I recently accomplished that with a plastic cap and a hot water faucet. I squirted Automatic Transmission Fluid (substitute your favorite solvent) into the carb, and capped it with one of those red caps used for shipping hydraulic lines and other things. It fit nice & snug. I left the carb mounted on the engine.

Then, I turned on the hot water faucet, waited a minute, and held the carb, engine & all, directly under the faucet. As the carburetor heated up, the throttle shaft got unstuck. By working the throttle arm back & forth, the lubricant worked its way into the shaft & bushings.

After working it back & forth for a minute under the heat, I shut off the water, moved to the workbench, and pulled the cap seal off & ran some more lubricant through it.

Worked for me.

Good luck,
Dave Olson