72 455 GS with factory Qjet. Fast idle 1500rpm .Ideas?

Still sounds like a vacuum leak somewhere...if not in the carb then somewhere on the intake manifold. When you have it idling, spray carb cleaner around the outside of the carb and see if you get a RPM increase.

At 1500 RPM you'd certainly have the main system engaging, i.e. nozzle drip. 1500 RPM at curb idle is way too high, your engine temp and fuel mileage will go south in a hurry. At idle, the throttle blades are almost completely closed, and all the fuel is being delivered via the idle tubes which exit below the throttle blades at the mix screws. You'd see no fuel dripping from the main nozzles.

I had idle problems similar to this, and I had both vacuum leaks and a fuel delivery problem happening at the same time. Higher RPMs will kick up the pressure on the fuel pump, so if your fuel sock is clogged, it'll have trouble getting fuel to the carb at idle. When engine is cold, remove the fuel line from the carb, place it in a bowl or other container and crank the starter...ensure you're getting some fuel flow. If not, check that all your rubber fuel lines to the pump are tight and not cracked/dried out. I had to overhaul my entire fuel system before I started getting consistent fuel delivery to the carb.

 

Stock distributor, 7042940 carb. Crane cams Fireball points conversion ignition.

 

Weak distributor springs, worn distributor parts, grease dried up and crusty on the distributor advance mechanism, and/or the installation of any aftermarket distributor springs/weights will cause all of the problems that you are experiencing.

I would verify with a timing light that each time the engine returns to idle speed, that the timing is returning to the base setting. Most likely it isn't when things heat up, and this put the idle speed all over the place.

Heat shields, insulating spacers and other associated parts are a waste of time/funds, IMHO. The stock parts have worked well on those engines for decades. I run a factory intake with a stock gasket on my own engine, zero issues now for nearly 30 years, no matter what engine I've had under that intake......Cliff

 

Primary throttle shaft will need bushings & Primary throttle plates worn . I have seen this all too often . Especially when the engine gets hot .
Either it wont idle or it will be over 1200 to run & eratic

 

Carb was rebuilt by John Osborne 100 miles ago, so I'm guessing he replaced anything that needed to be. I'll give him a call.

 

How is the first cold start of the day? Usually if there is a significant vacuum leak, it will start and stall a lot before it will hold an idle.

After it warms up and the idle goes high, you must verify both the primary and secondary throttle plates are returning against their stops. If they are, then chances of a vacuum leak or timing issue are more likely. If not, find and correct the reason for the throttle plate(s) not closing properly.

 

First start up is fine. Set choke with 1 pump, starts within 2-4 seconds. Idles fine. I found a couple of suggestions online from a Camaro site. Q jet with same issues. Gonna try them in the A.M. Thanks.