AFB hits a wall at 3000 rpm

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.

Loren

 

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 sock...you 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.

 

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 .

 

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.

 

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.

 

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.

 

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