Re: 5.7 alpha with thunderbolt ignition will not run when throttle is reduced.
Here's a thought:
Most tachs are driven from the - side of the coil.
If you pay very very close attention to your tach, you should be able to tell if it's an ignition problem or not.
Try to recreate the problem while staring at your Tach. If your tach slowly goes to 0 as the engine dies, than your ignition is okay.
But, if your tach suddenly flat-lines, and then your engine dies and spins down, then there's a good chance your ignition cut out, and that's the cause of your problem.
Don't know too too much about marine ignition triggers, but if it's variable reluctance, it could be a weak pickup. The voltage output of a VR sensor is proportionate to the RPMS. Meaning that lower RPMS give a weaker signal. If your pickup is faulty, it may not be working in the lower RPM range. That's IF your using a VR sensor to begin with.
If looking at your tach doesn't give you a definite answer, try using a timing light. You don't have to point it at your crank pulley, but just at something that you can see. If your not getting any flashes as your engine dies, than you know your ignition is cutting out.
Another thought. Have you checked to make sure your shift interrupt isn't kicking in? A poorly setup shift cable or throttle cable could cause this.
