MPS mystery, huh Options

[r3dplanet]

Last winter I had some fun rebuilding my MPS unit, which is for a '73 1.7 liter engine. The MPS part number ends with 049.

According to the calibration tests on the pbanders website in conjunction with my trusty Meterman LCR55 and MityVac, the unit is perfectly calibrated. It holds vacuum without issue. The readings at 15, 4, and 0 in.Hg all match perfectly. And I mean perfectly. No variation from what the LCR55 shows to the pbanders table.

The mystery is that when I hook it up to the car the engine won't start. It tries to start and I can sort of force to keep going by blipping the gas, but it won't idle on its own. However, when I install another verified trashed out unit, the car starts right up and idles and climbs smoothly.

Um. Huh. I'm hoping that there's some other adjustment elsewhere in the system that my tired brain won't remember. Somehow maybe my car is adjusted to work around the (leaky) old MPS? Or maybe I didn't rebuild the MPS correctly. But it was in and out of the thing for days and I'm convinced the physical operation is correct. But I'm always willing to be wrong.

Confused.

-m.

[JeffBowlsby]

Hey Stu,

Sounds like you are deeper into the theory and electronics than me, I am mostly a wiring guy with a casual interest in the electronics. No worries.

I assume the 'known signal into the primary' means setting the vacuum at known levels, then adjust the secondary inductance. I think there is more to it than that though. Brad has suggested that each MPS was custom calibrated to each engine with its specific sensors and ECU, which could affect the actaul calibration of each MPS, and make each one unique.

I intend to research this more carefully when I get the chance to spend some time with the new-to-me EFI Associates analyzer I picked up from the Cap'n that displays actual injector pulse width. Then I should be able to generate improved calibration data with a more precise degree of precision fo each engine.