You ask we answer. Here's the latest in our "Ask a Mechanic" series.

Q: I keep fouling the spark plug on my 2008 YZ250. Like after every ride. What's going on?

A: Old gas!

But Probably Something Else

Running old gas is one possible answer though if you ride as often as it sounds like, you probably are not using old gas. So, you have some work ahead of you to determine why you keep fouling the spark plug in your 2-stroke dirt bike. Worn or damaged pistons leak oil which fouls the spark plugs. Also, check the exhaust smoke. A too rich air/fuel mixture is a great way to ruin a spark plug. If your bike releases a lot of black smoke you might have a "rich fuel" environment. Bad jetting, bad gas and an incorrect gas to premix ratio all result in fouled spark plugs. Check your air filter too.

Check the Power Valves

Clogged power valves eat spark plugs for lunch and if I had to guess based on how often it sounds like you are changing the plugs you probably have clogged power valves. If you have trouble getting up to speed that's another sign you have clogged or stuck power valves. If so, you'll need a top end rebuild kit and of course a new spark plug. Lastly, check the reeds, float height on the carb and the crank seal on the clutch side. These crank seals when leaking suck in transmission oil.

Causes of Fouled Spark Plugs in a 2-stroke

  • Old Gas or bad gas
  • Clogged air filter
  • Incorrect gas/premix ratio
  • Worn or damaged pistons
  • Rich air/fuel mixture
  • Clogged power valves
  • Incorrect float height on carb
  • Leaky crank seal
  • Cracked, worn or chipped reed