Here is some data
I got off of the NKG sparkplug
manufacture site that may be of interest relative to SAG
Pre-delivery fouling
Carbon fouling occurs when the spark plug firing end
does not reach the self-cleaning temperature of approximately 450°C
(842°F). Carbon deposits will begin to burn off from the insulator nose
when the self-cleaning temperature is reached. When the heat range is too
cold for the engine speed, the firing end temperature will stay below 450°C and
carbon deposits will accumulate on the insulator nose. This is called
carbon fouling. When enough carbon accumulates, the spark will travel the
path of least resistance over the insulator nose to the metal shell instead of
jumping across the gap. This usually results in a misfire and further
fouling.
If the selected spark plug
heat range is too cold, the spark plug may begin to foul when the engine speed
is low or when operating in cold conditions with rich air-fuel mixtures.
In some cases, the insulator nose can usually be cleaned by operating the
engine at higher speeds in order to reach the self-cleaning temperature.
If the spark plug has completely fouled, and the engine will not operate
correctly, the spark plug may need to be cleaned / replaced and the fouling
cause identified.
Misfires
A misfire occurs when the spark travels the path of least
resistance instead of jumping across the gap. Misfires can be caused by
the following:
1.
Carbon
fouling
2.
Worn
or deteriorated ignition system components
3.
Too
large of gap size
4.
Spark
timing excessively advanced or retarded
5.
Damaged
spark plugs (cracked insulator, melted electrodes, etc)
6.
Mismatched
ignition system components (plug resistance / wire resistance, ignition coils /
igniter modules, etc.)
7.
Insufficient
coil primary and/or secondary voltage – voltage required to jump the
spark plug gap higher than coil output
This is what I found relative to Lead Fouling
Two types of lead fouling
1. Deposition of metallic lead or lead salts in the spark plug gap
causing a direct short.
2. Deposition of metallic lead or lead salts between the insulator and the
shell, gradually shortening the flashover path until misfiring occurs. Metallic
lead deposits reduce the plug's shunt resistance.
Statistical Analysis of Airline Spark Plug fouling presented
By Shell Oil Company in 1950
The choice of operating variables could exert an appreciable effect on
fouling.
- The use of abnormally long rich-mixture idle
periods
- Extremely lean mixture during cruise which
results in poor combustion
- Low mixture temperatures and low cylinder head
temperatures
My interpretation of the above data still leads me to
believe that fouling is the most likely cause of SAG. But, of course, that is
just my opinion.
ED
Ed Anderson
Rv-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
http://www.andersonee.com
http://www.dmack.net/mazda/index.html
http://www.flyrotary.com/
http://members.cox.net/rogersda/rotary/configs.htm#N494BW
http://www.rotaryaviation.com/Rotorhead%20Truth.htm