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Do not have any power on the left side of the coil

When using spark plug tester, there is no fire on the left cylinder coil on a Briggs&Stratton 21 HP, OHV, twin cylinder engine. The model is 407777 and type is 0124E1. What could the problem be?

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Check that the air gap between the ignition armature not getting spark and the flywheel are between .008 and .012. Also note that the engine must be producing, by Briggs standards, 350 rpm to produce spark. It's rare for an ignition armature (coil) to go out, but it does happen. If a new plug on that side and correct air gap don't solve the issue, hit me back up on it.

Posted on Mar 13, 2009

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Only firing one cylinder, replaced both plugs and one coil? What else could be causing this?


Justin, Did you replace the right coil, are the leads good,
Are both the triggering magnets still in the fly wheel,are they clean? Touch any piece of metal to both of them and make sure they are equal strength or close to it. If one magnet weak replace.

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1 Answer

Backfires after running and stops, must shut off to stop the backfireing. I have replaced the fuel filter, and checked the gas cap vent,what else could cause the backfire?


For a more specific answer, you should post the applicable elements of Make/Model/Year/Engine Model/Transmission Model/Deck Size, and hours on meter if you have one. A short description of the events preceding the beginning of the problem is also helpful.

Several possibilities for small engine backfire:
  1. Needs new plugs. Start with new plugs.
  2. Shear pin under flywheel is slightly damaged throwing off the timing. Remove flywheel examine shear pin for any damage. Just go ahead and replace it; if it even looks nicked or sliced.
  3. Coil, or spark plug wire, is going bad. Inside the Coil are two windings of wire and no moving parts. With shorted (melted together inside) windings you can still get a spark, but it will be weak. With open (burnt/broken) windings, you get no spark at all. There are also electronic components like diodes which may be failing. Coils are subjected to heat and vibration and do fail on occasion. Another coil consideration is the spark plug wire. Back in the day, we would replace all our spark plug wires when we gave our car a tune up; because spark plug wires do fail. The only way to replace the spark plug wire on a mower is to replace the Ignition Coil.

    Note: Coils may not fail completely at first. They can be complicit in the situation where a mower starts and runs for a while.They may cause backfire. As it heats up the coil parts expand and cause the engine to shut down. It does not re-start until the machine has cooled enough to allow the coil to cool. This can be 30 minute to several hours; perhaps it starts again the next day only to repeat the failure.

    Short of a well running engine, with no special equipment, the only field-test for a coil and the spark plug wire output is to visually assess the color of the spark at the end of the spark plug. Alternatively, you may use a spark tester. A spark tester gauges the strength of spark by making it jump a bigger gap. This test calls for a gap in the range of 4.2 mm (0.166 in). It's important to note that setting a test gap beyond 5.0 mm (0.200) could damage the ignition components. Briggs and Stratton manuals, for the newer engines, specifically state that you should not test for spark with the spark plug out. I do not know why this is. They specify you should use a spark tester.

    B&S notwithstanding (at your own risk), with a new plug attached to the plug wire hold the threaded part against a ground point or wrap some bare copper wire around the threads and fasten the wire to the engine frame or other good ground. Make your shop or garage dark so you can see the spark easily, then crank the engine. The flywheel must spin rapidly (at least 350 RPM). The strength of the spark is revealed in the color. A red or yellow spark is weak and probably will not spark in the cylinder. A blue or white spark is strong and has enough voltage to fight across the spark plug gap even under pressure within the cylinder.
    Note: a good strong spark also makes a strong snapping noise as it fires.
  4. Needs a valve adjustment or a valve job. This is high on the list of the most common reasons for backfire. Start with adjustment.
  5. The other thing that causes backfire is fuel mixture too lean. On a small engine this is not usually an adjustment problem. It may be the fuel system/carburetor is dirty or partially clogged. It may also be an air leak in the gaskets of the intake manifold. It might be low fuel pressure from a bad pump or clogged fuel lines.The electronic fuel shut off valve may be malfunctioning. If nothing else solves the backfire, check fuel flow through the pump and lines, remove the carburetor and clean or rebuild it. This is the least likely of the options so save it for last.
Here is a link to a great essay on magnetos and spark plugs.
Village Science Magneto Spark Plugs

For Twin Cylinder Engines
A power balance, or cylinder balance, test should be run on a twin cylinder engine. If compression is good and cylinder leakage is minimal (in other words if the engine is not worn out, doesn't have too many hours), but a cylinder is misfiring or shows up weak in a power balance test, it indicates a fuel delivery or ignition problem (fouled spark plug or bad plug wire).

If an engine is hard starting, runs rough, misses or lacks power, a cylinder balance test will determine if both cylinders are operating to their full potential. You will need an engine tachometer, a spark tester, and an insulated screwdriver.
  1. Using the spark tester, with the plugs in, start engine and verify spark is equally good on both cylinders.
  2. Remove spark tester.
  3. Connect engine tachometer
  4. Run engine at top no load speed. Note rpm.
  5. Ground one cylinder spark plug with screwdriver. Note rpm.
  6. Ground other cylinder with screwdriver. Note rpm.
The difference in RPM between the cylinders should be less than 75 rpm. If the difference between the two cylinders is greater than 75 rpm, the cylinder with the least rpm loss is the weakest and is a potential problem.

