If there is rust blocking the passage of water in the jackets, just having water get to it will help soften it up. For that reason, I don’t necessarily agree that blowing them out with compressed air when dry is the better approach. I’ve unclogged these things with the motor running by letting the water do the work for me, but its not easy to explain and best demonstrated live.
As Joe suggested, I agree with taking the motor far enough apart to remove the elbows out of the cylinder castings. You have to be careful doing it, but it needs to get done. The reason I say that is because some rust flakes are too large to be able to pass through the elbows plus work their way through the plumbing.
The Model 12 cooling system requires attention in other areas. If you lose too much water at the pump area due to leakage, it won’t be able to supply the engine properly. The main water tube moves up and down inside the water pump gland nut and packing, and the packing can break down and leak. I’ve had a couple where the packing was rotted so bad it couldn’t be re-used so I just put a rubber O-ring in its place and it worked fine. And when you’re looking at the water pump, verify that the check valve in the pump is working and not loaded up with grease from the lower unit (it happens). The pump piston can also wear to the point that too much water leaks around it instead of getting pushed up to the powerhead.
Hope this helps.
Best,
PM T2
He's livin' in his own private Idaho..... I hope to go out quietly in my sleep, like my grand-dad did..... and not screaming, like the passengers in his car...
