1980 kz750 no start only backfire

When i try to start the bike all that happens is an occasional backfire while the engine cranks over. If i spray starting fluid in the carbs it will start backfire and then die. I check the splugs and re-gaped / cleaned them and they all spark fine.I have been messing with the carbs only to get nowhere. Any idea on where i should start

First thing to check is the timing.

Then check where the sparkplug wires are going.

On the Z1, KZ900, KZ1000 and GPZ1100 the left ignition coil wires to the sparkplug #1 and #4 and the right coil go to #2 and #3.

The sparkplug caps can fail intermittently and the mechanical advancer may be sticking.

Make sure coil trigger wires are not switched - that will cause backfire for sure (don't ask how I know)

I know that the wireing is correct for the coils and the spark plugs. it ran fine before and no on has worked on it. and im not to sure of how to check the timing on a motorcycle. how do i know if the camshaft and crank are lined up? do i go by the #1 piston being at tdc and then see where the check the marks behind the ignition control module?

Simple question, is your vaccume line from the petcock in good shape? how about your boots from the airbox to the carbs?

If the vaccume line is disconneced or the line is cracked and pulling air, then no gas is getting to the engine (only air)= no start and backfire.

KR

there isnt any vacuum line coming from the petcock there is vacuum ports on the intake boots but they have rubber caps on them the vacuum ports are just there for syncing the carbs i think. and the intake ports are in good shape there arent any cracks anywhere and no leaks.