Dapokeman

I've been having problems starting my 2000 Tahoe lately, and when I finally starts, it runs rough. Today i couldn't start the truck whatsoever. Called a local friend mechanic to diagnose it but couldn't start it. The things he checked and did: swapped the crank sensor, Inspected and changed the plugs, cleaned the distributor cap and wires and checked for spark which there was, checked for fuel through the spicket that looks like a tire stem and fuel spet out. So now the mechanic thinks it's the timing. Any suggestions anyone????