DON'T USE MR MUSCLE! or any other caustic alkaline cleaner! NaOH aka sodium hydroxide is great at dissolving tar & soot.Thanks for this - right now the only concern I've got is rounding off or snapping the allen bolts, but given they're allen bolts, I'm not expecting anything too strenuous. I've got a ball-ended 6mm from my bike toolbox, and I've also got normal ones that can go on my 3/8" ratchet with three different length extensions, so I should get enough leverage. Gives me a chance to use the gasket I've had in my parts box for the past couple of years too.
I don't need the car for a few days, so will probably put it in a bin bag with Mr Muscle sprayed liberally in there and just give it a few rinses then fill it with oven cleaner again until it looks clean. Maybe a long-handled bottle brush or something too.
With any luck, it'll improve emissions at any stretch, but it'll be nice just to have an engine that's breathing the way it should be. If this doesn't fix it I'm not really sure what next, but at least it'll be clean.
BUT
It is also great at dissolving Aluminium.
I used a blowlamp to start & hot air paint stripper gun on my manifold, very effective & dramatic.
The ports? Trickier, I didn't have time & a bad back didn't inspire me to be leant over the engine for hours cleaning them out.
If the relevant cam lobe is pointed up, and you are dextrous with a load of oversized dental tools to pull the shite off the port walls & valve stem, and have a small enough vacuum cleaner to pull the pile of loosend flakes out it probably can be done.
A more fun way was used in the 1920's on the Le Mans Bently's to decoke the engines,
(tricky job as the head and block were cast as one, no head gasket to fail, but no access to scrape away the pre-detonating soot.)
they stuck a metal tube with a gentle flow of pure oxygen into the spark plug hole, and lit it somehow, this would allow the soot to be burnt away.
you need bottled oxygen, (gas axe, oxy-acetalyne gear, saturation rebreather dive gear, mad rocket scientist etc.)
Be careful as pure oxygen is absolutely evil stuff if it builds up in a confined space. You will spontaneously burst into flames if you get it wrong.
On second thoughts, a BAD IDEA.