How to Repair a Four Stroke Lawn Mower

At times, you may find yourself with a lawnmower issue that is more difficult than replacing the oil, plug, or air cleaner. This manual explains procedures for repairing these issues in a Briggs and Stratton engine. This article assumes you have a basic knowledge of how an internal-combustion engine works and the parts it uses. Note: if you have an engine that is not a Briggs or a special Briggs, the procedures should work, but descriptions may not be entirely accurate in some places. Additionally, if you have a two-stroke engine (gas and oil mixed)this article won't apply as they are totally different machines.


  1. 1
    As with all lawn mower repairs, remove the spark plug lead, and ensure that it is a safe distance from the plug.
  2. 2
    Check the oil, plug, and air filter to rule out these simple repairs.
  3. 3
    Check compression. With the plug in the engine (disconnected lead) and the safety kill-switch bar pushed down, pull the starter cord. You should feel a quick, tight resistance. If you do, skip to step five.
    • If you felt little or no resistance and heard an asthmatic-sounding puff noise, you have bad compression.
    • If you felt and heard nothing, ensure that the starter cord system is working. If it is, you may have a thrown rod and should replace the mower.
  4. 4
    Troubleshoot bad compression, if you had it. Bad compression could be caused by either a bent/broken valve, or a blown head gasket. Remove the eight large bolts from the front of the engine (and anything in the way) and remove the head.
    • Check the head gasket for any missing chunks. If you found missing chunks, remove what is left and obtain a replacement from a small engine shop. Then use steel wool to remove any stuck on pieces of gasket, place the new one on, replace everything and try to start it.
    • If the gasket was intact, inspect the valves and ensure that one isn't bent or not fully closing. If it is bent, gently pry it straight with a slotted screwdriver and try to get it to start.
    • If a valve was straight but not closing, you need to remove the valve and, depending on severity, either use a bench grinder to shorten the back end, or replace the valve entirely. Both of these are difficult and complex procedures and you may prefer to replace the mower unless you have a lots of time, patience, and experience.
  5. 5
    Check for a bent crankshaft by tipping the mower with the exhaust up (gas will drip out, but this cannot be prevented). MAKE SURE THE PLUG LEAD IS OUT and turn the blade slowly. If you see the bolt in the center shimmy from side to side, discard the mower because it has a bent crankshaft and will not operate correctly. If it remained centered, proceed.
  6. 6
    NOTE: Everything beyond this point should only be done by those who know what they're doing and have dismantled and engine before.
  7. 7
    If compression was not the problem, check for spark by removing the plug from the head, replacing the lead and grounding the tip against the block. If you pull the starter with the safety kill switch down, you should see a small spark jump the gap on the end of the plug. If you did, continue to the next step. If you did not, the magneto (Y-shaped piece of metal with plug lead coming out) may be bad and should be cleaned or replaced.
  8. 8
    If you had spark, check the carburetor by removing the air cleaner and its assembly from the carburetor and check the following.
    • If your mower has a choke, ensure that it moves without obstruction and can close fully.
    • If your engine has a primer bulb, push it to ensure that gas can enter the carburetor.
    • If this didn't work, ensure that there is no dirt or carbon deposits inside the carburetor.
    • If it still won't start, remove the carburetor from the tank (5 screws) and inspect the two screens for blockage.
    • Inspect the rubber diaphragm and paper gasket for holes and replace it if necessary.
    • If there is a wide spring below the gasket, place it above the gasket, not in the metal pocket below.
  9. 9
    Inspect the throttle with the air cleaner still off, move the throttle knob to see if it fully opens and shuts and, if necessary, clean it out. If it still doesn't fully open and shut, replace the carburetor.
  10. 10
    Inspect the manifold check the intake manifold for holes and try to wiggle it. If it wiggles, tighten the two bolts on the left end. If there are holes or any melting, replace it.
  11. 11
    Check the flywheel key, the small, removable notch in the flywheel. If it is cracked, shattered, or missing, set the engine to TDC (top dead-center) exhaust stroke (both valves will be opened slightly). Align the flywheel magnets with the magneto and replace the key.
    • To remove the flywheel, pry up from the bottom while striking the top of the crankshaft with a hammer.
    • If the two notches in the flywheel and crankshaft were misaligned, then you have a timing problem that should be fixed by a professional.
  12. 12
    Recheck your work. If at this point the mower still won't start, the cylinder may be scored from heat or it could be something worse. Either way, it is pointlessly expensive to continue repairs. You should check one last time for any mistakes you may have made and attempt a start on fresh gas. If it still refuses to turn over, it may be time for a new mower.


  • If at any point you feel overwhelmed, don't make an irreparable mistake. Take it to a mechanic and explain the situation.
  • Unless you are an expert, don't open the crankcase or remove the piston.
  • Always properly dispose of engine oil at an auto parts store or garage.
  • In an engine, dirt is your enemy. Remove any debris you find, inside or outside of the motor.


  • Always disconnect or remove the plug when servicing a mower.
  • Don't let a mower lay on its side too long. You will likely lose your gas and/or oil, and foul up your plug, filter, carburetor, and clog your positive crankcase ventilation system.
  • Try to do repairs in a well-ventilated area due to combustible gas fumes.
  • Gas can cause a burning sensation if left too long. Always wash it off thoroughly with soap and water if you get any on your skin.
    • Clean up any oil puddles as soon as they form, so that you don't slip and hurt yourself.
  • Avoid touching the blades without gloves. Even dull ones can cause nasty cuts and slashes.
  • Never work on a hot engine as temperatures can reach 1,200 °F (649 °C) and can cause serious burns.
  • Never tip a mower with the plug lead on the plug.

Things You'll Need

  • Vise grips or duct tape (to hold down the kill-switch bar)
  • A small socket wrench set
  • A hammer (to tap on top of crankshaft while removing flywheel)
  • A pry bar (to pry up on bottom of flywheel while tapping crankshaft with hammer)
  • A medium Phillips-head screwdriver
  • A medium slotted screwdriver
  • A plug wrench or large plug socket
  • An illustrated parts list to help with reassembly and to identify parts' names. See if you can download one.
  • Owner's manual for oil specs and location of engine numbers.

Article Info

Categories: Lawn Care