Yatzee
Well-Known Member
Ok, my '68 GTX. Basically stock 440 w/mild Comp Cams, headers w/3" duals, stock intake with Edelbrock 650cfm AVS jetted "fat", Carter "hemi" fuel pump. 4 speed, 3.55 gears.
Has been converted to Mopar electronic ignition, all in at 36 degrees advance - vacuum advance plugged off, the car hates it.
Entire fuel system, including tank and 3/8" lines, is new. Hell, most everything is new.
Engine checks out at 130psi on compression test for all cylinders, not so much as 5 psi difference between cylinders.
Y'all may recall my various threads bringing this beast back from the dead....if not, lots of background to be found in those threads.
Current situation is that she has really crisp throttle response off the line and pulls like a freight train up through 1st, 2nd and into 3rd gears, pulling to 5500RPM before shifts.
Once into 3rd and hard into it, she pulls fine until she gets over 4000RPM, then she noses totally over as if it has literally run out of gas.
I'll get off the throttle and give it a second and it comes right back, doesn't even stop running - it literally runs out of "breath" (I'm assuming fuel).
A few seconds later, everything returns to normal once I've stopped thrashing on it.
Yes....I know the carb is undersized....sigh.
Question: I know the fuel pump and the rest of the fuel system is plenty big/stout enough for this setup, may even be overkill - and that I'm likely totally emptying the fuel bowls on the carb when it noses over like that - but is it literally possible that the fuel pump isn't able to supply enough fuel "straight up" to meet demand when it noses over?
In other words, shouldn't the carburetor just take all the fuel pump can deliver "straight shot" when under that demand, regardless if the bowls are empty, and keep on trucking?
OR are the fuel inlets not capable of allowing enough fuel straight through to meet that high demand?
This may well just be how the carburetor demonstrates that it isn't big enough to handle the demand, but I know it's jetted big enough for the situation and the fuel delivery system is big enough surely?
No not at all. The size of the carb with it's jetting will only allow a certain volume of fuel through the orfices which will never exceed the amount of fuel in the bowls can deliver regardless of engine vacuum. If the carb is too small it may not be able to supply enough fuel for the engine at high rpm's, or if the deliver fuel volume to the carb is not to spec you may not be able to keep the bowls full, but the engineering of the carb is such that the bowls will always have enough fuel to supply the various circuits of the carb even at wide open throttle. Perhaps you should look into a larger carb, better volume of fuel delivery that's constant, or, undersized/collapsed/ partially plugged fuel lines or restricted pick-up sock back at the sending unit. Also check the push rod length to determine if your getting full stroke of the pump lever or a deffective pump.