top of page
Search

John Deere X300 Runs Rough or Pops Under Load? Here’s What Fixed This Kawasaki Engine

  • Writer: Individualized Repair
    Individualized Repair
  • Jul 31
  • 5 min read
ree

If your John Deere X300 or other mower with a Kawasaki V-twin engine runs poorly under load—especially when engaging the PTO, climbing hills, or mowing thick grass—you're not alone. A customer brought in their X300 with that exact complaint: loud popping, loss of power, and poor performance when under strain.


In this post, we walk through the step-by-step diagnostic process and uncover the real issue: a failing ignition coil that was still producing weak, misleading spark. Whether your issue is misfiring, bogging, or backfiring under load, this post will help you track it down and fix it.

Want to see the full repair process? Watch the full video on YouTube here.


Step 1: Rule Out Fuel Problems


Before jumping into ignition or mechanical issues, check for fuel quality and delivery:

  • Inspect the fuel filter and make sure it’s flowing freely

  • Test fuel for water contamination by draining into a clear container

  • Use ethanol-free gas if possible to reduce long-term issues


In this case, the mower had clean, good-smelling fuel and strong flow from the fuel pump—so we moved on.


Step 2: Rule Out Airflow and Choke Issues


If your mower runs rich or blows black smoke:

  • Check the air filter for clogs

  • Verify the choke linkage isn't stuck partially engaged

  • Clean or lubricate the choke shaft if needed


Even though this X300 had a clean air filter, the choke was sticking slightly—so we cleaned it out with compressed air and carb spray.


Step 3: Cylinder Isolation Test


To find out which side of the engine is causing the problem:

  • Unplug one spark plug wire at a time and run the engine

  • If one side runs fine solo and the other doesn’t run at all, you’ve likely found the bad cylinder

  • Try partial choke—if the engine runs better, it’s likely a fuel/air issue


On this X300, one cylinder failed to run independently and showed inconsistent spark with a spark tester.


Step 4: Confirm Spark Problems


We used a spark tester to verify:

  • One coil was delivering consistent, strong spark

  • The other coil was intermittent or completely dead

  • Even though it sparked visually, the plug had no usable spark under load


This pointed to a failing ignition coil, not a fuel or valve train issue.


Step 5: Replace the Bad Ignition Coil


After removing the blower housing, we:

  • Unplugged and removed the faulty coil

  • Installed a known good coil (with OEM spec gap of 0.010")

  • Used a business card for spacing during installation

  • Added anti-seize on mounting bolts to prevent future seizure


With the new coil installed, the engine immediately ran smoother and no longer popped under load.


Final Result


After replacing the ignition coil, the X300:

  • Ran perfectly on both cylinders

  • Engaged the PTO smoothly with no bogging

  • No longer produced popping or cracking noises

  • Had full power under load


This fix works for any riding mower, zero turn mower, stand on mower or walk behind mower with a Kawasaki V-twin, not just the X300—especially if you're experiencing misfiring, popping under load, or uneven cylinder performance.

Want to Watch the Full Repair?



We show you how to:

  • Isolate the faulty cylinder

  • Test for spark properly with and without a tester

  • Remove the flywheel housing and install a new coil

  • Confirm proper operation before reassembly


Pro Tip:


If you're troubleshooting a V-twin that runs poorly under load but shows “some spark,” don’t rule out a failing ignition coil. Spark testers are the best way to confirm whether it’s strong enough to fire reliably under compression. You can also use an in-line adjustable gap spark tester to see if it has enough power to brigs a larger gap than normal


Tags for Search Engine

Optimization


John Deere X300 runs rough, Kawasaki V-twin misfire fix, mower pops under load, riding mower loses power when mowing, spark tester no spark, ignition coil failure John Deere, X300 PTO pops and cracks, how to test spark on mower engine, lawn tractor bogs down, mower backfires under load


Frequently Asked Questions FAQ:


Q: Why does my John Deere X300 engine pop or crack under load?

A: Popping or cracking sounds under load typically indicate a misfire. This can be caused by a failing ignition coil, weak spark, or poor combustion due to a clogged air filter, bad fuel, or issues in the valve train.


Q: What causes a Kawasaki V-twin engine to run poorly when the PTO is engaged?

A: When a Kawasaki engine runs fine at idle but bogs under load, it’s often a sign of one cylinder not firing properly. A faulty ignition coil, bad spark plug, or intermittent spark delivery is a common culprit.


Q: How do I test spark on a John Deere X300 mower?

A: You can test for spark by using a spark tester or grounding the spark plug against the engine block while cranking. Reliable testers will show green for good spark and red or nothing for weak/no spark.


Q: Can one bad ignition coil cause the entire mower to run rough?

A: Yes. On a V-twin engine, if one coil fails, the engine will run on a single cylinder, causing loss of power, uneven running, and excessive popping or vibration under load.


Q: What’s the correct spark plug gap for the Kawasaki V-twin in a John Deere X300?

A: Most Kawasaki V-twin engines use a 0.030" spark plug gap. Always refer to your specific model’s manual to confirm the exact spec.


Q: Will a clogged air filter cause backfiring or popping?

A: Yes. A severely restricted air filter or stuck choke can cause an overly rich mixture, leading to backfiring, black smoke, and poor performance.


Q: How do I know if the ignition coil is bad even if I still see spark?

A: Weak or intermittent spark can still appear visually but may not provide sufficient energy under load. A spark tester will often reveal this more clearly. Replacing the coil is usually the best fix.


Q: Can a John Deere X300 run with one spark plug wire unplugged?

A: Yes, but only briefly for diagnostic purposes. If the engine continues to run with one wire unplugged, the other cylinder is likely working properly. If it stalls or struggles, the unplugged side is critical to performance.


Q: How do I replace the ignition coil on a Kawasaki engine?

A: You’ll need to remove the blower housing to access the coil, unplug the kill wire, unbolt the coil, and replace it with proper spacing (0.010"). Use a business card as a gap gauge if needed.


Q: Why does my X300 still run poorly after replacing the spark plug?

A: If spark plugs are new and properly gapped, the issue may lie in the ignition coil, wiring, fuel delivery, or even a valve train issue. A full diagnostic is needed to isolate the cause.


 
 
 
bottom of page