MPS Inductance Curves and Tuning, Is Fuel Injection Corp USA Selling Rebuilt MPSs with Bad Transformers? Options

[Superhawk996]

During this troubleshooting I would disconnect both advance and retard on the dizzy if you haven't already. Plug the ports on the TB of course.

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Let there be static advance + centrifugal advance only without a vacuum canister compounding things based on vacuum and changes in throttle position.

[Superhawk996]

Fair enough on the resistance thing. I doubt a few dozen milliohms is the issue, but it's not much trouble to run a temporary wire to battery ground.

And the ignition theory is interesting. Although, wouldn't that be more RPM dependent than throttle position dependent? Like not enough dwell to charge the coil? I guess the advance is moving up a few degrees when manifold pressure goes to 0, so I could test with the vacuum retard hose disconnected and see if anything changes.

But otherwise, the timing is set to the notch at 3000rpm - once I got the mixture richened up on the MPS it gave me the full rev range so I was able to tune it no problem.

The thing is that your engine ground also is the path by which the coil draws current.

I’m going to use an extreme example - let’s say your initial statement of 1.2 ohms between batt negative and engine case was true.

The Bosch OEM coil has an internal primary resistance of 3-4 ohms.

If you add an additional 1.2 ohms in the ground between battery negative and the engine ground, the coil draws much less (~25% less) current and you get a much weaker spark. Could be enough spark to work at lower rpm and proper mixture but when rpm’s rise and mixture is off, you get misfire. Again not saying this is your problem, but it is why good ground matters.

Now if your coil happens to be a VW bus style mount that mounts the coil off the fan shroud housing and that has 15 ohms of resistance from shroud to battery ground, that could be a real problem. EDIT: coil has its own ground (not chassis grounded) - ground path still comes though engine case via points and dizzy to engine ground. So floating ground to shroud wouldn’t be so much of an issue for the coil.

The other thing that grounds through that fan housing is the alternator. Powder coating or painting the fan shroud without cleaning the mounting points to the engine case can cause this type of grounding issue to the fan shroud. Not sure what your situation might be causing that high of a resistance. I would want to understand this a little better to understand why that should is so far from engine ground.

Not trying to run you around endlessly on electrical hunches but they can’t be ignored when you have the unexplained issues you’re having. Even more so knowing that the car was once in a fire and knowing that fire history cars usually have unexplained wiring and ground issues.

[emerygt350]

When I was troubleshooting my 914 I ran grounds from everywhere to the battery negative. Shroud, block, transmission, etc. After everything was working great I removed them one by one.