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Why rear cylinder sparkplug fouls on 1996 harley?

I have a 1996 Harley Davidson 'BadBoy'. The rear cylinder sparkplug fouls out [solid black] about every 75-100 miles. I have to carry extra plugs, socket and ratchet. The front plug does good. Would carb need to be re-jetted. I had mechanic adjust jetting one time, [said I have the kind that has a slide or something to adjust jetting]. I am wondering if it is running to rich. Would that effect the rear plug, and not the front plug?
Back plug is black and sooty. Front plug is good. Both tail pipes are black and sooty, not black and wet. No smoke from pipes when bike is first cranked. Carb spits a little while riding.
It's a '95 bike. My son has a '96 Jeep Cherokee and I keep getting years mixed up between it and bike. Sorry.

There could be a couple different reason for your rear plug to be fouling.
Being that it is a 96, you could have a weak plug wire on the rear cylinder, especially if you still have the original factory wires. General rule of thumb is to replace them every 3-5 years. There could also be corrosion in the coil plug towers, causing a weak spark. If so, replace the coil and wires.

This is also a very good candidate for an intake air leak. The carb could be jetted too rich to compensate for an air leak. If it is leakiing on the front cylinder, this could be exactly why why your rear plug is fouling, because the front cylinder is sucking in air, cauaing a lean condition, evening itself out to APPEAR to be normal.
To check for this, warm up the bike to operating temp, remove the air filter and and use a can of Brake Cleaner with a straw to spray directly on the intake flanges at the head with the engine idling, if you hear ANY change in idle, they are leaking and need replaced.

I can recommend the proper jetting for you, but need to know exactly what carb and exhaust you are running.

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