I can agree that Muratic would work well, but how can one carefully do it? I used Muratic acid once to clean out a gas tank before treating it with an Eastwood gas tank treating kit. It was easy to handle / manipulate, so there was no spillage, slopping or spray anyplace I didn't want it. However, as many people have attested (and I know because I have done it), when you do the inside of an engine, it gets real messy. I think it would be difficult not to slop and spray acid everywhere; not sure if it would be worth the risk.