Mar 26, 2015 | Riding Mowers & Garden Tractors

1 Answer

Why briggs twin will only run with one spark plug wire grounded


I am not a mechanic however I am assuming it is a two cylinder unit. I am guessing but when the spark fires it goes to both spark plugs (cylinders), one has fuel the other is in the exhaust stage. This keeps the ignition system very simple and reliable. However if the spark gaps get to big it will not fire. It is also possible the coil or associated driver is failing. This would hold true if both spark plugs were in the same cylinder.

Aug 08, 2014 | Briggs & Stratton Riding Mowers & Garden...

1 Answer

Briggs and Stratton hunting and surging.


How about a leak down test?? sounds like blow bye. is it consuming large amounts of engine oil???

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I have a Briggs & Stratton 18.5 HP Twin Cylinder engine that has no spark. How can I check the magneto? And, there is a wire that goes to the magneto. Should there be 12v to that wire? If I replace the...


It all depends on the model of engine. If it is a twin engine doesn't it have two magnetos.Very unusual for both to go bad It will depend on the type of ignition(points or solid state) The wire going to the magneto could be the kill switch (kills when grounded) Disconnect it and see if you get spark. IF so then it is in your safety/ignition circuit. If you replace magneto usually the gap is .008-.012

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1 Answer

Briggs and stratton 24 HP model ELS 725 mower stopped running have power, turns over, does not fire. please explain your answer with instructions for ELECTRICAL (IGNITION) SHUTDOWN,CHECK WHEN IT SHUTS DOWN...


A great expedient spark tester can be made from an old spark plug that you know is GOOD.....clip or grind the grounding terminal ( the terminal which goes over the top of the electrode) back until you have a gap of 1/8 to 3/16 of an inch between the terminal and the electrode...Remove both plugs from the machine and attach your new test plug to the plug wire. Ground the plug by placing it on the head or the shroud. Turn the machine over. If you are getting fire you should see a strong blue spark shoot from the electrode to the terminal. If you have no fire then you probably have a bad ignition coil, bad plug wire, or maybe just a bad seat, brake, or PTO safety switch...I'd check these first. Also check your ignition switch in the run position with a test light to be sure it is good....hope this helps... ;-D

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1 Answer

Briggs vtwin 21 hp - Only running on one cyclinder. I think- started up let run for 10 minutes one valve cover warm the other nothing still cool. mower has half the power as it did a year ago- ? help


Pull the spark plugs
Attach back to the plug wires and ground the metal plug body to the engine casting one at a time

Crank engine look at he spark
Does it fire equally on both plugs
Does the spark go only to the electrode or does it shoot sideways to the edge of the plug?
If one plug does not fire at all then you have a bad coil on that cylinder

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1 Answer

Briggs & Stratton 6.75 lawnmower. Won't start.


1) Determined that there is spark?
2) Have you used a type of spark tester?
3) What is the condition of the plug. Brown or dark brown is OK. Black or silver is NOT Ok.
*If you do not have a spark tester,
a) remove the plug, and with it connected to the wire,
b) find a portion of metal around the cylinder that you can ground it on, and then pull the rope.
c) If the plug is black, you may see spark, but you should replace the plug.

*** PLEASE NOTE ; if there is excess fuel in the cylinder or on spark plug,
it can catch fire, so be carefull to cover the hole with a heavy rag so that any fuel
cannot be exposed to spark plug durning test !!!!
Get back with me on how this issue works out.
thanks, rwvanlue

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1) remove air filter assembly
*be cautious of tearing the mounting gasket. If it does tear, you HAVE to replace it. It is part of the priming system
2) remove the float bowl fastener. This is the main jet
3) carefully remove the float bowl, the rubber o-ring/gasket will not be able to be used if it is removed, you WILL have to replace it
4) using a spray 'carb cleaner'
spray up thru the center of the carburetor
spray and clean the main jet with cleaner and 'tag' wire
(tag wire is very thin, and can be used to clean the hole in the center of the main jet)
5) clean any debris from inside the bowl or carb body
6) re-install bowl and center bolt, air filter assembly

*if you need gaskets, your local lawn & garden dealer can supply. The kit is cheaper than individual gaskets, and all the parts needed to finish the above repair are in the kit.

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1 Answer

Shuts off after running a while


Given the fact that you offer absolutely NO information about what "IT" is, I am going to guess at a solution for you.
I am assuming that you have a one cylinder Briggs and Stratton engine mounted in a riding lawn mower that sounds like you are dealing with a defective ignition armature, commonly know as the coil.
You can verify this by obtaining an in-line spark tester, the kind with a neon bulb that will show a visible spark while the engine is running,
Insert your tester in the spark plug wire and connect it to the spark plug, start the engine and run it until it "shuts off". Immediately try to restart it while watching the spark tester for a visible sign of spark. If you see no light flash or a very weak looking light in the tester, replace the ignition armature/coil.

Oct 29, 2009 | Briggs & Stratton Riding Mowers & Garden...

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