Replaced the head gasket and now it won't start

make sure the compression is good first. the backfiring could be you have the plug wires on the wrong plugs. check your firing order. i would help but you did not identify your motor size.

Roy

I have the leads in the right place and it is a V6. There is one thing that bugs me is that when I line up the camshaft on the rear I have to advance it half a tooth forward then put the timing belt on, crank it over two revolutions and all timing marks line up. If I have all camshafts at their timing marks then put the timing belt crank it over two revolutions the rear camshaft is behind one tooth. Which is correct or are neither, and it is someting else causing this to happen?

it is about the slack in the belt. that why it is off when you set it initially. you did it right. once it is tensioned up, it moves one of the cams off. you have to compensate for it like you did.

Roy

Could the rear cam be on the exhaust stoke and how many teeth should the timing belt have?

Just letting you know that I towed the car the mechanics and after 3 hours he worked out that the Front timing belt pulley was put on back to front making that head 180' out, he has since corrected this and it starts fine everytime but when the revs are increased it backfires through the intake he then did a compression test and found that the whole motor has no compression meaning there is something wrong with the bottom end of the motor.