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Discussion Starter #1
Hi everyone.

I have here a B&S 18.5 HP opposed twin (model 42A707) that has been sitting since 3-4 years maybe less maybe more. I'm trying to bring it back to life, the starter motor was seized up so I changed it. The engine is cranking, and have very good compression. The fuel is reaching the spark plugs in the cylinders. It doesn't make fire. So I have here a brand new ignition coil so I installed it on the engine. No spark on both cables (tested with a spark tester). I thought maybe the new coil was bad (things like that may happen) so I took another one from another engine. It's the same. No fire.

I reinstalled it on the engine it took it and it fired right up. My question here what am I missing? All the wiring is good, the grounds are good to (nothing in excessive rust), I've checked the safety switches ( brake and seat), I have a tractor here of the same model so I have compared the wiring and everyting is there.

Can the ignition switch cause a problem? It's the only thing I don't have a spare it has more connectors than the ones I have on all my other tractors...

Thanks in advance for your help
 

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Kill switch or wiring? (Actually the ignition switch IS the kill switch.) But could be the wiring to the switch. :tango_face_wink:
 

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Funny timing, I just replaced the coil on my 42A707 this past weekend :) I'd never done it before, but it was pretty straightforward, I'm trying to think of what could accidentally go wrong.

Coil is right-side up, and air gap is 0.010-0.014"? Flywheel magnets are still magnetic? Wires are going to the correct cylinders? You'd still get spark that way, admittedly, but it wouldn't actually run. If there's no spark with the kill wire disconnected, I can't see how the switch would have anything to do with it. Coil has a good ground connection where it mounts, and the magnets aren't too rusty?

As was mentioned, I'd try the replacement coil on the other engine (if you haven't already), to see what that shows. See if the problem stays with the coil, or with the engine.

Perhaps something like the plug wires are grounding out somehow? The wires aren't pinched going through the sheet metal shrouds? I'd check continuity from the plug clips to ground, in case of a shorted wire, bad insulation, or something. Bearing in mind that a meter might not show a connection, but a high-voltage pulse might still be able to jump it. Your spark plug won't show continuity by itself, but it still sparks :)
 

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If the wire for cylinder A was going to cylinder B, I was thinking it wouldn't run.

But in thinking about it some more, I think I realized my mistake. Even if the cylinders do have their power cycles on opposite revolutions (my guess), it would still be ok, since each plug presumably fires on every revolution anyhow. And since there's only 1 coil, and 1 magnet (I believe), it has to be firing both plugs at the same time.

It's not like there's 2 coils, 30 degrees out-of-phase on the flywheel, or something like that. In *that* scenario, I believe there could be a chance to have cylinders firing at the wrong times, if your swapped wires, such that it wouldn't run.

Did I figure out my mistake correctly?
 

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Try this. Remove both spark plugs so as the engine spins really fast. Take spark plug (any old plug) and set the gap to about 1/8 inch and ground the plug good to the engine block and look for a spark in a dark place, not in sunlight, etc. Look very closly for a sharp white spark.

I've seen what you describe and actually have a white spark with no visible orange especially on the Briggs twins.

Here is a spark tester that you can actually see the spark if in a shade or dark place by hooking it up and either ground the end of the plug wire or just hook it in series with the spark plug and the engine will run with it in series. This one has the wide spark gap.
Spark Plug Spark Tester Oregon 42-087 Briggs & Stratton 19368

I've seen some of them Briggs twin spark so white to both plugs that it's not visible in sunlight or lighted room and a spark tester that uses a light would not indicate such.

Then if you see you do have a white spark the engine will be severely flooded, install new spark plugs and full throttle with no choke or let the engine dry for few hours with the plugs out and go for start. If you get a start with the old plugs but it won't re-start after it get hot, install 2 new plugs. Do not operate the engine over about 20 sec's with the air cowling off.

