don't think there is any way you can get it to work at part or closed throttle. The carbs are not designed to run without those plugs.
    Last edited: Sep 19, 2014
  14. djnippa

    djnippa Donation Time

    The original mechanic who worked on it custom made, 4 little plugs that blocked up each of the four separate areas where there are three progression holes.
    The carbs were designed to run without these, not with them. Hence why he removed them.
  15. RootesRacer

    RootesRacer Platinum Level Sponsor

    The newfound horsepower has NOTHING to do with the removal of the plugged progressions holes.

    Power comes from the mains and chokes, not the progression.
    The progression is (now) why it stalls and runs poorly at idle and light load.

    You may well have messed up carbs but one thing I can say for sure is you need to fire the tuner and find another.
  16. djnippa

    djnippa Donation Time

    Yes I realise that. He was shocked that the plugs were actually there, and was glad I told him about them, as he would never have checked them.
    However by the end of the session he said he could see why the other engineer had made the plugs.

    This is what he did to it.
    It had 115 Main Jets (which he is returning to me, so I can at least drive the car)
    It’s now got 140 Main jets (I now know this is where the power comes from)

    It also has:-
    50 f14 idle jets (he did try 45 and 40 - but still running way too lean)
    F34 emulsion tubes
    210 air correctors

    I'm actually thinking that there are three possibilities now.
    1) There is something wrong with the carbs.
    2) The Timing Chain has been put on a link out.
    3) The Cam-shaft has not been correctly ground.

    What's your money on?
    Last edited: Sep 20, 2014
  17. RootesRacer

    RootesRacer Platinum Level Sponsor

    Is this "carb" or "carbs"?

    Title is "jetting dcoe 40's..." which implies a plurality.

    Its very unlikely that something is wrong with all bbls of the carb(s).

    One bbl may have a problem but DCOEs have absolutely separate circuits
    so unless one cyl or a cyl pair (for single DCOE) has oddness in its spark plug,
    I'm going to have to go with tuner error.

    Those are odd emulsion tubes and idle jets for a single weber config.

    I'd begin with a conservative/common set of jets/chokes, hire an experienced tuner and have a do-over.

    It would be nice to get some degree data on the cam. This you can do yourself with a dial indicator and degree wheel. This will tell you if you are straight up on your overlap or have a considerable offset (like if you were off a tooth).
  18. djnippa

    djnippa Donation Time

    It's Carbs.

    I'll try to get you more info on the cam on Monday.
    Thanks for taking the time to look at this.
  19. Rootes 66

    Rootes 66 Donation Time

    Noticed a photo on FB where plug lead No1 appears to come from distributor cap in 8 O'clock position........Alarm bells..
    My bet is your "professional engine builder" has aligned the cam timing sprockets and wrongly assumed that position to be No1 TDC, when in fact its No4, you need to turn the engine through a further 360 degrees for No1 TDC.

    Check the cam is properly timed before spending a Penny more!

  20. djnippa

    djnippa Donation Time

    WOW, if that's the case, then that is a major F**K-UP.
    I'll email the so-called 'engine builder' your information straight away.
    Huge thanks.

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