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Thread: Backfire .........POP ???

  1. #1
    blownbbf
    We are having problems with a 406 small block Chevy. It has a 420 mega blower, victor jr heads, small roller, twin holley 650's dps.
    Even after it has heat in the motor..... when you spike the throttle its giving a lean backfire POP. eek! It has 15 deg initial with a total of 34.The power valves have been removed and jetted up 5 or so.
    We are thinking of going to 50cc acc pumps? Could the problem maybe be the squirters being too small or maybe does it need a more aggressive cam on the linkage for the acc pumps?
    Any suggestions?
    ps What do you think of the size of the carbs being used? 750's would probably be a better choice ehh? I wonder how much power we could pick up with extra 200+cfm?
    Thanks in advance

  2. #2
    Hotcrusader76
    Are they HP650's or #4777 650cfm's?

  3. #3
    blownbbf
    4777... Help !!

  4. #4
    Ian
    Now I am the first to admit that I do not know much, but it could be that you need a more aggressive cam on your acc pump. Are the carbs brand new?

  5. #5
    Hotcrusader76
    Could you also please fill us in more with the engine specs? Fuel pump type? Fuel filter's condition? Fuel line size? Etc.
    [ January 21, 2003, 04:48 PM: Message edited by: Hotcrusader76 ]

  6. #6
    Craig
    Start with bigger shooters, then you can dial it in with a Holley pump cam kit. I spent a lot of time dialing in my big heavy Buick Gran Sport at the drags. I were running slicks so when you stand on it from off idle, you instantly get that lean condition. Once I started getting it dialed in, it had a great 1.5 60' times, and this is just a street car. You're should be on the right track with it being lean condition, but also make sure you're running enough adavnce in the ignition.
    [ January 21, 2003, 04:51 PM: Message edited by: Craig ]

  7. #7
    Hotcrusader76
    Most likely you have the stock little red 240 cam which pumps out approx ~18cc of fuel for every 10 or so pumps (average). Going to position number two with that cam should give you another 2cc's of shot.
    Now if you bought the lighter red cam 330 (3 possible positions), you could jump to a total 30cc shot or 37.5cc (pos.2)all with keeping the 30cc pump mechanism.
    Otherwise you can foot the bill and get a 50cc conversion for all four corners. A waste in my eyes for that application. Then your dealing with the turd brown cams. Way more shot than you need right now.
    Just look into purchaseing some 330 cams I think they are commonly called the pink ones. I do carry these cams and could have them shipped out tomorrow if ya like. Send me an email.
    ~Ty

  8. #8
    GofastRacer
    OH well, here goes all my stars!!.....
    The simple cure!.. wink
    [img]http://free.***boat.net/ubb/Avatars/Forum_Graphic_Depot/Toilet.JPG[/img]

  9. #9
    Craig
    The Flying Toilet Bowl?
    Supposed to be good full throttle, but have heard verying degrees of success with idle and part throttle charcteristics?

  10. #10
    blownbbf
    I think we have the pink ones already,with a 28 squirter. Should we go bigger with squirt?
    Also can a person change just one acc pump on each carb, or do you need to do all four?
    Thank you
    Rick

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