wondered if the experts have any interest building a list of causes for bogging / hesitation between 3K and 4K rpm .

My problem has a previously smooth running 79 carb added to a swapped 84 12A with a 84 snorkel (no charcoal cannister) Not sure which dizzy was put in the swapped engine . 79 alt and seperate voltage reg still connected . Rats nest removed , running RB header. Runs strong but has trouble btween 3K and 4K .

Only does this under load . Revving in neutral has no hesitation.

 

1. Fuel filter
2. Fuel pump
3. Carb problems

 

3a. Vacuum secondary leak
3b. Mismatch between carb and carb spacer

 

You mught want to try asking our resident expert (Sterling). www.sterlingmetalworks.com/bymc


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Weak fuel pump maybe. Couple years ago my car had similar issues. Changed fuels pumps and problem vanished. Always remind yourself that these cars are 25+ years old and things can get tempermental quick.

 

Does the FB carb have the same spacer as the SA ?

My car is out province and the swap was done by others. I think the SA spacer is still on the removed SA 12A. The Sa carb may have been mounted directly on the FB intake . This might be the problem

 

Carb spacers varied from year to year, and even within some years (CA vs Fed vs Cali builds). I don't have a detailed listing of all the changes, but have personally seen 3 different variations, so there are probably more. The changes revolved around the vacuum ports in the spacer "gasket" and in the number of vacuum pipes fitted.

Most of the differences were smog-related, but you could get problems from them not being properly sealed off.

Carb directly to intake could likely be your problem - - Probably won't seal right, and could have covered/uncovered some ports in unexpected ways.