Engine not running right, overheating

OK, the car was not idling and was overheating at the drop of a hat at the start of Summer. Why is that? The answer wasn't obvious at all!

Compression test.
Most cylinders showed 135PSI, one at 105, lowest at 100.
I suspected a blown head gasket, so off with her head!
The gasket was OK, but the valves looked a bit the worse for wear: Pitted and with black crap coating the back.
More disturbing was that there were two water holes which were blocked by the gasket. No wonder it was overheating. The new gasket had the same blockages on both sides (front and rear) of the engine. I made holes for it, as you can see:

Engine with new gasket

It's the big one with the big red ring around it. No idea why it was not punched out, it might have been a tooling error.
Jobs to do:
1. Make holes in gasket. Your gasket might already have them if you're lucky. My dad was very surprised this one didn't.
2. Clean the head.
3. Remove then grind in valves.
4. Reassemble

The longest task is #3. Grinding or lapping in the valves is very time consuming. The head was unlikely to have cracked or warped so I didn't get it checked. Cast iron is very rugged.

The gasket was very hard to make holes in. A drill with the round hole attachment is a good choice. It's not like a regular gasket, it has metal in the middle.
The head cleaning just required a wire brush attachment. I did this then removed the valves, which requires a special tool that looks like a big G clamp. I cleaned up the valves and inspected them. They were OK, but pitted and they needed to be lapped in again.
This is the time-consuming bit. It requires valve grinding paste, a little light oil or thinners and a sucker.

Place sucker on valve

paste

Place the sucker on the valve and a small amount of the coarser paste around the valve, where it contacts the head with a drip of something lighter (thinner is ideal) which reduces the friction. Grasp the stick between your palms and rub like you're starting a fire with it. Also, lift the valve every so often. Not sure why but my dad swears it's important. When it feels smooth, remove the paste and inspect the surfaces. If they aren't completely smooth (you can still see the pits) repeat. I had to do this between 6 and 10 times before it was smooth. When it is, finish it with the smooth version at the other end of the tube, just the once. It took me two hours to do 12 valves.

OK, reassembled the head, connected everything and dad told me the valve clearances will need to be set. This is another place where you really need to be careful. The slant six requires the engine to be running and hot to adjust this. The setting is 0.010" (ten thousandths of an inch) on inlet valves and 0.020" in the exhaust valves. Now this is important don't fail to set the valve clearances again when the engine has warmed up.
I did, and had it running on five cylinders until I'd worked it out. The adjuster is 7/16" so get a ring spanner and re-adjust. You'll find the idle speed will change radically if yours was anything like mine.
I'd also bought a tune-up kit at the tip shop and it included a vacuum gauge. This is useful for getting the idle mixture right. Maximise it and your car will love you.
Mine seems to love me now. She idles better than ever (as in has literally never been this good) and is smooth and powerful. Also doesn't overheat as easily. Have to get the radiator sorted out next and I just might get through Summer!