sounds like you may be running a little rich. also may be needing a tune up is your tailpipe all covered in black soot? course it matters on the size of the stripes also a little black may be okay matters on your elevation and such but air cleaner and filters could need some changing as well cant hurt anyhow. how many miles are on the engine? also remember these cars dont burn fuel like newer cars and if they sit for a while in between start ups a little carbon is a natural thing. But not being there check your plugs, timing and filters and if that doesnt cure it try to lean it out a bit, not being there it is hard to really tell. but start there.
Modern unleaded gasoline has a lot of alcohol in it. Premium unleaded gas has even more. When you burn this stuff one of the things that happens is water in the exhaust mixture when the engine is cold. The water rinses out some of the soot from your exhaust system, causing the black spots. When your car is warm the water goes out as vapor because the exhaust system is warm.
My neighbor has a 48 plymouth and a 61 Bonnieville He put nothing but marathon in his cars says they are one o the few that dont put alcohol or corn pone in the gas I dont know how true this is but I didnt see it on the pumps.In that old plymouth he also adds -MARVAL MISTREY OIL- in the tank for a lead additive.I havent gone that far though
Marvel Mystery Oil isn't a lead additive. It's been used to promote lubrication in place of the lead that used to be in leaded gas. The lead molecules would bond to the seats slightly cushioning the the valve against its seat.
MMO is more hype in my opinion. Why would I want to run oil through my fuel system? Hmm...
basically MMO is nothing more than type F transmission fluid with a couple more additives so they can make claims. But you will get the same effects at a tenth of the cost by using the tranny fluid. As to if it works well some say it helps lubricate the top end, but with todays fuel additives i think it is a waste of money and total bunk. One of those great myths of folk lore kind of like using stuff with teflon in it, total myth and snake oil.