Muriatic acid will do no good if you can't get it in there and from the sounds of your restricted flow you can't. I suggest complete removal of the manifold so you can get after it on the bench, I think you're headed that way anyway.
Before anything else you should perform a pressure test. There are threads on this forum describing the technique. There's no point in cleaning out a manifold that's rusted through. If you are uncomfortable with this test an engine shop can do it easily. If it fails the test, replace it and be done.
There are 2 freeze* plugs on the side of the manifold than can be removed to gain access to the water jacket portion. Through there you can poke, prod or whatever to clean her out. Solvent won't do any good. You may even consider taking it to an engine rebuilder or radiator shop for a "boil out". Once the clean out is complete these holes can be filled with replacement freeze* plugs or tapped for threaded pipe plugs.
*The common term "freeze plug" is a misnomer, not there for freeze protection at all. It is actually a "core plug" that fills a hole necessary in the casting process. I inaccurately called it a freeze plug here only due to the common usage and recognizable appearance.
Before anything else you should perform a pressure test. There are threads on this forum describing the technique. There's no point in cleaning out a manifold that's rusted through. If you are uncomfortable with this test an engine shop can do it easily. If it fails the test, replace it and be done.
There are 2 freeze* plugs on the side of the manifold than can be removed to gain access to the water jacket portion. Through there you can poke, prod or whatever to clean her out. Solvent won't do any good. You may even consider taking it to an engine rebuilder or radiator shop for a "boil out". Once the clean out is complete these holes can be filled with replacement freeze* plugs or tapped for threaded pipe plugs.
*The common term "freeze plug" is a misnomer, not there for freeze protection at all. It is actually a "core plug" that fills a hole necessary in the casting process. I inaccurately called it a freeze plug here only due to the common usage and recognizable appearance.
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