what causes carbon fouled spark plugs

Classic rich condition. You added an exhaust and a K&N without rejetting. The carb is running lean on the main jet? Actually, with an open filter and free-flowing exhaust, the engine can pull more air, making the mixture lean—wait, soot means too much fuel. If you leaned out the main jet, you'd get less soot. But with those mods, you might actually need to richen the pilot circuit. Check the pilot jet—it’s probably too big or the air screw is too far out. Turn the air screw in to lean the idle mixture.

Hate to disagree with the previous reply, but carbon fouling usually means the mixture is too rich, not lean. If the engine runs rich, unburned fuel leaves carbon deposits. With your mods, you definitely need to rejet. Start by checking the spark plug color: black and sooty = rich, white or light tan = lean. If it’s black, you’re rich. Try a smaller main jet. Also, make sure your choke is fully releasing—stuck chokes cause rich running.

Another thing: plug heat range. Too cold of a plug won’t burn off deposits. If your plug is a colder heat range (e.g., 7 or 8), try one step hotter (6 or 5). A hot plug runs cleaner at low speeds. But if you run it hard, a too-hot plug can cause pre-ignition. So find a balance. What heat range are you using now?

Are you running a stock pilot jet? I had the same issue on my 450. Took off the air filter lid and put on a slip-on, then the carb needed a 42 pilot (up from 38) and a 155 main (stock 148). The fouling stopped after that. But my elevation is 500 ft. Where do you ride? Elevation matters.

Check your float level. If the float is set too high, it raises the fuel level in the bowl, causing rich mixture and fouling. Measure the float height from the gasket surface—should be around 16-18mm for most carbs. Also make sure the needle and seat are clean; debris can cause flooding.

I had carbon fouling from worn valve guides. Oil was leaking past the guides and into the combustion chamber, burning and leaving carbon. Check your oil consumption. If you’re burning oil, the plugs will foul with black soot but also have a oily residue. If it’s dry soot, it’s fuel. If wet, it’s oil.

Also check your ignition timing. Retarded timing can cause incomplete combustion and carbon buildup. But that’s less common on modern CDI quads. More likely it’s the carb, as others said. Start cheap: clean the carb thoroughly, replace pilot and main jets with stock sizes, then tune from there.

Don’t forget the air filter. Even with a K&N, if it’s over-oiled, it can restrict airflow and richen the mixture. Clean it and apply just enough oil. I’ve seen people drench them and cause all sorts of running issues.

Could be a stuck ring or low compression. If the rings are worn, oil gets past and burns. Do a compression test. Anything under 100 psi is suspect. Also, check the spark plug tube seal for oil leaking down.

I disagree about the heat range being the primary cause. Heat range can mask the problem but won’t fix it if the mixture is way off. The plug should be in the proper heat range for your engine. Stock heat range is fine for most mild mods. Only change if you’re consistently fouling after fixing the mixture.

Another possibility: you’re not riding it hard enough. Carbon fouling is common in ATVs that only do slow trail riding or idling. The engine never gets hot enough to burn off deposits. Take it out on a long wide-open stretch and run it at high RPM for a few minutes. That can clean it temporarily.

Don’t ignore the choke. If the choke plunger is sticking partially open, it will enrich the mixture. Pull the cable housing and lube it. Check that the plunger returns fully when the choke lever is off.

I had the same problem on my 250. After everything I rejetted, replaced the plug, still fouled. Finally found the intake boot was cracked, causing a lean condition but then the ECU added more fuel to compensate? Actually, a lean condition would cause the engine to run hot and possibly not foul. But a crack can let in extra air, making the engine run lean, but the carb isn't compensating. Hmm, maybe it was an air leak making the pilot circuit act weird. Anyway, check for leaks.

Check your fuel. Ethanol attracts water, which can cause poor combustion and soot. Run non-ethanol if you can. Also, old gas can varnish and mess up the carb. Drain and fill with fresh gas.

Consider the plug wire and cap. A weak spark can cause incomplete combustion, leading to soot. Measure the resistance of the plug cap (usually around 5k ohms). If it’s out of spec, replace it. Also check for spark gap—use the recommended gap.

I’m a mechanic. In my experience, 90% of carbon fouling in stock-ish ATVs is from a rich pilot circuit. The pilot jet controls from idle to about 1/4 throttle. If it’s too big, the plugs foul at idle and low speed. Turn the air screw in to lean it. Or get a smaller pilot jet. Then adjust the needle clip position for mid-range.

Another vote for checking the float height. I set mine 2mm lower than spec once, and it caused constant flooding and fouling. Dropping the float level (lower fuel level) leaned out the mixture. Fixed it.

I’d also check the valve clearance. If the intake valves are tight, they don’t close fully, allowing intake charge to leak back into the manifold during compression? Actually, tight intakes can cause loss of compression and poor idle, but not necessarily carbon fouling. Exhaust valves being too tight can cause them to not open fully, reducing power but also maybe not fouling. Anyway, just do a valve adjustment if you haven’t.

Some people say running a hotter plug helps, but I’ve heard the opposite: if you have a rich mixture, a hot plug will just burn hotter and may not help with soot. Actually, a hot plug burns off deposits better. But it’s a band-aid. Fix the fuel mixture first.

Before buying anything, confirm your ATV model and year. Some models have specific carb settings. Also check the service manual for stock jet sizes. Don't just buy a jet kit; start with the stock settings and adjust from there. And always test ride after each change.