Engine erratic & rev limitation

Are you sure of that Holley model number? - I can't find it on the list I have.
Is that the number that follows the word 'LIST' off the choke horn?
Do you have a vacuum or mechanical secondaries?

Assuming it's not something simple like the choke being stuck on.
1 & 3 could be a sticky throttle cable, or may be the cable is too tight and it's holding the butterflies open.
2 - Could be fuel pump or pipe.
Could be ignition timing.
Could be plugs.

Yes it sounds like you have a vacuum leak. Look to see if a vacuum plug has popped off.

Julian,
First start off with the timing. Could be set way advance in the effort to get it to pass smog. You might be able to hand "tune" the timing to get started. Losen the clamp and turn the distro. Set it in the position where the engine idles the best rotate left and right until it seems to run smooth. No need yet to pull the firewall and perform an accurate timing. Just get a good baseline. This should get the engine to idle to a reasonable speed.

WD-40 is a great tool to track down vacumn leaks. Spray it on the various hoses and fittings looking for a leak. It engine will speed up when a leak if found. Do not forget to check the large brake vacumn hose from the back to the master cylinder (really the vacumn booster). Also spray along the manifold to the heads to makes sure the manifold is not leaking. Doubtful but worth a check.

My initial feeling is that the timing is set too advance. Both the high idle and power drop off at RPM could be linked to too much advance. Sputtering at load and RPM can also be a fuel issue, either jets too small or power valve of the wrong size or spark (bad plugs or wires). Let's take if from there before we get into the messy expensive stuff. Call if you need more.

Rapier,

Sorry for the confusion, fingers hitting too many keys at once, Holley Carb is a 4150 model type and was new on the car when I got it.

Running vacuum secondaries
Timing was set with strobe at 10 degrees BTDC, with vacuum plugged.
Distributor is new as are wires & plugs.

I'll get the WD40 out tonight and check for vacuum leaks, I'm erring toward that being the problem too.

Will keep you posted.
Thanks, Julian

Check your fuel filter and fuel pump. Sputtering between 4500 and 5000 rpm's sounds like it is running out of fuel! (running lean) Your high idle sounds like a lean condition -like a vacuum leak or lack of fuel.

Just a thought.

Julian,

Check to verify the choke is "de-choking" all the way within a couple of minutes. If the choke butterfly remains partially closed during normal operation, it will make the motor idle fast, run rich, blubber as you described. A rich motor will refuse to rev just like a lean motor, but how they do it is different. A lean motor surges, a rich motor just runs out of steam. A rich motor will refuse to rev even parked in the driveway, a lean motor may rev just fine parked in the driveway. Also verify the electric choke was wired directly to the alternator, and not to a circuit common to the ignition.

The primary & secondary butterfly stop screws should be set so that the transfer slots are barely visible below the butterflies, the idle jets about one turn off their seats to start. Normally idle jets screw out to become richer, except for some emissions calibrated carbs. Fuel bowl settings also need to be checked. To really get that carb to work well, your "timing" needs to be optimized first. If your ditributor has a vacuum advance mechanism, I recommend it is connected to "ported" vacuum. I have described how to optimize the static ignition timing (at idle) in the past. Your static timing setting should end up somewhere around 20 degrees BTDC. Stock Cobra Jet motors like to idle at about 1000 rpm. After timing, check the fuel bowls, then optimize the idle jets. Idle rpm is then set with the primary butterfly stop screw.

your friend on the DTBB