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Thread: carb trouble

  1. #1
    Junior Member SHOW GUEST
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    carb trouble

    hello everybody, i have 327 motor, 9.1 CR, comp cam {274-286} {230-236} {487-490}, vortec cylinder heads, edelbrock vortec performer rpm intake, 575 demon mech sec carb, pertronix flamethrower hei ignition, turbo 350 trans, 2800 stall, 4.10 gears. car is running so rich at idle, it will burn your eyes, it has a new power valve, and i know what the vacuum is, so i know it is not opening, the front idle mixture screws have hardly any effect, i have 18 initial timing with vacuum advance plugged, then when i hook it back up i have 44 at idle, i run manifold vacuum for advance, does that seem like to much? any suggestions?

  2. #2
    Senior Member RACING JUNKIE fla1976's Avatar
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    rich

    What jets are you running?

  3. #3
    Junior Member SHOW GUEST
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    69 primary and 78 sec with 6.5 power valve.

  4. #4
    Senior Member RACING JUNKIE
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    The vacuum advance should only be hooked to a ported vacuum source (no vacuum at idle), not manifold vacuum (vacuum at idle).
    There should be no vacuum present at idle but when you start to open up the butterflies then ported vacuum will become present.
    The vacuum advance works in conjunction with the centrifical advance and the total of these is added to your initial, this is your total timing.
    For instance I.E. 10 initial, plus 18 centrifical, plus 10 vacuum advance will net you a total timing of 38 degrees, which is about where you want to be.
    The may not be your problem but its something that needs to be addressed so you can get the most performance out of your engine.

    After getting the timing squared away adn the engine off run all your idle mixture screws in till they bottom out then unscrew them all 3 full turns, this is just a starting point. You will probably end up somewhere between 2 1/2 to 1/ 1/2 turns out.

    Sometimes you have to drill some air bleed holes in the primary butterflies, a very small drill bit will work, usually less than a 1/8 bit will do the trick, start small and work your way up no sense in getting crazy here, causes theres no backing up once you go to big.
    Drilling the holes will add extra air necessary to get carb to idle correctly when using a high lift cam.
    Goodluck, Cp

  5. #5
    Senior Member RACING JUNKIE
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    If you have the idle speed screw on the throttle linkage turned up enough to uncover too much of the transfer slot you will be pulling fuel from the main circuit not the idle circuit...That is usually what causes one not to respond to the mixture screws. Get the timing ironed out and you may be able to back off on the speed screw and get back on the idle circuit. Drilling the holes Charlie mentioned will help this too..more air gets through so you maybe can back the screw out some.

  6. #6
    Senior Member RACING JUNKIE
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    I read what CP & Tod said about the primaries & drilling. I had the same problem w/a 850 DP on a BBC. I had a internet documentary about this very thing & I drilled 1/8" holes in the primaries (one per) in the corners farthest from the idle transfer slots. This helped a great deal. Later, I replaced the carb with a QF/HP which has secondary butterfly/adjustments which does the same thing. I want to note the obvious however: if you are in a shop, you are going to get burning eyes & a rich smell anyways while tuning/running on a HP motor. In addition, I also run a primary PV. although my dist. is manual, I run 2 to 2.5 steps lower than my inches of vac at idle which works good for me. Good Luck!! Scooter
    "I would walk through hell on Sunday before I fear the enemy"

  7. #7
    Senior Member MASTER BUILDER
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    Are you sure your float level isnt set too high??????????
    Unpredictable Pro Mudd Racing
    2009 MRA Pro Mod Champion
    3 time National Record Holder

    http://i251.photobucket.com/albums/g...mud/3158-1.jpg

  8. #8
    Senior Member MASTER BUILDER
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    Get rid of the carb, we had 3 of them on the dyno the other day, they all sucked, rich at idle or way lean on the top end, BG needs to go back to the drawing board.

    Buy an AED 650 HO, problems will be solved.
    Competition Engines & Hardcore Engine Parts
    www.cnc-motorsports.com
    info@cnc-motorsports.com


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