Spent the whole of last night in the piddling rain, sleet, dark, and cold, getting the cam timing bang on and putting the car back together.
Bearing in mind I was working under a dodgy bit of tarpaulin, in the pitch dark, with only a maglite to see by - the job took I would estimate just over three hours.
I cranked the engine over by hand again once timed up properly, and I could again feel good compression.
Double checked everything was done properly, filled up with coolant, and Fired her up.
My three bitterly cold and wet hours paid off. She roared straight into life. Sounded a bit tappety to begin with, but I wonder if that was the OS manifold gasket, been suspicious of that for a while.
Once she got a bit warmer, and the SAI also shut up, she purred like a happy kitten

Then died again. Tried cranking, and she fired up, coughed, and died. Went and got a can of petrol, and all was well again

Gave it a couple of gentle revs, sounded OK. Couple of hard revs near the redline - again, no problem.
A slight "knock" when I reved it, I Traced to a loose cable tray - whoops! fixed that.
Road test - seems the same as before. The flat spot at 4krpm has now gone, so I just wonder if it didn't slip all those teeth in one go. Maybe it had slipped only three teeth instead of four, and I got away with it??
I'll compression test it again, but the way it's looking, is that I had a very lucky escape!
In addition to it now running OK, it has a brand new genuine GM cambelt kit, water pump, aux belt, and aux belt tensioner, all fitted and set up properly by myself.
I've now done approx 300 miles, all between 70-90mph, and I'm averaging 30 miles per gallon.
Fingers crossed. Looks like my LPG install can continue this weekend...
