250 rough shut down

I'd bet it's a simple fix, up the octane.

Back in day of leaded gas, my 250 Granada would occasionally chunkty chunk at shut down. Never did it running premium.

 

Around 700 or 800 rpm in park. Around 500 or 600 in gear

 

Is there a dashpot solenoid on the carb?...

 

A guesstimate of 100 RPM difference in idle is huge.

 

Try checking your spark plugs. Too much carbon build up on the plugs and cylinders can cause that’s types of symptoms.

 

Theyre brand new plugs. I did stick an endoscope in to look at the cylinders. There's carbon but not bad enough to cause a problem in my opinion

 

Advance your ignition timing a few degrees and then back off the idle speed screw to bring the idle back down to 750-800 in Park/Neutral...

 

I used a vac gauge to tune the carb and put some 93 octane fuel in the bowl. Seems to be fixed for now