AFB hits a wall at 3000 rpm

Discussion in 'Carter' started by FormerGNowner, Apr 4, 2014.

  1. FormerGNowner

    FormerGNowner Well-Known Member

    Finally doing some tuning on the 300...
    30* dwell, 2.5* initial timing, 600 rpm warm idle, hot air choke works
    Two stabs of the throttle and it fires right off. I'm happy so far.

    Even at half throttle, I hit a huge dead spot as I get to 3000 rpm. Is this the secondaries not opening, or accel pump out of kilter? I haven't had the car long, so I can't say how clean the carb innards are or what jets/rods we're dealing with.

    If I can make external adjustments, I'd rather do that before the full teardown. Thanks for the direction.

  2. Aaron65

    Aaron65 Well-Known Member

    There's little chance that the secondaries aren't opening on an AFB...they almost have to open if there isn't a mechanical obstruction. Is your choke opening all the way?

    I'd almost guess you have low fuel pressure from a plugged filter, line, or tank could also have some plugged secondary jets or passages, causing you to go way lean when the secondaries start to open...I'd think that would be more of a stumble though.

    Do you have a fuel pressure gauge/tester? You could probably rent one from the bigger auto parts stores. I'd think that anything over 3 psi would be enough to keep you in good shape.

    DEADMANSCURVE my first word : truck

    i agree - a pressure check would be a good idea ( 5-6 lbs ) . if bad start rear at tank and work your way front lookin for problems . if good then maybe pop the top of carb off at least and check bowl etc condition . if its kinda gunky lookin its gonna be that time .
  4. Jim Cannon

    Jim Cannon Loves that Dynaflow hum!

    There is a very small, easy to miss, secondary lock-out tab on the AFB that will not allow the secondaries to open until the choke is fully open. If that little linkage down from choke to lock-out is not adjusted correctly, your secondaries can be locked out even though the choke is open. This is an external adjustment, as you requested, and easy to check/fix.
  5. FormerGNowner

    FormerGNowner Well-Known Member

    Thanks for checking in guys. Sometimes it's the hard to find simple stuff. The in-tank sock was toast. The motor seems much happier today with a new sock and fuel filter. Jim-I will see what the choke lock-out looks like.
  6. philbquick

    philbquick Founders Club Member

    It shouldn't hit a wall at 3000 RPM even without the secondaries. Two barrel motors can go 6,000 without any problem. Is there any way you can monitor fuel pressure while you drive? If the fuel pressure gets to 2 PSI then it must be the fuel filter, the sock (as someone has already mentioned) or to fuel pump. Could also be crud in the float bowl and emulsion tubes.
  7. FormerGNowner

    FormerGNowner Well-Known Member

    Fixed this up too. The arm wasn't consistently dropping out of the way of the tab.

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