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Thread: Idle and main jet increments dcoe twin 40's

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    Idle and main jet increments dcoe twin 40's

    I jetted my carbs as per some recommendations from a fellow member on here,

    2.0 pinto some head work and twin 40 dcoe carbs.

    had it on the rolling road the other day, I knew it was rich at idle befor I went as it was black smoking at idle.

    the idle screw isn't even 1/2 a turn out and it's showing about 10.5:1 on the air to fuel ratio

    at full throttle it goes off the scale rich and won't even pull through, the only way to get a run was part throttle until 2500 osh then full.

    i know I need smaller idle jets and I understand that f8 a leaner jet than f9

    i currently have 55f8 idle and I think I have in my toolbox a set of 45f8 that I might try.

    my question is, is there a rule of thumb or a calculation to work out what would be best or is it a case of trial and error?

    also I have f11 tubes with a 135 main and 180 air corrector but it's running off lean between 3500 and 4500 to about 13.7:1 and then goes to 14.3 upto 5500 where we stopped as its lean.

    i think I need a bigger main and a slightly smaller air corrector, is this correct?

    the rolling road graph of only part throttle until 2500ish



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    Re: Idle and main jet increments dcoe twin 40's

    here is a chart for jets

    http://forums.190slgroup.com/showthr...0-and-3800-RPM

    btw, throttles has to be nearly closed at idle and adjust with mixture screw.
    Throttle stop half turn screwed from contact with lever.

    But ignition timing and carb balance has to be spot on, otherwise you'll never get it correct.

    No vaacum advance!

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    Re: Idle and main jet increments dcoe twin 40's

    There is no vac advance.

    I'm on holiday at the moment and will be having another go at it when I'm back.

    I'm thinking of changing the idle jets cause of the black smoke at idle and shutting the throttle bodies up would make the mix richer? On the rollin road in 4th gear you give it full throttle at anything less than 2.5k and it bogs down and kicks out lots of black smoke, and it's that bad it won't even pull through,

    It's got 32mm chokes and 175 needles and a newmans ph4 275 sprint cam

    Or are you saying if the throttle butterfly's are open to much it's sucking more fuel in? Ie like part throttle?

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    Re: Idle and main jet increments dcoe twin 40's

    Quote Originally Posted by Warrior View Post

    Or are you saying if the throttle butterfly's are open to much it's sucking more fuel in? Ie like part throttle?
    yes. At idle engine should be running mainly across screw mixtures. Throttles should be barely open with around half a turn of the idle screw

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    Re: Idle and main jet increments dcoe twin 40's

    I think that's how it's set but no harm in double checking,

    I think when I'm home I'll start from scratch as if I've never seen them befor and go from there.

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    Re: Idle and main jet increments dcoe twin 40's

    Hi, the jetting I give is a starting point to get the engine running and get it fine-tuned.
    If you have issues like the ones you write about, I guess there is something (fundamental) wrong with the engine; carbs or ignition.
    Have you checked the floatlevel and also that floats aren't leaking?

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    Re: Idle and main jet increments dcoe twin 40's

    Quote Originally Posted by Miniliteman View Post
    Hi, the jetting I give is a starting point to get the engine running and get it fine-tuned.
    If you have issues like the ones you write about, I guess there is something (fundamental) wrong with the engine; carbs or ignition.
    Have you checked the floatlevel and also that floats aren't leaking?
    Hi, thanks for the info in the previous thread it got me up and running and the car drives, far more than I would have got without your help,

    I did check them but like I put above, when I'm back from holiday I'm going to start a fresh and go from there, I'll check the timing again and check the cam is dialled in 100% then zero the carbs, ie mixture screws and the idle are at a zero starting point, like 1/2 turn out on the throttle stops and 2,2-1/2 out on the idle mixtures and check the float levels,

    Also what fuel pressure would you recommend I try as a start point?

    Could you advise of anything that could be fundamentally wrong? Timing was checked the night befor the rolling road and it was ok, 14 deg ar idle

    It black smokes worse than my 1987 diesel landrover at idle
    Last edited by Warrior; 20-06-2016 at 21:35.

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    Re: Idle and main jet increments dcoe twin 40's

    if it black smoke that much you have to wonder if the choke/cold start enrichment is stuck on is on

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    Re: Idle and main jet increments dcoe twin 40's

    Quote Originally Posted by Graham View Post
    if it black smoke that much you have to wonder if the choke/cold start enrichment is stuck on is on
    I did wonder this but I removed the 2 lever bits and checked there weren't on and the look closed,

    Think I'll remove the carbs and have a good look at them

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    Re: Idle and main jet increments dcoe twin 40's

    Quote Originally Posted by daniele View Post
    yes. At idle engine should be running mainly across screw mixtures. Throttles should be barely open with around half a turn of the idle screw
    Thanks for the above, I'm back now and had a look tonight, 1st I checked all the valve clearances 4 of them where out, then I've dialled the cam in to full lift at 110 deg atdc which is how newmans measure it, this means my cam is now 2 deg retarded, only thing not check was ignition timing at idle as I was struggling on my own tonight and I don't have enough hands,

    The idle speed screw held the throttle a fair way open and the mixture screws were about half a turn open,

    I backed the idle speed screws right off and then back in untill they contact and then 1/2 a turn more,

    Ive got the idle mixture screws set at about 2.5 turns out and finished near 3

    Now sounds a lot nicer but won't idle, I'm gona have another look tomorrow night,

    If I can get it to idle at say 4 turns out on the mixture I need bigger Idle jets yes?

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    Re: Idle and main jet increments dcoe twin 40's

    Going in holiday to Scotland now. Leeds is too far to come there. Good luck!
    check ign timing first. You are on the right direction but now you have to check that spark position
    55f8 seems ok to me. Once set the timing, play between throttle stop and idle screw. Check also for false air.
    ciao

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    Re: Idle and main jet increments dcoe twin 40's

    Quote Originally Posted by daniele View Post
    Going in holiday to Scotland now. Leeds is too far to come there. Good luck!
    check ign timing first. You are on the right direction but now you have to check that spark position
    55f8 seems ok to me. Once set the timing, play between throttle stop and idle screw. Check also for false air.
    ciao
    Ok thanks, enjoy you holiday

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    Re: Idle and main jet increments dcoe twin 40's

    I've check the timing and it's set at 14 deg at idle, I found that adjusting the idle mixture screws effected nothing, did not idle up or down and wouldn't idle without the speed crew screwed it to hold the throttle open,

    I've fit some 45f9 idle jets which I had that came with the carbs, I now have it idling on idle mixtures alone although it's low (600rpm) and less than half a turn in the idle speed screws,

    No black smoke now, but I suspect it might be lean now befor it comes into the main, I have also increased the main jet size to 150 as it was very lean, I also have some 140's to take to the dyno and a set of f16 tubes that I could use if needed,

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    Re: Idle and main jet increments dcoe twin 40's

    The car now has an mot, It drive much better now but still not 100%

    It spitting back when driving along at light throttle, and it hesitates when you make it pull hard from low rpm's

    I've made the idle mixture richer and it's better but still not there, I was thinking of goin to an 50f9 idle does this sound a logical step?

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