How to Repair a Four Stroke Lawn Mower
As with all lawn mower repairs, remove the spark plug lead, and ensure that it is a safe distance from the plug., Check the oil, plug, and air filter to rule out these simple repairs. , Check compression., Troubleshoot bad compression, if you had...
Step-by-Step Guide
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Step 1: As with all lawn mower repairs
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. , 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. , 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. ,, 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. , 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. , If it still doesn't fully open and shut, replace the carburetor. , If it wiggles, tighten the two bolts on the left end.
If there are holes or any melting, replace it. , 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. , 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. -
Step 2: remove the spark plug lead
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Step 3: and ensure that it is a safe distance from the plug.
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Step 4: Check the oil
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Step 5: and air filter to rule out these simple repairs.
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Step 6: Check compression.
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Step 7: Troubleshoot bad compression
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Step 8: if you had it.
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Step 9: Check for a bent crankshaft by tipping the mower with the exhaust up (gas will drip out
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Step 10: but this cannot be prevented).
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Step 11: NOTE: Everything beyond this point should only be done by those who know what they're doing and have dismantled and engine before.
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Step 12: If compression was not the problem
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Step 13: check for spark by removing the plug from the head
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Step 14: replacing the lead and grounding the tip against the block.
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Step 15: If you had spark
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Step 16: check the carburetor by removing the air cleaner and its assembly from the carburetor and check the following.
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Step 17: Inspect the throttle with the air cleaner still off
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Step 18: move the throttle knob to see if it fully opens and shuts and
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Step 19: if necessary
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Step 20: clean it out.
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Step 21: Inspect the manifold check the intake manifold for holes and try to wiggle it.
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Step 22: Check the flywheel key
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Step 23: the small
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Step 24: removable notch in the flywheel.
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Step 25: Recheck your work.
Detailed Guide
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. , 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. , 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. ,, 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. , 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. , If it still doesn't fully open and shut, replace the carburetor. , If it wiggles, tighten the two bolts on the left end.
If there are holes or any melting, replace it. , 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. , 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.
About the Author
Richard Robinson
Enthusiastic about teaching practical skills techniques through clear, step-by-step guides.
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