Leave the kill wire off when testing and if the engine starts with the kill wire off, choke to kill. Those engines will run even with the spark plug wires swapped but the longer wire usually goes to the left bank. Some of the later mags have a this side up etched in the black epoxy cover and some mags cannot even be mounted upside down.

and just another. A sheared flywheel key on those type Briggs twins engines WILL NOT kill a spark going to the plugs.
 

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Ok . step back. Install the coil on the engine that's subject of the post. Leave kill wire off to element wiring issue. Install 2 new plugs or the ones from the other engine that just started on the new coil. Remove air cleaner. Spray some carb cleaner into the intake. Crank and see if it fires. If not and if following suggestions above and you do see spark ( I suggest an inline tester) then remove whatever necessary to see if you have a sheared flywheel key. ( not common on the rider engines) You need compression,fuel and a spark (at the right time). How did you determine that compression was good and fuel was getting to cylinder?
 

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Discussion Starter #12
Ok . step back. Install the coil on the engine that's subject of the post. Leave kill wire off to element wiring issue. Install 2 new plugs or the ones from the other engine that just started on the new coil. Remove air cleaner. Spray some carb cleaner into the intake. Crank and see if it fires. If not and if following suggestions above and you do see spark ( I suggest an inline tester) then remove whatever necessary to see if you have a sheared flywheel key. ( not common on the rider engines) You need compression,fuel and a spark (at the right time). How did you determine that compression was good and fuel was getting to cylinder?
Thanks for the help :tango_face_smile:

I came back from doing that you said: Brand new working spark plugs from the engine that works. Compression is good around 100 psi on both cylinder. (The engine that works the compression is not good 100 and 75 psi and it fires right up with both used and new ignition coil and both sets of spark plugs). I have a spark tester (the one you connect on the plug and on the other end the cable from the coil. There's no light. (I thought maybe it was not working, so I tried it on the "dying" engine, and there was light). The fuel is reaching the plug, and yesterday after multiples cranking attempts it was leaking from the muffler. A rodent made a nest on both sides of the engine ( cleaning not finished yet), I checked the wire and none are broken. It's like there is no current passing in the coil.

Here some pics that maybe will help (or not)
 

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Here is a spark tester that you can actually see the spark if in a shade or dark place by hooking it up and either ground the end of the plug wire or just hook it in series with the spark plug and the engine will run with it in series. This one has the wide spark gap.
Spark Plug Spark Tester Oregon 42-087 Briggs & Stratton 19368

I've seen some of them Briggs twin spark so white to both plugs that it's not visible in sunlight or lighted room and a spark tester that uses a light would not indicate such.
That's a cool spark tester, thanks for the reference! It's reasonably priced, too.

The only downside I see (if I'm correctly interpreting the picture on Amazon) is that you can't leave it on the engine while actually using that spark plug? In other words, you couldn't run a single-cylinder engine while using this tester?

I have a light-style inline tester, which has been a very useful tool. I used it to help diagnose my ignition issue with my 42A707, I left it connected inline while mowing, etc, and used the flashes to determine when I was actually getting a spark, vs not getting spark.

FWIW, my light-style inline tester did show flashes with my Briggs opposed twin when I was getting spark. It worked for me, at least, but it's good to know that maybe that's not a perfect test for every ignition system/engine.
 

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That is kinda baffling :) I'm grasping at straws a bit, but just to ask.

If you remove the spark plugs to make it easier to spin the flywheel (and listen), the coil is never actually touching the flywheel, is it? No high spots on the flywheel? No shiny spots on the coil surface? In your last pic, the coil looks pretty close to the flywheel. I'm not an EE, but perhaps if it touches the flywheel/magnet, it makes the coil not function properly.

Whatever you're using to set the gap (I used a business card), can you keep that between the coil and the flywheel, as you rotate the coil through a full revolution, to make sure the coil is never getting too-close?

