Outboard won't start?

After rebuilding a 1989 Johnson GT 150 I found that it will not fire. It has perfect compression and is sparking in all 6 cylinders but all it will do is backfire even with starter fluid sprayed into the sparkplug hole. So iv decided that the problem must be the timing. So the question is is there any thing else that could be going wrong if not how do you adjust the timing?