Just can't my 340 to run well on 91 octane.

[fishy68]

IIRC, I've got a 6.0 pv. The engine has 11" of vacuum at idle in gear.
I'm pretty sure I have 72 jets up front. It's been a while since I have been into the carburetor.
It's got a metering block on the secondary side.
The choke plate and shaft have been removed. It's got the tan accelerator pump cam and a 32 squirter.
Plus, it's got the white vacuum secondary spring.
The spark plugs are Autolite 66s and are very clean.
Judging by the gas smell out the tailpipes I have always wondered if the set up is a bit too rich.

As Moper said it sounds lean by the spark plug description. That smell you smell out of the tailpipe doesn't always mean it's rich. Sometimes what your smelling is oil burning. A very small amount of oil burning can stink yet not show up on the spark plugs. Also, running lean can cause higher combustion temps and make it spark knock worse. I'd jet it up 2-4 notches all around and see how it acts. Just as a side note, in the 750 vac. sec. carbs I've done I always seem to have to put more jet split in them than what they came with from the factory. They come with a 3 jet diff. but when I tune them I always seem to be on the order of 6-8 jet differences front to back. But nearly every application is different.

 

[340_833 dart]

Would spark plugs that are too hot cause some of the problems mentioned?
The Autolite 66s cross reference to Champion N12Ys. That's the recommended plug for the stock '73 340s.
However, my '72 Duster 340 called for N9Ys. So, I bought a set of those this afternoon.
I'm waiting on some carburetor parts to come in tomorrow and will change jets, the power valve and primary squirter, along with the vacuum secondary spring.
I was mistaken in saying the carburetor had 72 jets in the primary side. They are 70s.

 

[RustyRatRod]

What am I missing?

You said it yourself. Cylinder volume and combustion chamber volume.