Engine bogging down on acceleration

Gregg Benedict

Gregg Benedict

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The engine is a stock '68 440 non-HP 350hp. It has also been converted to electronic ignition a few years ago. I'm starting to get some hesitation on acceleration from a stop. The harder you try to accelerate, the more pronounced the hesitation. The engine runs great at idle. The engine also runs great while accelerating when moving, even while kicking down the transmission. I can easily go down the road at 70-80 mph without a problem. Before I start to replace random things, I'm looking for the main problems that may be causing this. Could it be the plugs? Carb rebuild? Fuel pump? Fuel filter?

-Gregg B.
 
Morning, I had similar problem with similar symptoms on one of my cars in the past, turned out to be accelerator pump of carb. There are quite a few other things that could cause what you’ve described, but it is on the list of simple and cheap things to diagnose and fix.
 
Unless the gas tank and sending unit have been replaced in the last few years, check the sock/filter on the sending unit and make sure it isn’t clogged or falling apart if everything else in the fuel system checks out as being good.
 
CBODY67 said:
What carburetor?
Click to expand...
Stock Carter. Part number below:
1717699063536.png
 
FURYGT said:
Unless the gas tank and sending unit have been replaced in the last few years, check the sock/filter on the sending unit and make sure it isn’t clogged or falling apart if everything else in the fuel system checks out as being good.
Click to expand...
Fuel tank was boiled out/cleaned like 6 years ago. New sending unit was installed at that time as well.
 
Gregg Benedict said:
Yeah... I was told that. Apparently, the original Holley carb was replaced at the dealer in '69. Any idea what the cfm is for this Carter?
Click to expand...
750 CFM.

From the intrawebs, taken from Dave Emanuel Carter book.

Carb._________Venturi dia._________Bore dia.

CFM__________Pri.____Sec._______Pri._____Sec.

750 ................1 7/16.....1 9/16 ......1 11/16....1 11/16
625.................1 3/16.....1 9/16.......1 7/16......1 11/16
575.................1 1/4.......1 9/16.......1 9/16......1 11/16
500.................1 3/16.....1 1/4.........1 7/16 .....1 11/16
400 ................1 1/8.......1 1/4.........1 7/16......1 7/16
 
You can check the Factory Service Manual to see what throttle bore and venturi diameters your carb has.
 
Gregg Benedict said:
Yeah... I was told that. Apparently, the original Holley carb was replaced at the dealer in '69. Any idea what the cfm is for this Carter?
Click to expand...
Back then, the durability record for Holley gaskets and seals was not good on Chrysler-spec carburetors. Holley had attained the "high performance" orientation that Carter AFBs used to have, but their gaskets and seals did not hold up, needing a carb kit about every tune-up. Imperial owners were not happy with those added expenses!

It was somewhat common to swap out a "multi-rebuild" Holley for a "last forever" AFB, even by many dealers, back then. So your swapped AFB is not that uncommon.

From my experiences, Holley gaskets and such have improved markedly since the later 1960s.

CBODY67
 
CBODY67 said:
Back then, the durability record for Holley gaskets and seals was not good on Chrysler-spec carburetors. Holley had attained the "high performance" orientation that Carter AFBs used to have, but their gaskets and seals did not hold up, needing a carb kit about every tune-up. Imperial owners were not happy with those added expenses!

It was somewhat common to swap out a "multi-rebuild" Holley for a "last forever" AFB, even by many dealers, back then. So your swapped AFB is not that uncommon.

From my experiences, Holley gaskets and such have improved markedly since the later 1960s.

CBODY67
Click to expand...

This complaint about Holley carburetors still circulated widely during the daze of my adolescence, the 1970s.
 
I'm suspecting the Edelbrock item is an exact replacement for an original Carter item. BUT many carb parts places probably sell accel pumps for the Carter without getting into the Edelbrock issues. Additionally, you can probably buy a Carter carb kit with an accel pump in it, too.

CBODY67
 
The last time I replaced an accelerator pump with one from Napa, the thing failed within a couple hundred miles. The valve material on the assembly looked like it was made of dried out leather. I thought a modern one from Edelbrock would work better.
 
Gregg Benedict said:
The last time I replaced an accelerator pump with one from Napa, the thing failed within a couple hundred miles. The valve material on the assembly looked like it was made of dried out leather. I thought a modern one from Edelbrock would work better.
Click to expand...
ALL accel pumps were of leather prior to about 1970, before the orange silicone types appeared. Leather has "oil" in it and can be harmed by ethanol'd fuels, I suspect. A friend was restoring an old 1955 Rochester 4GC carb. In order to deal with an off-idle situation, he finally went back to the orig leather accel pump and the problem vanished.

It was common practice to "roll" the leather to soften and flex it for a better seal, prior to installation, back then. Which also made them fit a bit tighter, although the "cup style" pump is supposed to be wear-compensating, spreading out as it pushes gas into the accel pump system.

Might see if NAPA can provide the orange silicone pump?

CBODY67
 
CBODY67 said:
ALL accel pumps were of leather prior to about 1970, before the orange silicone types appeared. Leather has "oil" in it and can be harmed by ethanol'd fuels, I suspect. A friend was restoring an old 1955 Rochester 4GC carb. In order to deal with an off-idle situation, he finally went back to the orig leather accel pump and the problem vanished.

It was common practice to "roll" the leather to soften and flex it for a better seal, prior to installation, back then. Which also made them fit a bit tighter, although the "cup style" pump is supposed to be wear-compensating, spreading out as it pushes gas into the accel pump system.

Might see if NAPA can provide the orange silicone pump?

CBODY67
Click to expand...
One more thing... I think I may just go to a new Edelbrock carb, as I was thinking doing that next year anyway. Is it your opinion that the AVS2 is better than the Performer AFB? Maybe it just doesn't really make a difference for a big c-body anyway. Since I currently have a 750 cfm Carter on it now, should I just go with the 800 cfm AVS2 or the 650 cfm AVS2?
 
The AVS2 can be better, as the basic AVS came after the AFB in product evolution, which can mean some incrementally-better venturi shapes. There is nothing wrong with any AFB, though, as almost everybody that bought one of the 1400s afternarket versions raved about how well they performed.

The AVS2, with its improved annular discharge venturis (watch their video on the Edelbrock website) makes for a more homogenized mixture, which has to be better. Many have mentioned "EFI-like throttle response".

Why the perceived need of 800cfm? Just curious, as the main driving is done on the primaries. I might head more toward the 650 range if it were me. Just curious.

I would recommend using the thick, OEM-style base gasket/insulator, though. Edelbrock and Holley both make them, so they are available. With the phenolic bushings in the mounting holes, they greatly reduce the change of uneven torques and cracking the baseplate area.

Just some thoughts,
CBODY67
 
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