'74 FJ40 - Sudden idle issue (1 Viewer)

[Swede4477]

Howdy. My ’74 FJ40 developed an idle issue this afternoon and I’m looking for some guidance on where to start. I recently rebuilt the Aisan carb and had a ported vacuum port installed in the throttle body (thank you FJ40Jim). I also installed a new HEI/DUI distributor along with plugs, wires, a fuel pump, and fuel filter. I set the timing to 12 degrees BTDC and adjusted the idle mixture and idle speed screws using the factory service manual. It’s been running like a charm for the dozen times I’ve started it until today. I set the choke and let it warm up as usual. Once warm, I opened the choke and it idled down to 650 where it should be. After a few minutes of running fine, the idle suddenly dropped and it would barely stay running. It seemed to either be running rich or seemed like the choke was closed (though it wasn’t). I also noticed a sound in my fuel tank after shutting the engine down, almost like it was releasing pressure. I started it again a few times, and it would idle fine at first, but then something would change after 30 to 40 seconds, and it would be back to barely idling. I do have the charcoal canister hooked in where it will purge to the manifold vacuum base port below the throttle body, via an Echlin vacuum valve hooked in to the ported vacuum port. I did check the canister and check valve prior to hooking up and they both seemed to be functioning properly. Any ideas on what is causing the sudden rough idle?