Freewheel
 

Why does my freewheel go 'snap, crackle, pop' when it gets wet ?
(It also misses on rare occasions, when wet)

How do I deal with this ?
Assuming it's water that does this, why ? It isn't rust because once it dries out it's fine.

It's not a Shimano, it's a chinese freewheel. The derailleur, which is the same brand ('Soleus'), looks like a copy of an old Japanese Sunrace derailleur.

Have you oiled the freewheel mechanism? If not, then it's likely that the freewheel is at the end of it's useful lifespan. These Chinese derailleurs are, almost without exception, a cheap piece of cr@p. You should replace it with a decent (meaning SRAM, the only good manufacturer of freewheels left) new one, or else scour eBay for a passable used one. New, a 6-speed will set you back $35, and a 7 or 8 speed derailleur will cost you $40, both at Nashbar. There are often specials on these, too.

John,
I am just going to throw out a few ideas. First, is the sound like 1) metallic popping/crackling
2) like grit
3) greasy gunk that has gotten wet

Have you ever cleaned this freewheel? If you want to try to continue using it you might clean it by soaking it in solvent for an extended period, swishing and turning it periodically. I have seen written that this should never be necessary, but other sources describe this process. After thorough drying you dribble a few drops of thin oil throught the juncture between the fixed and turning part.
Assuming the freewheel has some miles on it and has standard threads, I, personally would just buy an newer SRAM or Shimano freewheel. Harriscyclery.com has a nice selection of both. Nashbar and Performance have SRAM. If you don't have the appropriate removers, just get the freewheel from a shop, and they will spin it on for you. The shop can also look at it and make sure they get you the right thing. New freewheels are not that expensive - $20-50, depending on what you want. Replace the chain with a good SRAM chain at the same time.

Did you ever spray silicon on your chain/freewheel? IF yes, they will squak when they get wet.

A coat of chain lube fixes that problem.

This freewheel has done quite some mileage since it was last wet.

I live in Darwin : the wet is from December to April, and it doesn't rain AT ALL any other time.

This freewheel goes okay now, but it's coming up to the wet season. The freewheel made a popping, clacking sound, when it got wet, when it was BRAND NEW. Since it still worked, the shop refused to replace it under warranty.
But then it went okay during the dry.

When it was oiled, it made the sound MUCH worse, and missed more (but, in reality it missed only very rarely). It sounds like it's building up pressure in the sprag clutch and this is sticking the mechanism ? How would you relieve this ?

Has anyone had this experience with old Japanese Sunrace freewheels ?

There is probably corrosion and/or grit inside the pawl mechanism. Inexpensive chinese freewheels are notorious for this. You might be able to pull it apart, and if you have the patience of a saint, even get it back together again! Is it worth the time and trouble on a $20 freewheel that wasn't very good to begin with?
Considering how cheap a new freewheel is, as well as the vast number of slightly used decent freewheels available, I wouldn't bother fixing the old one. It's toast-toss it. Replace it with a decent one, not the first chinese piece-of-$h!t they try to sell you. Those chinese freewheels are made solely for the department-store bike crowd. If the local shop doesn't carry SRAM, go to one that does, or look on the used market.

Bottom line without getting too technical as many love.

The freewheel is dead. Too much usage. These things are not meant to last forever.

You can try squirting some light oil inside but it will still wiggle as the bearing simply are a bit worn and there is room.

DO NOT open it to try to fix it.

Just get a new freewheel.

Any freewheel with a history of skipping or locking up is dangerous to use. Replacement is good insurance against injury. You will want to replace the chain, as well, if it has stretched by more than 1/16" per foot.