Stovebolt engines have a nasty habit of building up a thick layer of crud in the bottom of the water jacket that shortens the area where coolant can contact the cylinder walls. Short of removing the head and manually dislodging the sediment with a long probe, it's difficult to get rid of it. I've had some degree of success by running a pretty strong solution of muriatic acid (technical grade hydrochloric) and water in the cooling system followed by a strong dose of baking soda to neutralize the acid. About 2 cups of the acid that's sold in the big box hardware stores for masonry cleanup to a cooling system full of water- - - - -run the engine for 30 minutes- - - -then add a whole box of baking soda to a fill with clear water and run for 10 minutes as a neutralizer. Then flush with clear water and refill with a 50/50 mix of antifreeze and water. The only downside is the possibility of opening up thin spots in the freeze plugs and creating a leak.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!