I have an old 7-speed freewheel that came with a wheel I bought recently. I cleaned it up and realized it's a Suntour Winner Pro that looks to be in great cosmetic shape. Unfortunately, the freewheel mechanism doesn't work. The thing will spin, but not well and without the ratcheting sound. Is it worth trying to fix?

No.

First try the easy fix:flip it over and drizzle in some PB Blaster or WD40 from the back side to free up the pawls (they make the clicking sound). If that works blast out the residue with compressed air and drip in some plain 'ol 30w motor oil.
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This worked pretty well for an old Maillard freewheel I have. It was almost silent (not in a good way )before but afterward it sounded almost like new. It's worth a try.

Can I watch you put all those little balls back in?

That's the HARD way!
Save it for plan 'B'........
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buy a new one.

Hard to find a 7 speed SunTour Winner Pro to buy nowadays.
Besides, you're not a real C&V type unless you've rebuilt a freewheel, relaced a wheel, straightened a frame/fork, or machined a part to fit obscure/obsolete specs (like Schwinn). I won't even mention setting up centerpull Mafracs or rubbing oil/other concoction into an old leather saddle.
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Have you tried what top506 suggested and did it make a difference?

Otherwise, look closely at all the sprockets for wear. If the sprockets are good but the innards of the freewheel is not (no clicking=bad or damaged pawls) , it is possible to rebuild it by substituting the freewheel body.

Is this a two-notch or four-notch body? I might have the fw body you need, but you'll need to have the correct remover, two chainwhips, the motivation and patience.

In the 1970s I rebuilt both Regina and SunTour freewheels; the latter are actually pretty easy. However, for the past 20 years I have resorted to filling a sticky freewheel with WD-40, followed by 3-in-1 oil, followed by automotive motor oil or ATF. If that fails to repair it satisfactorily, I figure it's time to replace it, since replacement springs and pawls are unavailable.

I have no reservations about opening a freewheel up, the only problem comes when it is stuck and my cheap spanner won't do the job.

I open it up over a 5 gallon bucket to catch everything, clean it all up and use grease to hold the bearings in place. I have never had a problem with it.

Why bother if it can be fixed?

Ditto. I mean, if it's broken currently, you're not losing anything by trying to mess with it. Might be worth doing just to see what it looks inside.

I love this quote from Sheldon Brown's website. It is like he is daring you.

Might be easier to replace with N.O.S. that you might be able to locate by searching around.

It would be worth the education. Heed what Rabid Koala says. I use an oil pan my self. You may need to buy new bearings and the freewheel wratched mechanism may be just have completely stuck by gunk problem that is eminently fixable. Go for It but, check out the tools required.

It isn't THAT intimidating:

In a workshop environment a freewheel rebuild is not that bad, though it is time consuming and may not be worth the expense given the current cost of entry level freewheels.

However, Stacey's comment reminded me a terrible experience I had about 25 years ago. I was participating in an autumn Century tour that was cold and wet, when a friend's freewheel dissasembled itself. There we were, miles from nowhere, kneeling on the road, with near frozen fingers, trying to find and pick the ball bearings out of the ripples in the asphalt. The passing car drivers would slow down and roll down the windows to see what was going on. A few laughed at our predicament. It was very frustrating that none offered to transport my friend and her disabled bicycle to the nearest town. Somehow we managed to find enough bearings and get the freewheel assembled. I used a rock as a hammer and took apart my chain tool, permitting me to use the pin as a punch to tighten the cover plate. It seemd like we were out there for hours. Fortunately the roadside repair held up. It's not an experience that I don't want to go through again.

wow, dude. Stripping and assembling a freewheel in the cold, by the side of the road, with the pin from a chain tool. This is true caveman biking talent! I don't think I've ever heard of anyone else attempting anything, anything in the world, harder than this. Well, maybe that guy in the canyonlands that cut his own arm off with his pocket knife...that would have been equivalent if he'd reattached his arm after freeing the moving parts.