Okay so I bought one of those HF 21gal 2.5hp oiled air compressors.

https://www.harborfreight.com/21-gal-25-hp-125-psi-cast-iron-vertical-air-compressor-61454.html

If it's hot out, it's fine.

When it's not hot out (like 80s+), then it fails to restart when tank pressure drops below regulator pressure. (ETA, fails to start meaning the pump tries to turn over but cannot spool up, and it pops the breaker on the unit)

I think the cold start valve closes too quickly. I just replaced the one it came with with a CDI CS-25, and that didn't fix the issue.

What can I do to create more of a buffer for the pump to spool up before closing the valve on the head? If I unscrew the cold start valve a bit and then simultaneously flip the compressor on and begin to screw the valve in, it works fine, so I think it just needs "a little bit" more time than the valve provides.

It's plugged directly into a 20A outlet -  no extension cord or anything. It isn't popping the house breaker, it's popping the breaker on the compressor itself (I don't know how many amps the breaker on the unit is rated at). I changed the oil in it after break-in (per the manual), and just changed it a second time to be sure. I don't know the weight of the oil (because HF doesn't say) but it's the HF brand compressor oil that it came with.

What are my options here? Yes I know it is Harbor Freight (hurr duurrrrr) but the thing works fine for what I use it for aside from cold restarts. Do I need to add some kind of expansion chamber between the head and tank? Make a tee and use two cold start valves? Tear the cold start valve apart somehow and add a stiffer spring?

From the picture, it looks like it uses a standard pressure switch with an unloader. That's the little black box with the red lever and the small metal tube going to it.

My guess is the contacts in the pressure switch are damaged from arcing.  Very common problem.

I just replace my switch last week with a Square D brand. My compressor is an Gardner Denver, but they all use a pressure switch to stop and restart the motor.

Sounds like the bleed off for the head isn't releasing. May be full of gunk. Can't see where it's at on the picture but normally attached to the pressure switch. Take it apart and spray it out with Carb cleaner.

Change out the start capacitor, I did it on a sears and its about $10.  Mine would pop breakers during startup or once it dropped and tried to refill.  Works fine now.

Probably an unloader valve issue as previously mentioned. That is the PSSSHHHH of air you hear when it switches off after reaching pressure. It unloads the pressure from the compressor piston. The cold start type of valves in oiled compressors are mainly for really cold weather so you are able to get the pump warmed up for a bit before making it start pumping pressure.

You can check the pressure switch contacts just remove the cover. Most of the switches are 2 pole so on a 120V unit just move the black wires over to the un-used pole and you are GTG but that does not sound like your issue.

Could be a start capacitor or the centrifugal or solid state start switch but that is not usually temperature dependent. You can test the Capacitor if you have a meter with capacitance function.

Thanks, I'll take the unloader assembly apart and clean it, and buy a replacement if needed. Can someone give me a brief explanation of the 4-port vs single port variants? I'm guessing I'll be able to tell what I have as soon as I get home and look at it.

No capacitance meter at home but we may have one at work.

A 4 port pressure switch has 4 openings. One to attach to the tank, and three others, to attach accessories, such as an auto tank drain system, gauge, etc.

Usually, single port is all that is needed.

You can use a 4 port in place of a single port, but you would then have to cap off the unused ports.

Just make sure that you get switch rated the same or better electrically. Also, make sure the port size is the same, or you'll have to use adapters to attach it.

So I replaced the unloader and put a bigger starting capacitor (240uF, compared to 120uF original) on it.

It now starts when cold, but blows the onboard breaker after a few seconds. If I swap the original capacitor back in, it behaves exactly like it did in my OP.

According to my KillAWatt, it is drawing 24-25A before dying.

Any ideas on what else to check? I think what I'm going to do is vent the tank and leave it open, and see what the draw is when it is running with no load. The user manual states 14A when running.

It still feels to me like the cold start valve is closing too early, before the pump is really up to speed, but I don't know how to source a "slower" one, that stays open longer.