If I'm re-reading the thread properly, you've never gotten spark on the troublesome engine, or at least never gotten the engine to run, right? Just want to make sure I've got straight which combinations do what.

The flywheel, and the flywheel magnets, are common on all of this, with the "bad" engine, and the various coils. Can you hold a piece of steel against this flywheel magnet, and see how strong it feels? Then hold the same steel against the flywheel of the good engine? Maybe the magnet is weak somehow.

A crude test that I'll do, when trying to see if the spark is just weak, is to decrease the spark plug's gap significantly. A smaller gap is easier to jump. So if the coil is producing *something* (vs being grounded out, for instance), just not enough to jump the normal plug gap, you might then show a spark. Which would then help better-understand what's going on.
 

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Not many options left at this point.

If you have a known good coil and a properly set air gap, either the coil is not getting a good ground or your flywheel magnets are fubar'd.
 

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Good point. Maybe add a separate ground wire for the coil, like from under the head of one of the mounting bolts, to a nice ground on the engine block.

I had an old Stihl chainsaw that lost spark, while sitting on the shelf. After a bunch of trying stuff, I added an extra ground wire to the ignition, and it fired right up. It was weird to me, it seemed like it should have still had a good ground with the way it was mounted, but that suddenly cleared up the problem. At least it was a cheap fix, in my case.
 

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Discussion Starter #18
Whatever you're using to set the gap (I used a business card), can you keep that between the coil and the flywheel, as you rotate the coil through a full revolution, to make sure the coil is never getting too-close?

If I'm re-reading the thread properly, you've never gotten spark on the troublesome engine, or at least never gotten the engine to run, right? Just want to make sure I've got straight which combinations do what.

The flywheel, and the flywheel magnets, are common on all of this, with the "bad" engine, and the various coils. Can you hold a piece of steel against this flywheel magnet, and see how strong it feels? Then hold the same steel against the flywheel of the good engine? Maybe the magnet is weak somehow.
Yes I use a buisness card too and all the engines here I installed a coil there was no problem. The coil doesn't touch the flywheel, there is non friction marks, I always check that before cranking it. For the magnet, there is some force when I hold a piece of steel against it, I will compare with the good engine tomorrow after work and I'll let you know if its the same.

It's the first time an engine is giving me trouble like that... I had one seized up with a mix of gas and water in the crankcase, with rust on every part, I "unseized" it with a ratchet and once the oil changed I cranked it and it started right up... I mow 2 hours a week with it...
 

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That's a cool spark tester, thanks for the reference! It's reasonably priced, too.

The only downside I see (if I'm correctly interpreting the picture on Amazon) is that you can't leave it on the engine while actually using that spark plug? In other words, you couldn't run a single-cylinder engine while using this tester?

I have a light-style inline tester, which has been a very useful tool. I used it to help diagnose my ignition issue with my 42A707, I left it connected inline while mowing, etc, and used the flashes to determine when I was actually getting a spark, vs not getting spark.

FWIW, my light-style inline tester did show flashes with my Briggs opposed twin when I was getting spark. It worked for me, at least, but it's good to know that maybe that's not a perfect test for every ignition system/engine.
Yes you can use it on a single cylinder engine while the engine is running. The picture is misleading.

I do not like to use the light type ignition testers mainly because I've seen them flash and indicate a good spark and the spark would be too weak for good engine starting or performance and if the flasher indicates no spark I would have to make a test on a good engine to see if it's indicating correctly.
The one I listed is a actual spark jump test where you can visually see the spark. I also use it on chainsaws while they are running to watch the spark.
 

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Not many options left at this point.

If you have a known good coil and a properly set air gap, either the coil is not getting a good ground or your flywheel magnets are fubar'd.
Right and always leave the kill wire disconnected from the coil when testing for now.

I see what appears to be a smudge rub mark on the magnet. Check the flywheel sideways slack and or set the air gap greater, like use two business cards and test.
 
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