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!

Compression Test

At least it's still November, so this is still "shortly" to me.
Performed a compression test early September to find out if there was anything weak in the engine. If you don't know what this is all about, buy a compression tester from your local auto parts shop.
Compression test
Remove all the spark plugs and screw the tester into #1 spark plug hole. If you didn't buy a screw in one, you will need to hold the tester in place and get someone to help you. Look, just get the screw in one.
With it firmly in place, put your foot on the accelerator and press it to the floor. Now, turn the engine over. You'll hear the compression strokes move the needle of the gauge (sounds like puffing). When you've gone through three engine cycles, check the gauge. Repeat for the other cylinders and write down the results.
As mine is a 160HP slant, compression is expected to be higher then the 155HP version (red motor). I was hoping for 180psi, and got the following:
1 138
2 110
3 155
4 155
5 148
6 152
Urgh! Bloody awful. Definitely something (or things) wrong here. With these results, we look for a couple of things, being firstly the number, which is obviously quite low here and secondly whether the readings are similar to each other, which they are not in my case.
If there's any low numbers at all, you can determine if it's the head or piston that's causing the problem by doing a second test. This time, grab an oil can and squirt some in befrore screwing the compression tester in. The reason that we do this is that squirting oil in will seal any misbehaving rings and bring up the reading significantly if the problem is with piston rings. If the problem is with the head (e.g. valves are damaged or valve stems worn out) the reading will still be low.
With mine, the compression came up significantly as expected, since I had the head resurfaced, bought new valves etc when the LPG buggered it up about two years ago.
So I was going to have to remove the engine. Did this on 6th September.