slant six starting problems

Changeneering it is called.
First off, diagnosing with this small amount of info is difficult.
Did you do any work to it recently? Any other symptoms?

With your diagnosis, try to be methodical. You need to determine if it is fuel, electrical or timing related.
First off, try using starting fluid to see if you get any ignition. If so, you likely have a fuel delivery or timing issue. If it backfires, check the distributor timing. If that is ok, maybe time to check valve timing.
If you get no fire at all, a multimeter can be used on the wiring up to the coil input. I am not sure with the ECU's since I have no experience with them, but I believe with the key on, you should have 12 volts coming in to the coil. You may have to crank the engine for the voltage to turn power on to the coil. If not, trace it back and find out which component is getting but not forwarding voltage.
If you have have good spark, is the carb squirting when the throttle is actuated? If not, your carb is fuel starved. Crank the motor and try it again. If no fuel, trace it back to where the fuel stops. Typically a bad float setting, fuel pump or clogged fuel filter etc.
If you have spark and fuel you need to check the ignition timing and possibly the valve timing. These typically wont change unless you have something worn or broken or they were recently adjusted. I would rule out all of the other stuff first unless you made some unreported changes recently to the distributor or valves.
Assuming you have spark, check the ignition timing.
These are some basics to check. Report back and let us know what you find. From there we can dig deeper.