502 mpi hard restart
 

Did a mild build - rebuild two years ago with eddies fuel return to tank trick and a computer reprogram. Ran great for a couple months then became hard to restart and had to have it full throttle to start. Its been getting worse to start some times starts great but next time it almost starts at full throttle but not quite ( when it does start she always runs strong). There are days it has 43 psi on the rail and spark but wont take off. I have been throwing lots of parts at it on weekends but no luck. I have a MEFI pro scan hooked up to it but still confused. Any advice would be appreciated.

Full throttle is clear flood mode. Ie: no fuel (or very reduced) thru injectors.

Injectors were cleaned and tested at time of rebuild. I never could see fuel in the sight line to throttle body and both vents on vst held vacuum but i replaced the float seat and diaphragm anyways yesterday. Rail pressure seams to hold good. Started right up today and took for a ride when i got back on trailer it would not restart. Got home and vst tank was about 2/3 full but had a little pressure when i pulled the fill port plug.

How do you know it has spark when it won't start?

Used one of those inline spark checkers

someone else had a similar issue turned out to be the gas line from the tank to the filter head, it was collapsing internally

I can measure it for a new one this weekend but it ran about 69 mph yesterday parked on the trailer and would not restart and the vst tank was about 2/3 full.

where are you picking up fuel psi schrader valve on the rail ?

If you were coming off the coil, then it has to be fuel

Injectors are batch fired, I use a Noid light and check a couple when cranking to see if there firing.
Might be something simple as the Tach pulse is to weak for the ECM to see. If it doesn't see that the Dizzy is firing it won't fire the injectors

Yes. about 35 with vacuum and 43 without.