Roger on the sandblasting idea. If you have a single side electrode plug, gap it to .010" and stick it back in. If the SAG goes away, it is a plug getting carbon tracks and needs discarded. If the ignition system uses 12 volts to the coil primary, and an open circuit to fire the plug,

( a Kettering system) this secondary voltage break down is typical.

Slow rise times alloy for a number of alternate paths to form and drain off high voltage until no spark happens. A CD system supplies 400 to 600 Volts to the primary, and the plug fires on the way up to saturation, and again during secondary collapse. So you get two hits every time. Plus so much energy is available that carbon paths are burned away and the plug fires no matter what. MSDs for 40 times at low RPM, so shorted plugs become a thing of the past.

Another theory on SAG is that it is an overheated plug tip causing early firing. (preignition) which kills off torque and adds much more heat to cause more misfires. Racing plugs are NGKR6725-11.5. The 11.5 is the heat range. The higher the number the colder the plug (In NGK plugs). Use the coldest plugs you can get to work.

 

Lynn E. Hanover

 

Bill, there are 2 possibilities. First you will hear from many people that you should never sandblast plugs. The reason is that the porcelain is pitted and the glaze on the plug is broken and then the plugs can embed carbon. The plugs can then track carbon and the spark will travel down the carbon track. On fine center wire plugs blasting can round the electrode which increases the voltage needed to fire the plug. Both conditions are bad, tracking is the worst. Pull your plugs and look for what appears to be a pencil line down the insulator. That is tracking. It likes to hide in the area near or under the sidewire which doesn't clean up as well when the blasted.
Bill Jepson

On Sep 3, 2012 8:47 PM, "Bill Bradburry" <bbradburry@bellsouth.net> wrote:

Has anyone ever noticed a hiccup associated with sparkplug SAG?  I have a hiccup that is occurring at random intervals from seconds to several minutes.  Sometimes it is pretty mild and sometimes it shakes the plane!  I also seem to be down in power and I am beginning to suspect plug SAG.  I have 38 hours with the Iridium plugs plus a lot of time over the years of running the engine before flight.  I had a couple instances of flooding which required that I take the plugs out and sandblast them to get them to fire.  Just drying them out didn’t work.

 

Thoughts??

 

Bill B