In a message dated 10/19/2006 1:32:36 P.M. Eastern Daylight Time,=20 ALVentures@cox.net writes:

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I have been doing a lot of= =20 re-tuning of the mixture which may account for some of the dark deposits,=20= but=20 I'm considering a change of plugs soon. Just wondered what other=20 recommendations might be out there regarding plugs, heat range, gaps.=20 Lynn; I recall you weighed in on this before, but I didn't find it i= n=20 the archives.

Thanks,

Paul Yaw is on the dyno every day, and has forgotten more about rotarie= s=20 than I have learned.

So, whatever he says to use, is the plug to use.

Judging the plug's (and engines) performance by looking at it aft= er a=20 full power run and a clean cut is still a viable way to recover data on= =20 mixture and heat range. However, at aviation RPM, there is not much need for= a=20 real (Lethal) ignition system. Just about anything will work. We had the two= =20 distributor 12As turning over 9,000 RPM with good reliability. So, for racin= g we=20 use R6725-115 NGKs with the gap at .010". This is an ice cold plug in heat r= ange=20 for a 12A (smaller than a 13B) making about 250 HP.

So, why not use that?

There is no need or advantage. These plugs are $25.00 each. They are ha= rd=20 to find. You will not be turning RPM where lighting the mixture is becoming=20 difficult. At even 7,000 RPM the engine is hardly running at all. They will=20= foul=20 quickly. Running car fuel gets you a very dark clean cut plug=20 reading as in fluffy black more like oil fouled. Most of that wil= l be=20 from just one second of idle power on car gas. Avgas works just fine for plu= g=20 readings.

A clean cut is from cruise RPM to stopped at the same throttle setting=20= and=20 ignition off.

The guys looking through the eyepiece at the track are running racing f= uel=20 with lead in it. They are looking for cement boil around the center electrod= e.=20 Flat black to no color on the shell face.

No rounding of the electrodes. A hint of tan on the side of the porcela= in=20 facing the intake valve. Good luck on that one. Fractured porcelain and so=20 on.

From 9,000 on up, a good CD or CD with multi spark is a must, both powe= r=20 wise and misfire wise.

So, a dark plug that has idled for just a second or so will look bad=20 because the nose temp has dropped out of the operating temp range and collec= ts=20 phosphors from car gas or lead compounds fro avgas. it just looks bad. It wi= ll=20 clean up when used hard again.

Lynn E. Hanover