1979 Yamaha IT250 Smokes on Startup – Causes and Fixes

Shop parts for a 1979 Yamaha IT250 Dirt Bike.

Understanding Startup Smoke on the 1979 Yamaha IT250

Seeing smoke from your 1979 Yamaha IT250 when you first fire it up can be concerning, but it isn’t always a sign of trouble. Knowing when smoke is normal and when it points to a problem helps you keep your IT250 running strong and reliable. This guide breaks down the most common causes of startup smoke and offers practical fixes tailored to your bike.

When Is Startup Smoke Normal?

  • Cold Starts: A brief puff of smoke is common when starting the IT250 after it’s been sitting. This is especially true in cooler weather or after the bike hasn’t run for a while.
  • Oil Residue: If the bike has been stored on its side stand, oil can pool in the exhaust or crankcase, burning off as smoke on startup.

In these cases, the smoke should clear up within a minute or two of running. If it persists or is excessive, further investigation is needed.

Common Causes of Excessive Startup Smoke

The 1979 Yamaha IT250 is known for its robust performance, but like any machine, it can develop issues that lead to more than just a harmless puff of smoke. Here are the most likely culprits:

  • Premix Ratio Issues: Using too much two-stroke oil in your fuel mix can cause heavy, bluish smoke. The IT250 requires a specific oil-to-fuel ratio. Double-check your mixing process to ensure accuracy.
  • Oil Pooling in the Crankcase: If the bike sits for extended periods, oil can seep past seals and collect in the crankcase. On startup, this excess oil burns off, creating noticeable smoke. This is more common if the bike is stored on its side or at an angle.
  • Worn or Leaking Crank Seals: Crank seals keep transmission oil and crankcase pressure separate. If they fail, transmission oil can enter the combustion chamber, causing persistent white or blue smoke. This issue won’t resolve on its own and can lead to further engine damage if ignored.
  • Rich Fuel Mixture: An overly rich air-fuel mixture can cause incomplete combustion, resulting in extra smoke. Check your carburetor settings and air filter for blockages or excessive oiling.
  • Excessive Idling or Low-Speed Riding: Prolonged idling or slow riding can cause unburned oil to accumulate in the exhaust, which then burns off as smoke when the engine is revved or restarted.

Diagnosing the Source of Smoke

To pinpoint the cause of smoke on your 1979 Yamaha IT250, consider the following steps:

  • Observe the Color:
    • Blue or white smoke typically indicates burning oil.
    • Black smoke suggests a rich fuel mixture.
  • Check the Duration:
    • Smoke that disappears quickly is often harmless.
    • Persistent smoke after warm-up signals a deeper issue.
  • Inspect the Spark Plug: Remove and examine the spark plug. Oily or wet plugs point to excessive oil in the combustion chamber.
  • Monitor Oil Consumption: If you’re topping off transmission oil more frequently, suspect crank seal leakage.

Practical Fixes for the IT250

  • Adjust Your Premix: Use a precise measuring cup and follow the recommended oil-to-fuel ratio for the 1979 Yamaha IT250. Avoid guessing or “eyeballing” the mix.
  • Drain Excess Oil: If you suspect oil pooling, remove the spark plug and kick the engine over several times (with the ignition off) to expel excess oil. Clean or replace the plug before restarting.
  • Replace Crank Seals: If you notice persistent smoke and rising oil consumption, replacing the crank seals is necessary. This job requires splitting the cases, so consider your mechanical comfort level before proceeding.
  • Clean the Carburetor and Air Filter: Ensure the carburetor is clean and properly jetted. Wash and lightly oil the air filter—too much oil can be drawn into the engine and burned as smoke.
  • Run the Bike Regularly: Regular use helps prevent oil from pooling and keeps seals lubricated. If storing the IT250 for long periods, keep it upright and start it occasionally.

When to Seek Further Help

If you’ve addressed the common causes and your 1979 Yamaha IT250 still smokes excessively on startup, it may be time for a more thorough inspection. Persistent smoke can indicate internal wear or a more complex issue that requires professional attention.

Keep Your IT250 Running Clean

Startup smoke on the 1979 Yamaha IT250 is often a normal part of two-stroke ownership, especially after periods of inactivity. By understanding the difference between harmless smoke and signs of trouble, you can keep your bike in top shape and enjoy more time on the trail. Regular maintenance, careful premix preparation, and prompt attention to changes in smoke behavior will ensure your IT250 remains a reliable companion for years to come.

Related Shopping Categories

Shop Oil for a 1979 Yamaha IT250 Dirt Bike.

Shop Spark Plugs for a 1979 Yamaha IT250 Dirt Bike.

Shop Carburetor Accessories for a 1979 Yamaha IT250 Dirt Bike.

Shop Gaskets for a 1979 Yamaha IT250 Dirt Bike.

Shop Air Filters for a 1979 Yamaha IT250 Dirt Bike.

Disclaimer: This guide is provided for general informational purposes only and is not intended to replace your motorcycle's official owner's manual. Always refer to your manufacturer's documentation for model-specific instructions, torque specifications, safety procedures, and maintenance requirements. If you are unsure or inexperienced, consider seeking assistance from a qualified mechanic or technician.