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Small Engine Repair: Free Help

Your machine will not start or runs poorly? Try these tips before you take it in for service. READ YOUR OWNER'S MANUAL. Remember that our service manuals quickly pay for themselves. The step by step troubleshooting procedures eliminate guesswork and show you how to fix it right the first time. Small engine repair and lawn mower repair make great do it yourself projects.

Gasoline can ignite very easily. Work only in well-ventilated areas and away from sources of heat, sparks, and flames. Always wear safety glasses, and be careful of sharp blades. Keep a fire extinguisher nearby.
Trouble Diagnosis Procedure

If you follow these easy steps in order, you will be able to locate the problem. This is the same procedure we follow when diagnosing engines.

A word of caution here: We have developed this method over many years and recommend that you do not assume the trouble is a certain part, as a problem in one area will often show up as a symptom somewhere else. An example is carb malfunctions are often incorrectly assumed to be governor problems. Let's roll up our sleeves and get to work!

Decide if it is worth repairing! Old worn engines are not easily fixed as there is not just one problem, but a combination of many problems! More serious problems such as a scratched or worn bore, worn piston rings and worn valve guides are best repaired by installing a shortblock or complete replacement engine.

Engine will not start:

Most "it won't start" problems can be divided into two areas: fuel related or ignition related.
  • Be sure that the machine has an ample supply of fresh fuel. For 2-cycle machines that require it, mix the proper amount of 2-cycle oil into the gasoline and shake to mix completely. Never use old or leftover fuel. Store fuel in an airtight container in a cool, dry place away from sparks and flame, and keep all vents on the can closed . Bad gas is more common than you may think, especially when starting a machine for the first time in a season. Do not store gas over the winter, pour it in your car and start fresh next year!

  • If the engine is a 4-cycle, be sure the engine oil is the right type, is clean, and is at the proper level.

Next step is to check for spark:
  • Make sure that the ignition switch is "on", that all attachments are disengaged and that the transmission is in neutral. Most machines have safety mechanisms that will not allow them to start otherwise. Consult your owner's manual for the starting procedure for your machine. 

  • Next- check for spark by removing the spark plug and grounding the "hex" part of the spark plug to a bare metal part of the engine. Remove any spilled gasoline that is nearby, then spin the engine by pulling the rope or turning the key. You should see a blue spark jump across the plug gap. If you have a good hot spark, skip to the carb section.

  • If the spark is yellow and weak, or there is no spark then the first thing to do is try a new spark plug. Do not try to clean the old one. Set the new plug gap to the proper spec (usually 0.030") and check for spark again. If you do see a spark, install the new plug and try to start the machine. If you still have no spark next try cleaning and setting the points on older machines, or replacing the electronic ignition module on newer machines. Before replacing the old module test it by disconnecting all the  wires from the module terminal that connect to the kill switches and recheck for spark, if you see a spark a wire or switch is defective.  Also check the flywheel key. That affects the spark timing, usually the key will need replacing if you strike something while mowing and the engine stops. We can provide the parts for your ignition repair

 Next step is to check the carb & fuel system:

  • If the machine won't start and the spark is good, you must make sure that fresh gasoline is getting to the carb. Make sure that the fuel shutoff valve (if present)  is opened, and that the fuel line is not plugged or kinked. Also be sure the fuel cap vent is open and any screens in the tank are clear of debris. If fuel can flow to the carb, carefully place 1 teaspoon of gasoline down the spark plug hole (or a add shot of starting fluid into the carb throat). Re-tighten the plug and try to start the machine. If it runs for a second or two, then quits, chances are the carb needs to be serviced (disassembled and cleaned & inspected, then rebuilt) or you have an air leak somewhere. Make sure all mounting screws or bolts are snug and gaskets are not missing sections. If the carb is a bowl-type check for water in the bottom of the bowl (but do not let the float swing all the way down or the float needle will fall out!). It is easy to install a carb kit yourself.

Engine will start but runs poorly or dies:


Carburetor problems produce different symptoms. There are many different types of carbs, but they all operate the same. The main types are the bowl and diaphragm. For info specific to your carb, we recommend that you purchase the proper repair manual for your engine. The manual contains exploded views of the carb, to help you get it back together properly the first time. They also contain diagrams of where the linkage goes, cleaning and adjustment procedures, and help you to spot worn parts. Most often a manual and a carb rebuild kit are cheaper than 1 trip to the shop. General problems and solutions are presented here, but due to the many variations of carbs out there, if you remove the carb, or disassemble it, be sure to write down how it is set-up now, to save fumbling later! No matter what type you have, a rebuild kit is inexpensive and will cure 90% of all carb problems. The first thing to do if the engine will run but acts funny is to try to simply adjust the carb slightly. Note the position of the screws now, in case you have to return them to the original positions. It is recommended only experienced do-it-youselfers attempt carb repair with out a manual.

  • Bowl-type carbs: Runs then dies: Check for stuck float, plugged fuel filter or fuel line. All tanks have a screen inside them to catch larger particles, and some also have a inline fuel filter. If everything checks ok, the passages inside the carb are plugged, follow the cleaning procedure outline in the manual to be sure you only have to do the job once!  Surging, only runs on choke, sputtering: Is the air filter dirty? With the bowl as the lowest part of the fuel system, any dirt or water will wind up here. Turn off the fuel valve, or drain the gas tank, then place a rag under the bowl to catch the gas that will run out. Loosen the "nut" that holds the bowl on, and after the gas has stopped draining remove the nut and bowl, but do not let the float swing all the way down! Wipe out the bowl. The "nut" usually contains 1 or 2 very small passages, soak in (or spray with) carb cleaner and use a wire bread-tie or other soft wire to poke out the holes. Blow out with compressed air, if available. Put the bowl back on and try to start it. If it still will not start, it needs a good soaking and a rebuild kit. Drips, or crankcase fills with gas: Check the float first. Remove the carb, and then the bowl. Remove and shake the float, it should not be full of gas, if it is, replace it. If it  is ok then the float needle/seat assembly should be replaced.

