Blower motor seems run weak - any suggestions?

How old is the system?
This system is for the second floor only, meaning just one thermostat controls it?
You mentioned capacitor, but its definitely not an ECM motor?
Does the motor run at all?
Low refrigerant can cause little to no air flow due to the evaporator coil freezing.

Any # of things can cause the air flow too drop...…..
Plugged Filter
Collapsed ducts
Restricted ducts
Evaporator Frozen
Evaporator dirty
Dirty Squirrel cage
Insulation blocking ducts

You need someone to inspect the unit!

Measured voltage? Measured amp draw? Capacitor checked? Delta T across coil?

Ok you’re saying you have a belt drive? That’s real old school. Is the blower wheel all gooped and dirty? Coil is clean you said?

If you have a belt that’s a pain to change air speed, gotta get different pulleys

You’re sure there aren’t manual dampers that may have inadvertently closed in the duct system?

If it’s direct drive then it should have different “taps”

Paste a pic of the blower motor and or the wiring diagram

And check the cap like we discussed

> I can not feel any air stream from vents. ...told me ...low refrigerant but I don't think so.

You should be able to "feel air flow". Hot/cold air systems are never designed for very low flow because that would require very large ducts and registers.

And certainly "low refrigerant" is not the cause of low airflow. Think of the A/C as a block of ice with a fan to spread the coolth around. Even without the ice you should feel the fan airflow.

As said, some experienced person should stick nose and hands inside the blower unit. There is danger. Electric shock, and getting fingers or neckties caught in spinning parts. It also helps to "know" how fast a motor typically spins and how tight a belt should be. If you are not experienced, it may indeed be wise to "just call the pros".

The other place to look is the ducts. Go in the attic or crawl-hole with the fan running (cooling optional). Your nose-hairs will tell you if there is a leak in the ducting. Remember that many duct installers just buy the right material, go in the attic, and come out when the estimated time is up. Most systems are left with unsealed joints; the home horror sites have pix of totally bogus "duct work". You may be throwing all your cool air in the attic.

The return system is also important. My house had marginal supply ducts but very undersize returns. It was a whole lot happier when it did not gasp for air. Return registers may have neglected filters, or prior children may have been dumping toys down the grill.

A belt-drive system, I think, will not be capacitor-Run but may be capacitor-Start. If the starting cap fails, the motor may erratically "turn" but at a mere fraction of its true speed. I *think* in that case the motor current will be far higher than rated running current (and might burn-up).

Belt tension- there are rules of thumb. If too tight the drive works good but belts and bearings fail quickly. Which leads to too-loose. If 25% of speed is lost in loose slippage, air-flow is down to 50%. Where I worked the HVAC techs ignored the blower-belts in my building and one hot summer I "noticed" that I could stop the slow blower with my hand (do not try this at home!!) When they tensioned the belts it blew good.

The state of the freon system has nothing to do with the fan. Get it blowing before you ask about temperature drop. Running either heat or cooling without proper airflow in the house blower will burn-up the fire or freeze system. (I had an overlarge burner on an undersize duct system- the blower strained but also the fire-box was failing from overheat.)