The metering rods are down during idle as far as I can tell and I took a look down the carburetor while it was running and gas was pouring out of the boosters so definitely either the needle and seat or fuel pressure. . Im going to pick up a regulator tomorrow and try that next.
Did you loosen the screw and slide the cover off to the side so you could see the rods with the motor idling? Rev it just a bit and watch them jump up?
I have been using 600 like yours since like 1985. Been on every 340 I ever had at one point or another. I have tuned it quite a lot since I trailer it on vacations all over the country including driving it to the top of Pikes Pike twice. They got to be lean to make up that mountain.
Anyway just recently put a temporary fuel gauge on my mechanical fuel pump equipped 340 and had 6-8 PSI depending on RPM and throttle position.
My car also has the hemi vapor separator mounted on the fuel pump connected to a factory 1/4" steel return line. It has a 3/8" suction line for the fuel pump. It was the first time I checked fuel pressure under load with the hemi VS/return line and I had plenty of pressure even under WOT. Been driving with this fuel setup for 10 years and 25K miles since 2007 on my most recent 340 with my trusty 1405 600 carb..
IMO 7-7.5 psi fuel pressure is not your problem.
What do the spark plugs look like? Get used to pulling them out, you need to keep track of them.
