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  1. #1
    Senior Member GASSD6's Avatar
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    Default 750 quick fuel carb 308 tuning question

    HI guy had eng built and eng dynoed, quick run down 308 288 cam alloy heads harrop hise 2'' spacer 750qf carb.

    right heres my question for a carb expert or i might be barking up wrong tree. motor made 460hp

    starts and runs from cold ok better when warmed up, drive it seems good pulls hard doesnt miss, flat spot or anything silly but when i turn it off it will not fire up again. it just turns over an over to the point of battery going flat and then it will just cough enough to give it couple pumps on accelerator and it might fire up. so i thought i was not get spark or 12v when cranking, its running a ready to run msd and a blaster3 coil no box. i was getting voltage drop at coil so i ran a relay straight to coil from battery. didnt help. optimum 750cca battery

    so carby from what iv done, has 2.5pv 77air bleeds and std jetting from box, carb is brand new. im lost......

  2. #2
    Part of the furniture gtrboyy's Avatar
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    Found the engine harness plug was brittle on my vh causing a voltage drop or sometimes really poor connection acc power to coil that it wouldn't start.Most times it was good then 1 out of 10 starts it played up like it was just about to fire but wouldn't.

    Other thing didnt help was 55amp alternator at night....

    Usually when cold just 2 small pumps,crank it...when it fires just dab throttle untill gets some heat or idles by itself.

    After that usually just a few cranks it fires then dab throttle untill it idles on it's own.
    Last edited by gtrboyy; 16-12-2016 at 08:50 PM.

  3. #3
    Senior Member GASSD6's Avatar
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    Quote Originally Posted by gtrboyy View Post
    Found the engine harness plug was brittle on my vh causing a voltage drop or sometimes really poor connection acc power to coil that it wouldn't start.Most times it was good then 1 out of 10 starts it played up.

    Other thing didnt help was 55amp alternator at night....
    yeah out of the vk loom im only running the thick pink wire to coil, to which now its my trigger (ign) wire to relay and new wires to coil and battery. and i have converted a vn 85amp alternator which when runnign is charging at 14.2 volts

  4. #4
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    Try foot flat next time it will be flooded...

    I have a qft650 on mine (blow through)i have just got around to purchasing a phenolic spacer.
    PHENOLIC CARBY SPACER 2" 4 HOLE - RPCR9135 | eBay

    Mine boils the fuel out of the bowls

  5. #5
    Do you ever leave? EH179's Avatar
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    Make sure you have good power to the ignition first.
    What fuel pressure do you have?
    Sight plugs for fuel bowls fitted? where are they at?

    2.5 Pv, really? Not liking that, but that is not your problem here.

  6. #6
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    Normally too much initial timing.

    There wont be many street holdens with 750's that need less than 8.5" pv, hell, my shitter engine with 224@0.050 needs a 10.5.

  7. #7
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    What lash settings you running ?

  8. #8
    Senior Member GASSD6's Avatar
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    Quote Originally Posted by HQ 368 View Post
    What lash settings you running ?
    20thou cold i think they were set.

  9. #9
    Senior Member GASSD6's Avatar
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    Quote Originally Posted by Toe Rag View Post
    Try foot flat next time it will be flooded...

    I have a qft650 on mine (blow through)i have just got around to purchasing a phenolic spacer.
    PHENOLIC CARBY SPACER 2" 4 HOLE - RPCR9135 | eBay

    Mine boils the fuel out of the bowls
    The first time it happened i thought it was flooded but pulled plugs out every one was dry no fuel at all. Slight dis color.

  10. #10
    Senior Member GASSD6's Avatar
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    Quote Originally Posted by EH179 View Post
    Make sure you have good power to the ignition first.
    What fuel pressure do you have?
    Sight plugs for fuel bowls fitted? where are they at?

    2.5 Pv, really? Not liking that, but that is not your problem here.
    Im sure i have good power now. Running 6.5-7fuel pressure. Fuel float levels are right on the arrows on sight glass. Because of lack of vacuum brakes aint the est so dyno man said just put 2.5 in it for now in case when my foots on brake it might be olening up tbe 4.5 i had in it.
    Is there any other why to wire up dizzy. Im using original vk pink thick wire as an ign trigger to a relay an all new wires str8 to battery an coil.

  11. #11
    in the shed buildin shit 3554me's Avatar
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    Might be a case of low air speed thru the intake and heads on wind up , has
    it got a reduction starter or is it winding over fast ?
    Last edited by 3554me; 17-12-2016 at 10:57 AM.
    10.44@128.90 on e85 26x10
    10.48@129.67 on e85 255 radial


  12. #12
    Not the Kingswood! hq308's Avatar
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    Unless there is a major issue with the carb the power valve will have no effect on the idle circuit, the whole "set your power valve at half your idle vac" is a myth. My old combo had 5" vac at idle in gear and I still needed to run a 6.5 power valve to get the AFR's right, you need to check your vac at cruise and see what it drops off to when you accelerate. If yours drives nice with the 2.5 it's likely over jetted to compensate, the power valve wont be opening till it's near full throttle. The power valve is part of the main circuit which has no effect at idle.

    With your hot start issue, have you tried giving the throttle a pump or two before cranking? You shouldn't need to do that but it will show if it's maybe a little lean when starting.

  13. #13
    Do you ever leave? HQ308Ute's Avatar
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    I'd go with making sure there's good power to the ignition.. i had the exact same issue at one time.. once its warm wouldnt fire. I was pulling my hair out ****ing around with the carb turns out it was a bad ignition earth. fired with a turn of the key after that.

  14. #14
    casual poster Kocho's Avatar
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    Quote Originally Posted by Toe Rag View Post
    Try foot flat next time it will be flooded...

    I have a qft650 on mine (blow through)i have just got around to purchasing a phenolic spacer.
    PHENOLIC CARBY SPACER 2" 4 HOLE - RPCR9135 | eBay

    Mine boils the fuel out of the bowls
    Mine did the same boiled the fuel out the bowls could hear it few minutes after shutdown I put a 1 inch phenolic spacer on mine seems to start easy now

  15. #15
    Part of the furniture gtrboyy's Avatar
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    Pink wire you're using comes off ignition switch block but you're using as trigger & power off battery for the coil so that should be good.

    Mine is crap vacuum too still fires & set up roughly same float levels etc



    Think maybe yours tad too much initial timing?

    ve sidi v6 vacuum booster will help get brakes back running that to booster although wondered if it helps or hinders if run to manifold.
    Last edited by gtrboyy; 17-12-2016 at 03:38 PM. Reason: .

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