vacuum issue, won't run, mechanic needs help Options

[enf2232]

Background: 1.7 liter which has been upgraded with new pistons and cylinders, original fuel injection. Car started and ran, then stalled and would not restart. Covid hit so it sat for a while. Now at mechanics shop where they have replaced the fuel pump (rebuilt original) and the manifold pressure sensor (rebuilt).

Engine has low manifold vacuum, 10-12" and will not start. If they apply 15-18" of vacuum with a vacuum pump it starts right up and will run fine for 20-30 minutes (on pump), then stalls. If they attempt to restart, it backfires and will not restart until cold.

Any suggestions on what to check or what next step should be?

[emerygt350]

Thats pretty low manifold pressure but intensely retarded timing could result in very low vacuum, that mixed with some vacuum leaks could result in your issues. May want to verify timing visually (quick and easy) while you are digging through the hoses. You don't even need to have the MPS vacuum connected for the engine to run. I have accidentally started mine many a time with the MPS vacuum disconnected. In fact, you can pull the TPS, the MPS, the plenum temp sensor, etc and still get the car to start and idle. The CHT and MPS need to be wired to the ECU for it to run though.