I have dismantled and assembled my engine doing a head gasket job, and when I try to start the engine it cranks but doesn't fire. On the first few attempts there were one or two puff sounds out the exhaust (e.g. link a back fire sound, but not as loud).

I am thinking it could be timing, but the manual doesn't give a detailed description of how to set this. Does this sound likely? What is my best bet at diagnosing the problem?

The full story is:
- Disco V8 3.9 94 (210K km)
- Months ago I had a regular overheating problem.
- So as I do, I DIY pulled the engine apart, deciding that everything needed a clean and it could be head gaskets as clay substance under the oil cap. Oh and also to learn about engines.

- So I today finished a 3 day weekend of putting everything back together after having heads machined and valve job, all new spark plug etc. and even dismantling, cleaning and assembling rockers. And doing it all by the offical workshop/engine overhaul manuals ...

Wasn't sure about setting timing, but assumed I had it pretty close. Lined up the groove in the crank belt wheel with the notch on the block below the distributor, seemed right position for cyclinder #1 at TDC. Then line the distributor rotor with the notch before plug 1 contact, which meant it moves to contact 1 when the distributor was inserted in place. I didn't expect the timing to be prefect but thought it would work.