  • Diaphragm type carbs: They are less prone to trouble as a rule. Hard starting, or will not start: (spark is ok) The diaphragm may act as an auto choke or a fuel pump. It is made of rubber and will harden over time and no longer flex. It is the first thing to replace. Next check the fuel pick-up tubes for blockage, the check-balls in the pick-up tubes for free movement, and the tank for water. If it still is acting up, install a rebuild kit.
  • Oil smoke or leak near the carb: The drain holes in the breather cavity (or breather cover on some engines) may be plugged. Access the cover and remove it. Wipe the cavity out and poke a small wire down the hole in the bottom, and check the cover for a small hole, and clean it out. The hole must be down when the cover is reinstalled. A new gasket is also required.
  • Oil  leak near the base of the engine: Most likely the fill tube o-ring is loose, loosen the dipstick and push down firmly then retighten. If that fails the sump gasket is loose or missing a small section.
  • Oil smoke thru the muffler: All the time -worn rings. Stops shortly after starting -worn valve guides.
  • Backfires thru carb or air filter: Intake valve not seating properly, check lash and/or reface valve & seat.
  • Kicks back when starting: Flywheel key is nicked, or, on points-type engines, the condenser is defective or point gap is too small.
  • Starter rope will not retract or is hard to pull: Usually the rewind spring or spring anchor points are broken. The starter must be disassembled and the parts inspected. Remember the spring is under tension!  There are many different types of starters, and a manual is a good idea if you are not familiar with them. If the starter is hard to pull, remove the starter and then try it, if the starter is ok off the engine, check the engine itself for for seizing or oil-locked cylinder. Remove the plug and pull rope -if it will now spin and oil blows out the plug hole then carb is leaking into sump or engine has been tipped for a long time.
  • Starter rope broke: On some types you must disassemble the starter to replace the rope, on others you simply wind the pulley up until it stops, back off 1 revolution and poke a new rope thru the hole in the housing and then into the pulley, tie a knot and slowly let the rope pull in.
  • Compression low. The most common cause of compression loss is the valves. Worn guides, warped, pitted, loose or burned seats, and improper lash are common valve problems. Backfiring through the carb is one symptom of valve trouble. If the valves are operating properly and the compression is still low, worn rings or bore are best fixed by installing a rebuild kit, shortblock or complete replacement engine.
  • Won't start when hot: If the engine spins too easily check for valves that are sticking open. If compression seems ok, then check for spark as detailed at the top of this page. Solid-state ignition modules commonly cause this problem when they start to fail.


  • If the battery will not hold a charge: The problem is either a  bad battery, or a problem in the charging circuit. First, charge the battery. If it is at all low, it will not start the engine. Clean all connections, corrosion is 80% of all electrical problems. Check that the fuse is not blown. In the wiring harness somewhere there is usually a fuse, look under the dash (If it is a push mower, there may be no fuse), or  follow the wires the come out from under the flywheel. There will be a fuse holder & fuse. The battery should read 11-13 volts (push mowers may be 6 volts), check it with a voltmeter and record the reading. Then check the battery voltage with the engine running (if possible) it should be higher, about 13.5 volts (7-8 volts for a push mower) if it is, the charging circuit is functioning. The typical battery lasts 4 years.  Most auto parts stores will load-test it for free, but charge it first for an accurate reading.
  • If the engine cranks too slowly, but all the above tests are passed, the problem may not be the starter, but too much drag on the engine. Try to turn it by hand... it shouldn't be too difficult. If it is very hard to spin, check that the deck is not engaged and that the transmission (if any) is not tight. With the tractor or mower OFF, put it in neutral and try to push the machine forward. It should move with moderate effort. Next, disconnect the spark plug wire and try to turn the blades by hand. They should not be tight. If they are tight, the problem is a bad belt or bearing, or grass buildup. If everything checks out, the starter motor itself it the problem. Look for worn brushes, seized bearings, or a burned commutator.
  • If the starter spins, but doesn't engage the engine: The gear that engages the flywheel is stuck or broken. Try to pry it upward towards the flywheel gently, then oil the starter shaft. If that doesn't help, replace the gear. Also check the flywheel ring rear for broken or worn teeth.


  • Test the solenoid: First determine the type of solenoid you have: There are 2 basic types of solenoids. One has 3 terminals and the other has 4. Two large diameter studs, and 1 or 2 smaller diameter studs. The 2 large studs connect to the battery and to the starter. The smaller ones are to the starter switch and the other is ground (if the solenoid only has 1 small stud then the ground is thru the mounting ears). To test: make sure the battery is fully charged. Next, short out the 2 large studs with an insulated screwdriver. If it cranks, the solenoid or starter switch is bad. If it won't crank, the battery or connections are the problem, clean all connections and try again.
  • Test the starter switch: Connect a piece of wire to the battery positive terminal and touch the other end to the small stud that is marked "+" or "s". If it cranks, the starter switch is bad, and must be replaced with the exact same type for proper operation..
NOTE: These are typical circuit layouts for reference only. Yours may differ.
Still stumped? Our Manuals, Videos and CD-ROMs pay for themselves on the first job!

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