OMC 7263, is this an original configuration?

Hello, Peter. You have an old, original Lawnboy there. The black sludge is probably oil from the gas/oil mix that was more than likely left in the tank. Start by cleaning that out along with the carburetor. Be very careful when taking the carb apart. Might be some delicate parts in there due to the age and condition of the machine, but all are still available if you look in the right places.

Good Luck! (And welcome to the forum!)

Thanks Brad,
Any idea as when that mower might have been produced?
There is a fair bit of corrosion inside the tank. Is there a good way to clean that out? Would a replacement tank be available?
I had the recoil starter apart for service, and have noticed that one of the little pieces that engage the recoil are missing. It still functions, but replacing the missing piece would be good. Where is a good place to find such parts.
Thanks, Peter

A Lawnboy dealer might be the best place to start. From there, Ebay, NAPA (great place to buy replacement coils for these machines), other enthusiasts.

I would try cleaning the gas tank with carb cleaner, then put some rust remover in the tank, or about 50 small nuts and washers and some water. Shake around until the rust appears to be cleaned out. Then rinse it really well. Also, you can put some vinegar in the tank and leave it overnight.

Thanks Brad, that's what I will do.
What would be the appropriate fuel mixture to use?
Thanks, Peter

My thoughts on that would be 16:1

Check this thread:

http://www.mylawnmowerforum.com/forum/39-lawn-mowers-parts-sale/591-omc-lawnboy-parts.html

It's old, but might still be able to get some great parts.