Jim
We can all learn something new every day if we look for answers. Me included...........So I looked deeper into why Autolite plugs do not work well in outboards as you claimed. Looks like there may be evidence of this.
I've always known CDI ignition systems (like those used on outboards and on some bikes) need a resistor type plug, but I didn't know there are different types of resistor plugs on the market.
There are the Monolithic Graphite and Glass type which are both high resistance type plugs and then there is an Inductive Coil type which offers a much lower resistance that is preffered on CDI systems.
The two types can have a noticeable effect on the performance of low spark voltage CDI systems, due to their internal resistance, especially if the ignition system produces marginal spark voltage. After researching different plug types and talking to three different Autolite engineers and an NGK engineer, Autolite admitted their plugs are the Monolithic Graphite type and not Inductive Coil type. They stated the Monolithic Graphite type is not the best type for marine CDI systems due to the noramlly low ignition voltage. That explains why they sometimes don't work as well with a marine CDI ignition systems that needs a lower resistance type plug to produce the best spark. The Autolite engineers admitted they have always just cataloged their Monolithic Graphite type resistor plugs for marine applications because that's the only resistor type they made, but as of just last year, they are now making and cataloging four new Inductive resistor type plugs specifically for marine applications. Looks like NGK and Champion are the only two other plug manufactures I found that make Inductive resistor type plugs. I always used NGK "Z" type plugs in my outboard boat and any motorcycles that ran CDI ignitions and I didn't have any problems. Now I know why. I've never had experiance with Autolites in marine applications so I haven't experenced the problems you've seen.
This is some of the info I found on the internet about the different type resistor plugs.
NGK's explination of Resistor type plugs for outboards.
Inductive Resistor Spark Plugs Specified as original equipment in a variety of late-model outboard engines, spark plugs with an inductive resistor are utilized when a CDI (capacitive discharge ignition) system is used. The resistor plug is necessary to reduce radio frequency interference (RFI), to prevent interference with the sophisticated electronics onboard modern boats.
Most resistor spark plugs use a "monolithic" resistor, generally made of graphite and glass materials, to filter the electrical voltage as it passes through the center electrode. This "filter" reduces RFI to an acceptable level. Although this type of resistor develops from 1000 to 5000 ohms of resistance, there is no significant loss of voltage to affect ignitability at the electrodes in modern automotive ignition systems.
However, outboard marine engines utilizing a CDI system have a much lower voltage output, which would be compromised by a standard resistor plug. Therefore, an inductive resistor is used. In this method, a coiled nickel wire is inserted into the center stem, and held in place by a spring.
This coil basically acts as an electromagnet, inducing a magnetic field around the center stem, creating a natural "field" resistor, and reduces RFI to acceptable levels. This method creates only 40 ohms of physical resistance, compared to the 1000 to 5000 ohms of resistance in "monolithic" resistor plugs.
While Inductive Resistor Spark Plugs cost more than standard resistor plugs, they provide major benefits in late-model marine outboard applications.
SparkPlug.com explination of Resistor type plugs for outboards.
CDI (Capacitive Discharge Ignition) for outboard marine motors is basically a refined version of the old magneto ignition system, except the points have been replaced with electronic circuitry. CDI is used on some (not all) late model marine engines such as Johnson and Evinrude, and some late model motorcycle engines. Do not use a "regular" resistor type plg on a motor with this type of Capacitive Discharge Ignition as this will cause misfire and poor performance. Rather, make sure to use a plug with an inductive type resistor (such as a Champion Q-type or NGK Z-type). Use of non-inductive resistor type plugs on these motors can create an open circuit within the spark plug (it will become a dead plug).
Modified automotive racing motors also have a form of CDI, (such as Mallory, MSD, Crane and Accel), these should not be confused with the marine or powersport systems, please refer to High output ignition systems for more information.
