1982 gmc truck engine trouble

I have a 1982 gmc truck with a 350 engine automatic I have new spark plugs,rotor,distributer cap ,wires,and something happend while driving in town it made a weird noise in the engine and staled after a few minutes started again but runs very bad and a lot of pigning from the engine I have to keep the rpm very hi and I have no power I taught I broke a piston or something in that line anyway I manage to drive about 12 milles home.I check the compresion right away nd to my surprise they are all ok between 160 and 180 ,I think its good for a 25 year old truck with lots of milles.Now it will start and run if I keep it to the floor and it makes a difference if I turn the distributer so I dont think that it would be the timming chain but im not sure it sounds like it has a very hard time exausting and there is alot of black suit comming out of tail pipe its not black smoke but dry black powder .I would like to know if this could be a problem with exaust system the cat or muffler mabe?How can I tell if its a timming chain problem or carb ,I have vacuum at the carb but something is rong with the vcuum at the brake booster it was very hard breaking on my way home .Now I dont know if its a fuel problem or ignition or even exaust problem I never had one shooting dry black powder from the exaust before and it also exaust at the manifolds on both sides and it wasent doing that before this happens.The pigning sound like a bad timming problem to me but would the timing make it sound like it has a plug exaust?In that 12 milles home I must of burn 5 gallons of gas its usualy hard on gas but not quite this bad.
I would appreciate any help I can get im not ready to retired my truck yet if possible and around here not to many mechanics left that knows something about carburated trucks the are more in the modern fuel injection vehicles.
Tanks in advance for any responces,have a good week everyone.