We have no strong reason to believe it is an ignition problem caused by putting 12 Volts on the kill wire Paul. The question needs to be cleared up by you checking for voltage on the kill wire of the second, or "Miller Falls mower" until you tell us what it actually is. What I think we have so far, is a mower that runs erratically. You have reported that the fault comes and goes with changes in the air cleaner, but I think more recently you have found that it is erratic all the time. If I heard it, I could probably tell you what is wrong. Erratic ignition is normally characterised by two features. First, the engine cuts in and out sharply: each shot is either fired or not fired. Second, there is usually some popping in the muffler under at least some conditions, due to the fuel that doesn't burn on the missed shots.
The way to find out for sure if it is an ignition fault that is gradually getting worse, is to put an in-line spark tester into the high tension wire. We discussed how and why, and how to make one, in a previous thread of yours:
https://www.outdoorking-forum.com.au/forum/u...amp;Main=9879&Number=59818#Post59818I think what should be done now, is to try that test on both your engines. Afterwards, keep the tester close to hand when dealing with engine faults. What I hope we've both learned from this thread, is that your descriptions of fault symptoms are so different from what I am used to, that we are at risk of going in completely wrong directions if we don't start with some basic hard data.
Applying 12 Volts to the kill wire is a serious abuse of the module, since it applies a potentially high, continuous current to a circuit not remotely intended for such treatment. I don't recall enough cases of it being done to have much idea how long it takes to destroy a module. It may vary widely.
