Mixture sensitivity

[Bryan Cotton]

Hey all,
One thing about our new "hot rodded" AeroVee is that it has gotten more sensitive to mixture. Meaning, especially on the primary only, if it is too rich it stumbles a little bit. Lean it out and it smooths out. On my flight today I also tried this on the secondary only, and it exhibits similar behavior but not as dramatic. The problem is that there is a fine line between happy, and EGTs over temp.

This is not too bad to manage in normal flying. We climb on both ignitions, and don't have to start leaning until 3000-4000' or so and then it's just a tweak. It is a lot less sensitive to mixture when running on both ignitions. When we level off, we lean out so the hottest EGT is about 1320-1350, and then shut off the secondary. Today I was doing slow flight, stalls, steep turns, rolls, and loops. Very unsteady flying with a lot of power adjustments. So it was a pain today. It didn't use to be this finicky.

I could go a tweak leaner on the needle, and that might make it run good. But I'm concerned if I will overtemp EGT during a full throttle climb to altitude. The good thing about trying this is that it is pretty easy, and I don't have to pull the intake manifolds.

New plugs about 35 hours ago, after I had the engine apart for new heads. I gapped & installed them per the manual. I have not checked the accuracy of my gapping tool. It's one of those cast round ramp type gappers. Also new magnetrons, and I had to replace the coils about 33 hours ago. One of them failed on one cylinder. If I was to try something with the plugs, I could either go with more gap or with less. Less would make sense if the coil or magnetron was struggling to make a hot spark. More would make sense if it just needs a bigger spark to get the flame front going.

Any experience or musings are welcome.

[funflyingguy]

hey bryan,
was there something wrong with the magnetrons when you changed them out or just because it was a convenient time? any idea about how many hours you had on them at that time?

[Bryan Cotton]

hey bryan,
was there something wrong with the magnetrons when you changed them out or just because it was a convenient time? any idea about how many hours you had on them at that time?

It was a convenient time, and we did have some missing on them before too. I changed them at 285 hours and saved them as spares because I think they are still good.

[Area 51%]

A little late now to compare, but the combustion chamber shape of aftermarket heads can vary greatly. It's entirely possible the flame front is not what it used to be.

Confronted with your issue, I would swap the primary leads with the secondaries as an experiment. A lot of fiddling for possibly no gain.

An issue I had was the heads I was using had a thicker cross section that buried the spark plug. A little grinding around the plug hole unshielded the plug tip and once again the world was spinning in greased grooves.

[Bryan Cotton]

A little late now to compare, but the combustion chamber shape of aftermarket heads can vary greatly. It's entirely possible the flame front is not what it used to be.

Confronted with your issue, I would swap the primary leads with the secondaries as an experiment. A lot of fiddling for possibly no gain.

An issue I had was the heads I was using had a thicker cross section that buried the spark plug. A little grinding around the plug hole unshielded the plug tip and once again the world was spinning in greased grooves.

That's an interesting thought - thanks. At the very least I can look up through the secondary plug holes at the primaries and see what it looks like.

[Bryan Cotton]

I wonder if I can remove the spark plug washer and just use the CHT sensor as the seal. That would get the plug in a little further.

[Area 51%]

The plugs on my 2008 Colorado are about a foot long. There might be a suitable longer reach unit available.