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I'm somewhat new to wrenching my bikes and I'm loving it! :D

I have a 74 Yamaha TX 500 that keeps carbon fouling her plugs :(. I've put on slightly hotter plugs (D7EA from D8EA), narrowed the jets from #132 to #120, and put on K&N filters right on the throttle body.

She STILL fouls (black soot) the plugs almost to unusability within 15 minutes of hard riding.

What could it be? I've heard it could be an ignition issue or that I need an entire new carb.

What are your thoughts?

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Do you still have this bike along with the issue? – DucatiKiller Jan 13 at 9:02
I sold it, but the problem was a slightly wrong sized carb installed by previous owner. – JSK NS Jan 13 at 9:19
Yeah. That would have been tough to figure out. At least your not burdened with the problem. – DucatiKiller Jan 13 at 9:43

Carbon Fouling is caused by incomplete combustion. The usual cause of this is an over-rich mixture, but it sounds like you've checked most of the elements that could cause that.

What condition is the ignition system in? Distributor, HT leads etc? I assume you've checked the mixture when you changed the jets, but what about the choke (if she has one)? What about the ignition timing?

The NGK website also suggests that it could be due to worn rings or low compression, so it might be worth doing a compression test.

This page has quite a nice guide to plugs, with photos of the different types of deposit and a list of potential symptoms.

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