So the snail won the drag race...
- It's a '76 with federal emissions.
OK, that helps.
Catalytic converter (PO installed) is newer
Newer than '76, that's good...how much newer? Many of the aftermarket converters on the market are junk, and most of them are not up to the task of cleaning up the very dirty exhaust from a nonfeedback-carbureted car like yours; they can easily overheat and melt down internally, causing severe exhaust restriction. More info on what to buy in a
good and capable converter on request. '76 was the last year for the way-too-small exhaust headpipe; for '77 they finally put a 2-1/4" headpipe on the six-cylinder cars which uncorked their breathing even with a stock-type muffler (speaking of mufflers, if yours is old, even with low miles, it very well may have collapsed baffles inside—that was one of the causes of your same symptoms in my '73 bought with 44k original miles).
About 5,000 miles on a tune up - plugs,wires,cap,rotor
Good (assuming good quality parts were used). But that 18° BTDC ignition timing setting means your carburetor adjustments are also going to be way out of whack. Eventually you're going to want to get everything set up so your base timing is 5° to 7° BTDC. Recommend you contact DusterIdiot over on slantsix.org, who is a wizard at setting up well-curved SL6 distributors.
Being a '76 it has factory electronic ignition
Yes. A "multi-spark" ignition won't do much but drain your wallet and reduce the car's dependability, but consider the
HEI upgrade .
Pulls 21" of vacuum while idling at 750rpm
Also good. Still, how long since a proper
valve adjustment was done?
I tried installing a Holley 1920 carb once to see if it would be better, and even though it had way too large of a jet in it and ran rich
...and acted like a blowtorch on the catalytic converter...
it actually fet like it gave the car some power from the driver's seat.
The first few years of Holley 1945 were notoriously poor runners. A 1920 is a few different steps backward. Really, the desirable and effective induction improvement is to go to a 2bbl, which tends to not only improve driveabilty and performance but also fuel economy.
Pull and plug the vacuum hose off the EGR valve; that'll considerably reduce the mushiness of acceleration. Check if your distributor vacuum advance hose runs directly from carburetor to distributor with no stops in between; if not, make it so for another large improvement in driveability. While you're paying attention to the distributor vacuum hose, give it a hard suck and cap it with your tongue to see if it and the advance pod it's connected to actually hold vacuum.
Sooner or later you're going to want to check the cam timing. Slant-6s of this era often came through with severely retarded cams, which make the engine into a gutless wonder no matter how you adjust things or what bolt-ons you install.
There is nothing such as "free energy", and so-called "hydrogen booster/generators" (also "Brown's Gas", "HHO", etc) are a lame scam that only make sense to those who can't (or won't) understand basic science. The human mind has vast power to fool itself, but not vast enough to break the laws of physics, no matter what some yahoo on some website claims. Keep your eye on the ball; the task at hand is to figure out the cause of your car's sluggish acceleration and fix it—save the tinfoil for kitchen tasks; there are way better hat materials out there.
And in the end, take note that the '76 6-cylinder engines were weak and the '76 Darts were heavy and undergeared; these cars were sluggish even when new and as perfect as Chrysler could manage to build them at the time; you are probably looking at some carefully-thought-out upgrades to get the car to where you want it.