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Tom..... ....you raise some interesting miss-conceptions..... Sooty plugs have NOTHING to do with sparkplug heat range. Sooty plugs denote rich carb mixture. There is NO RELATIONSHIP between soot and carbon. While soot is a soft black, carbon is generally hard gray/tanish deposit. The hotter AL-437 sparkie is good to use with todays no-lead gasoline. Gapp'em at 0.025 rite outta the box. Sparkplug gap does affect the tempature of the ionized electrons that jump the gap. As a general rule, the wider the gap, the hotter the sparkie..... .BUT..... the problem is the wider the gap, the more sparkie volts your ignition coil gotta generate to jump the gap. Your 9N frontmount sparkies are just addequate, to be charitable. That is why it is so important to remember the 6 volt mantra: "keep'em clean, bright, and tight". And if a wider gap than the specified 0.025 was good, then as the plug is used and it wears open to 0.028 or 0.030, you'd get a hotter sparkie wouldn't you?..... BUT.....instead you get mis-fire and hard starting. So wider gap isn't always the way to go, is it? Advancing or retarding the timeing, can marginally effect the sparkplug tip-heat. And when we talk about sparkplug heat range, its the tip-heat that we are talking about. And heat range is a function of sparkie design. You want it hot enuff to burn off carbon deposits that are a by-product of combustion..... .BUT..... you don't want the tip-heat so hot that it self-ignites the gas mixture before its suppost to be ignited by the distibutor timing, because that will blow a hole in the top of your aluminum piston. Its called pre-ignition. (or ping) .....nasty stuff. You can play around with your ignition timing if you want....BUT....UNLESS you have your tractor on a dyno, ya ain't gonna be provin' nuttin'. Setter for factory specs, you'll be in the ballpark. Ya ain't gonna change the carbon deposits muckin' round with timing. Automotive engineers spend hours on dynomometer, carefully plotting the optimum ignition timing for maximum hp or maximum economy, and then the EPA sez...no..no.. thats too much smog, and the the engineer have to obey the bureaucrats. Bottom line..... .if'n you want to know more about automotive/tractor ignition stuff, take a nite school automotive powder-puff course or brouse at your local hot-rod parts shop for ignition and carburator books..... ....respectfully, Dell
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