Fouled plugs were common when vehicles burned a little oil and electronic ignitions don't cause much erosion. About the only reason to carry spares in today's vehicles is if you break one taking it out or reinstalling it.

 

News flash. Had 42k on the originals and replaced them even though they looked new with good gap. Had 15k on the next set when the bike started having an intermittent miss while on a 1500 mile round tripper for the Laughlin River Run. Almost seemed like a fuel delivery issue, changed my fuel filter ran some Techron, no change, was actually getting worse. changed plugs, now runs better than new with 58k on the odo. looks like there may be a hairline crack in the porcelain around the electrode causing misfires

 

I carried extra plugs with my Iron Sporty and my shovel .

Occasionally had to use a match book cover to set the points ha

I have 41,000 on the 08 FLHRC guess I should replace the plugs they look fine .

 

I only carry spare plugs when riding my old Suzuki 2 stroke street bikes. As far as having spares for a modern fuel injected 4 stroke engine? You got bigger problems if you need spares. Get yourself a tire plug kit (if you have cast wheels) and you're good to go. If you have spokes with tubes; always have walking shoes.

 

I have had one fail. I replace mine every year in the spring and last August on the way to Sturgis I stopped in Rock Springs Wyoming (on a Sunday) for gas and lunch. Bike ran great from Salt Lake to Rock Springs. Stopped the bike and went in for lunch and 30 minutes later she would only run on the back cylinder.

Thankfully the dealer in Rock Springs is open on Sundays and they had spark plugs. I replaced both of them after going thru the power commander, and etc to make sure it wasn't something else. Turned out the plug was just dead, no fouling or anything unusual.

 

I've had several fail on the years. As in break. One had the center electrode fall out, another had the ceramic break up (that did damage the engine).

I've also had a fair number of then foul out or simply wear out. Mostly in old well worn engines. Oil burners, 2-strokes, etc. Not an issue with anything modern, or even vaguely modern.

I do carry spare plugs in the boat (2-stroke outboard) and the snowmobile (2-stroke again). These are vehicles that are prone to fouling and make like darn hard to deal with if I'm stuck because of a fouled plug. Otherwise, I haven't carried a spare set of plugs for many a year.