What is the best chemical or method of freeing a seized carb. I have a carb from a K&B Sportster that has seized and I would like to free it up short of using a large hammer and channel locks or such.

Soak it in glow fuel for a while, if that doesn't work then gently warm it with a blowlamp or in a hot oven. If your carb has a plastic throttle arm or has O rings then these need removing first of course.
Dave :^)

There is simple method, although you might not belive it is a serious one = use CocaCola, original one, and place the carb inside
and check time to time... You might not believe, but it is true that the original CocaCola is a very aggressive chemical and if you
place thin blade inside of CocaCola cup for overnight, next day morning you see what is the "impossible " thing, just at room
temperature, no heating needed.

I just cleaned a sticky carb on a 1.50 two-stroke. I used denatured alcohol, and a five minute soak made a huge difference on a well-gummed carb. I've soaked entire engines overnight in glow fuel or alcohol, and that is a slower but gentler alternative to heat.

I did find after the soaking (which I resumed for another hour just to be sure) that the carb needed to be disassembled and scrubbed clean internally (i.e. the barrel/rotary valve) before it operated smoothly. I don't know where all the crud came from, but it really liked a good scrubbing with an epoxy brush.

Denatured alcohol has no castor in it to start another gumming cycle. When it's clean and smooth I wipe dry, then blow dry, then drop in a little synthetic oil on the moving parts.

....soaking overnight in plain ol' rubbing alcohol works good too.

Laquer thinner

This will free it up in about 5 minutes and prevent it from happening again

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

Normally if you just heat it with your heat gun until it is good and hot, then use a gloved hand to turn the carb linkage. The gummed up oils seem to release easily with the heat. Then you can clean it with normal alcohol or whatever your favorite method is. This has always worked well for me.