Bigger Carb or Bigger Jet Size?

Discussion in 'Engine Topic' started by Cuzz, Jun 22, 2015.

  1. Cuzz

    Cuzz Member

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    Looking at the picture of the plug after about 50 miles of street driving would you think its under carbed or just needs bigger jets? The bottom of the threads is even whiter than the picture shows..

    Motor is a BBC 496, 10.5-1, eagle rotating assembly, modified Pro Comp heads with larger valves and matching cam springs, cam is a solid mechanical with .260 @ 050 and .677 lift.. Timing is locked out at 37 degrees as it was on the dyno... The motor made 667hp/650tq on the dyno and is topped with a un-modified Proform 850dp.. When i received the motor all 4 idle screws were only 1/4 turn out..



    [​IMG]
     
  2. hhott71

    hhott71 R.I.P 11/19/18 Lifetime Gold Member

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    Looks fine to me.
    Under carbed while driving around with 15+" of vacuum. A 2bbl of a 350 would be flowing the same amount of air with that small throttle opening.

    If you were spinning 6000 rpm constantly you'd still not be under carbed. At 7000 a Dominator might be needed.
    Unless you are always running above 4000 rpm there is no reason to lock out the distributor.
    12-16 inital, 36-40 @ 3500 is a normal curve called out by Chevrolet Racing books and MSD among others.
     
  3. 450bench

    450bench Veteran Member

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    Based on the photo, the plugs look perfect.
     
  4. Cuzz

    Cuzz Member

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    Looking at plug photos online, I couldnt find any that are as white as mine at the base of the threads (the flat ring around the porcelain), that's why I was concerned. Like I said, its actually whiter than the picture shows, almost like a chaulk white.. I thought that portion of the plug visually helps with proper jetting.

    Redline is 7200 but will only see that maybe once or twice a year at the strip.
    I like the instant throttle response of having the timing fully locked out. The motor was built and dynoed by Jeff Lawrence (super stock nation record holder) and he is the one that locked it out so I left it that way.

    Thanks for the help

    [​IMG]
     
    Last edited: Jun 22, 2015
  5. 77 cruiser

    77 cruiser Veteran Member

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    Cruising doesn't mean a lot as far as plug reading, needs to be done after a WOT run & cut off clean. As far cruising mixture they look fine. Only too lean at cruise if it surges or misfires.
     
  6. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    What type of fuel are you using?
     
  7. Cuzz

    Cuzz Member

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    Turbo Blue 110 and mainly because I also have an additional 150hp nitrous plate system too and I don't want to take a chance on detonating.
     
  8. DFWJ

    DFWJ Veteran Member

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    was this the carb that was used on the dyno? If so, I wouldn't touch it.

    That plug looks fine, for the driving/load you described
     
  9. Cuzz

    Cuzz Member

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    It is the same carb.. Its seems weird that the 4 idle mixture screws are only 1/4 turn out. Everything I have learned says if it runs best with anything less than 1/2 turn out, something isnt right.. I just dont want to lean this new motor out and hurt it because it wasnt cheap..
     
  10. CorkyE

    CorkyE Veteran Member Lifetime Gold Member

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    Have you tried to adjust idle with a vacuum gauge? Guys on the dyno aren't so concerned about a clean idle as they are WOT performance, two different circuits in the carburetor. Before you go making big swings, I think an AFR gauge would be the way to go. IMHO
     

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