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Brent_Baumer

Spark tester says yes, spark plug says no

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Brent_Baumer
Hi all. Got a new one on me. Need help. I gave a 16hp briggs to my high school nephew to rebuild for a shop class project. The engine ran before he took it but it smoked pretty bad. Anyway after the rebuild he says he rope started it one time and it ran good at idle. He tried reving it up and fiddled with the choke (he says) and it died. He could not get it started again. He said he was not getting spark and thought he was not spinning it fast enough with the rope. He brought it over last night. We bolted back into the 7016H and hooked everything back up. Tried to start. Nothing. I put one of those in-line style spark testers with the lamp on it. Cranked. The lamp was indicating we have fire. Pulled the plug and tested it the old fashioned way (touching the threads on a head bolt and watched the electrode). Cranked. No spark. Got an old spark plug out of my tool box. Did the old fashioned test on it. Got one nice spark at the electrode as soon as I cranked then no more. Points and condenser are new. Point gap is .020. Magneto and plug are re-used. Wiring is just like it was before engine removal. Gonna get a new plug today on the way home from work. Any suggestions? Thanks, Brent

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Charlieson
I dont suppose the key on the flywheel got turned? I had a similiar problem with my 3212 several years ago and it was the shaft key, that throws the timing out of whack. ddh

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slb04786
Something else to think about. I had the same problem with my Briggs in my 7116 after a rebuild. The tractor had sat under a tarp for better part of a year before I bought it and rebuilt the engine. Turns out it was the wire that attaches to the thumb screw on the carburator side of the engine. This is the wire that kills the spark when the ket is off. It has a partial short and the engine won't start or will start and run rough, like the timing is off. I went as far as repulling the engine and checking the flywheel key and timing gears. Everything checked ok, but the engine still ran rough or wouldn't start at all. Removed that wire and it runs like a champ. I still haven't had time to get into the wiring harness to troubleshoot it. I just leave the wire off and hook it up to kill the engine. That doesn't explain why it won't run out of the tractor but I would try unhooking the wire and put in a new spark plug and see if that is the problem before I pulled the flywheel.

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UCD
Spark tester says spark, spark plug no spark = Bad Spark Plug spark plug fires out side of engine but not in engine = Bad Spark Plug Trust in your test equip. that is what it is for.

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patrician12
With the plug out the motor will spin faster.If the motor is too tight it will not make 350 rpm.You need 350 rpm to make magneto spark with the plug in.The light has less resistance than the plug that has to jump the gap.

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Brent_Baumer
Well, new plug. No spark. Put a Kohler coil on it to run off the battery. Starts right up and idles fine. Runs good to about 1/2 throttle then starts to miss. Have not adjusted the carb or the points yet. Garage filled with exhaust so I am gonna take a break. Really windy, rainy, crappy day today outside. Will resume adjustments tomorrow when the sun is out and I can work on it outside. Brent

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Al
HI, DThe inline spark testers with the light have a NE2 neon bulb in them. These bulbs fire at 65 volts. This is nowhere the several thousand volts required to fire across a plug. If you have the high voltage to jump the plug when the neon "fires" it becomes a conductor and if the voltage is high enough, will then jump the plug, so the light can be misleading. Since you only got one fire and have tried another plug, the first thing I would do is run a piece of paper between the points. Next I would check the points to see that they are not mechanically binding. I have seen points that the pivots were binding and they did not close solid enough to make contact, yet looked closed. Next I would try just connecting the old condensor across the points. No fix, I would pull the flywheel and check the key. Al Eden

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patrician12
Even if the key were sheared wouldn't he still have strong spark just not synchronized?If he changes the points and condenser and still doesn't have spark I still believe the motor too tight to make the proper rpm.It must do at least 350 on start up.I would remove the plug ,attach it to the coil wire and ground the body against the block.Pour a liberal amount of oil in the cylinder for lubrication and spin the engine.Now check for spark.

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Charlieson
Brent, You made the right choice. Battery ignition is definately the way to go. You'll probably have to adjust the points a little to get the tractor to run smooth. But the battery ignition seems to start much faster. ddh

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Al
Hi, If you have electronoic ignition, the flywheel key can be sheared and you will have spark, but not when you are supposed to. When you have points, the mangets have to cross the pole pieces in the armature and the same time the points break. When the key shears even partially this doesn't happen and No Spark. Al Eden

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D-17_Dave
The small engines with the points off the crank fire every revolution. With points they won't fire witha sheared key. With Magnatron ign. they will continue to fire, just not with correct timeing. Points off the cam should only fire once during the 4 stroke cycle, and not at all with a sheared key. The later model big cast-iron model engines that used magnatron ign. loose the points trigger off the cam and will fire every revolution. These need the 350 or so rpm's to fire. Engines with a magneto and points will fire off about as slowly as you can pass the magnets by the magneto. Simply smoothly pssing the magnets past the mag. and the points grounding the secondary will produce resonablely good voltage to fire a plug. Faster magnet speed doesn't mean more voltage, it just takes a sertain speed from the magnets pasing the trigger device such as the magnetron to fire the newer systems. IMO.

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Brent_Baumer
I converted it to battery ignition. Starts kinda hard but runs good. Engine SEEMS to vibrate more than before but this may just be my imagination. I've run it about 30 minutes, just driving around. Have not put it under any serious load yet. Thanks for the tips. Brent

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BigSix
Al: I have a similar problem in a 725 that I acquired recently. It had no spark, so I put new points and condensor in last fall, and it ran fine. Didn't use it all winter and now, no spark. What is the significance of this test you mention, and I wonder if you could be a little bit more specific about how one hooks the condensor up? Do you leave the other condensor connected, or just swap out the new for the old? If it's obvious, I apologize for being dense. B)
quote:
Next I would try just connecting the old condensor across the points.
Just thought I might try that test, but can't quite understand it. Thanks, Peter

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