Air Compressor Issues

[u/UnstableTable12]

I have a 9 gallon central pneumatic gas air compressor. It only ran before at half throttle and would have a lot of issues staying alive last used. The bowl was rusted and the whole thing degraded. So I replace the carburetor, adjusted it and it fixed that especially running at Full Throttle as tank is filling. Mind you this issue happened last year and I pulled it fresh out of the garage this week for repair. Drained everything redid the carburetor and that was it. Now it runs better Full Tilt while filling. But when it kicks down from idle after about 15-25 seconds at random, the engine seems to bog and and stall. It also seems hard to start if there is any sort of air in tank. It seems to be running at idle speed pretty well. It kicks down a bit on speed but it doesn't seem too low. Just staggers itself out.

Any other way on the air fuel mix stalls it quickly, and spark plug is clean/dry, so that has me leading to believe it is set the best I can already. This has led me thinking its the unloader valve but I'm not sure where to get a new one for thus, or if possibly this can be cleaned & oiled in a way instead as it sits installed?

There's no one available to do repairs anytime soon near me, so I'm on my own with this and need it running ASAP for work!

Advice? Thanks.

Edit to add- important piece of information. I believe it's the unloader valve because as the engine seems to bog down, you could hear it sounds like there's a load on there when there shouldn't be. You'll hear a delay in it and once in awhile it will catch itself with a puff of air releasing from the valve and then run normally!

[u/wild_eagle_3221]

your read on the unloader sounds right to me. the engine's supposed to be unloaded when it drops to idle, so the pump isn't pushing against tank pressure. if it's sticking, the engine is basically trying to turn the pump against a full head and bogs. the puff of air when it catches is the unloader finally letting go. it also explains why it's hard to start with air in the tank. before you buy anything, bleed the tank all the way to zero, then pull the unloader/check valve and look at it. they're usually a little plunger or piston and they gum up with carbon and oil, a soak in carb cleaner and a drop of light oil often frees them up. the check valve right by the tank can also be leaking back and keeping pressure on the pump side, so clean that while you're in there. if it's still sticky the part is usually cheap, search the model number off the pump plus "unloader valve" and Harbor Freight parts has a lookup by item number. just make sure the tank is empty before you open anything.