stock L78 Pontiac 400, 7.7:1 compression with stumble off-idle problem. Car also does not seeem to make the torque it should off the line. Yes, I know it's 7.7:1.
Carb has been rebuilt with Cliff's parts and here's what's in it, what's been done. Also, can't seem to back out the idle-mixture screws enough to richen it up. The car does seem to run hot in the summer at highway speeds so perhaps my mixture is too lean? (radiator confirmed not plugged, thermostat good, cooling system flushed completely and 55/45 mix of coolant/distilled water).
1. completely disassembled. Cleaned all passages including blowing air through to make sure no crap left behind. Rods and jets cleaned and polished to nice shine with non-abrasive cleaner.
2. Verified no leaks in primary or secondary wells
3. #72 primary jets, .044 primary rods, DB secondary rods, "L" hangar (all factory-sizes for the 1978 L78 engine).
4. Primary hangar verified not bent.
5. Check ball in A/P well.
6. New ethanol-resistant A/P. A/P verified as working.
7. Float verified to not have hole in it (it floats!). Float height set to 16/32". Factory spec is 17/32" so it's 1/32 higher.
8. Secondary Flap spring set to factory setting (secondary air flaps close then 1/2 turn).
9. Previously (about a year ago), r&r idle air bleed tubes and replaced with factory-sized (from Cliff).
10. APT moves freely in bore, APT retainer ring is holding the apt piston all the way down as it should.
11. A/P set to outer hole per factory spec. Both sides squirt fuel when pump actuates. No play in the pump lever to accel so pump is 'there' right from the first movement.
12. Secondary cam is not worn. Bought a replacment but original is the same size so just left original.
13. Used the thicker gaskets and everything has been torqued in order, as the factory service manual says to do.
Both pull-offs are installed and operational. (front and rear of carb on right side).
Have idle screws at 3 1/4 turns out. Turning them out farther does not have any effect on engine idle speed or vacuum.
The only thing I did notice is there is a small amount of 'play' in the primary throttle shaft bore. It's not snug by any means and I can wiggle the primary throttle shaft just a little bit. Not sure if that's "normal" or not, or whether it means that it's time to sleeve the primary shaft bore.
I reassembled this super carefully this time around and am pretty sure there are no vacuum leaks anywhere. Car is factory stock so all the emissions equipment is still there, although I have the manifold and porter vacuum supplies plugged to the thermal switch that controls the EFE valve and EGR valve.
I don't see any fluctuations in engine vaccum at idle. i'd tell you an exact vacuum reading at idle, but my gauge seems stuck at 7 in when it's disconnected so if I just subtract 7, I get about 22 in hg at idle.
Any guesses on this one? I'm quite befuddled and don't know what to do.
