I started loosing power on my 86 Brat, so I checked the air filter and found it completely black inside the housing and carb.

I thought it may have been left over from the previous owner sense I never had a problem with the engine, so I cleaned it up and replaced the filter but a few hundred miles later it was the same.

What causes this?

was it oily film? if so maybe its pcv related. check all the hoses and make sure they are not clogged up

No, its soot, it looks just like velvet.

Its on the inside of the air cleaner and carb as opposed to between the air cleaner housing & filter.

is your carb getting WAY too much fuel when the engine is running?

My check mark goes beside the ASV reed valves not closing

 

thus exhaust gas is being introduced into the housing

 

the soot could be carbon from the exhaust

 

Please see Karin's post on backfiring for some good links to fixing this.

 

http://www.ultimatesubaru.org/forum/showthread.php?t=33253

My check mark goes beside the ASV reed valves not closing...

Exactly what I was thinking based on your title, and mostly because of Skip's help with Karin's problem.

 

Might be able to verify by looking where the ASV hoses connect to the intake system and seeing if soot is between there and the carb.

ASV reed valves?????

Please use a few minutes of your life

 

and read over the links I posted

ASV reed valves?????

Please follow Skip's advice and read the linked articles.

 

ASV (Air Suction Valve) reeds are part of a one way valve between the intake and exhaust, used to introduce air into the exhaust for pollution control purposes. The reeds are thin, and eventually break, allowing exhaust flow into your intake system... a likely cause of the soot you see.