Joe Horton wrote:

>>Mark L. suggested that I swap plugs just for laughs since they were all brand new. Instead of swapping i just grabbed another new one and gaped it and installed it. I jumped in and gave her a start up. It was incredible. She fired up fast and just sat there purring away.<<

One thing to learn from this example is that at one point the engine had a miss at idle, but ran fine at high RPM. That makes you think maybe there's a vacuum leak somewhere, because a vacuum leak has a big influence at low speeds, but very little at high speed. That issue was eventually fixed by tweaking the mixture a bit, but then there was still a stumble and a misfire at higher rpms, yet no problem at idle. It was bad enough that apparently it showed up in the EGTs.

I used to work at the Ramstein AFB Auto Hobby shop when I was stationed in Germany, and we had a spark plug tester that would both sand blast the electrode and check the spark, both with and without pressurizing the plug (to simulate the compression process). After cleaning, we'd apply 150 psi air to a chamber that it was screwed into, complete with a little window so you could see the spark. Sometimes it would spark great with no pressure on it, but intermittently or poorly with air pressure applied. It wasn't common to see this, but it does happen, so that's something to consider when you have a high speed miss with no other explanation. And now we know that it can even happen with a new spark plug! There's something to be said for saving old "known good" spark plugs, to do this kind of testing.

Here's an example I found: . Apparently the new plug's spark went out far below the expected upper limit though.

Mark Langford.....I still love Germany, and their attitudes on just about everything,...

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