Greetings.
Small engines as for yard equipment are typically designed to not start when needed; no matter how many times the cord is pulled, times you scratch your head or the level of frustration by not knowing what is going on -or what is not going on-
To rule out the presence or not of ignition spark, some monitoring gadget would be nice to have other than sticking your fingers...
A neon bulb at the high voltage lead is very hard to see during daylight and less at the location the spark plug is relative to the cord puller.
An acoustical approach of a piezoelectric speaker can solve the poor visual of the neon lamp, clicking at every spark presence, if the noises while pulling the start cord do not mask it.
As Murphy's law is always lurking, how would you implement monitoring spark presence ? A passive approach would be very convenient. Adding complexity of battery powered detection circuits can introduce more trouble than needed.

 

hi Ext,
A old AM portable radio, tuned off any station, gives a satisfying click on an ignition system
E

 

Small engines as for yard equipment are typically designed to not start when needed; no matter how many times the cord is pulled, times you scratch your head or the level of frustration by not knowing what is going on -or what is not going on-

The biggest problem in that situation isn't ignition. It is leaving gasoline in the equipment over a period of time without treating it with Stabil, Seafoam or something similar. Just draining the tank isn't enough though if not treated, you need to start and let the engine run till it won't start again, to get all of the fuel out of the carburetor. The modern day gasoline has alcohol in it and starts to go bad in just a few months.

If you still want a ignition test or one of these is good, it has or they did have a slightly tinted plastic covering the bulb that allows it to be seen in all but the brightest sun light. I say "did have" because mine is an older one, don't know how their made today
https://www.amazon.com/Lisle-20610-Inline-Spark-Tester/dp/B0002STSC6

 

In my neighborhood people are always giving away lawn equipment that won't start because they failed to winterize their equipment properly. As @shortbus said, you drain the fuel and run the engine until there's nothing left to burn. Every fall I find snow blowers free as well again, due to failure to season (summarize) their equipment.

This year I replaced the spark plug in my lawn mower. It was getting hard to start. A new plug - an E3 type - made a world of difference. That with an annual oil change and a new air filter, my machine is running top notch once again.

When having difficulties starting an engine the first thing I do is squirt a tiny amount of fuel into the carburetor. If the machine starts then quits I know it's not ignition related. If it doesn't start I try a few more times before concluding that something has gone amiss with the ignition. With lawn equipment the ignition systems are so simple that it is usually a fouled spark plug. So your machine could have a good spark but a fouled plug. Those two tests, fuel in the carb and several tugs on the lanyard will usually either solve the problem OR cause you to check the plug. On the rare times a magneto (ignition system on small engines) goes bad. They're not too expensive and sometimes you can find one, as I often do, for free on a similar machine. To me - that's a lot less trouble than trying to build a spark detector. Then again, like @ericgibbs said, an AM radio tuned to a quiet spot will make a detector for you.

Years ago we used to listen to a CB radio for the tell tale hiss and pop of lightning. We'd count the seconds till the thunder and know how far away it was. At night you didn't need to do that, but in the daytime we knew when lightning was approaching. Or nearby.

 

Back in my early 20's I worked for "Miami-Lawnmower-Company",
which was the largest sales and repair shop in Florida.
I was the go-to "2-Stroke-Guy" and did around 90% of their 2-Stroke maintenance and repairs,
including being "fully-approved" by the "Green-Machine" and Stihl-Chainsaw Reps.,
and my work included virtually every Lawn-maintenance Company in Miami.

Small-Engine Ignition-Systems are amazingly "Bullet-Proof",
across all brands, they very seldom fail before around ~5-years of hard usage.

If your Spark-Plug is fouled,
1) it means that your Carburetor is not operating correctly, or, is improperly adjusted,
2) or You have a severely clogged Air-Filter,
3) or, ( in the case of a 2-Stroke-Engine ), the Fuel to Oil Pre-Mix-Ratio is incorrect.

Most small 2-Stroke-Engines must use a Carburetor that contains 2 Rubber-Diaphragms that
"Pump" and "Regulate" the Fuel respectively.
These Rubber-Diaphragms are required to facilitate Engine operation in any position, even up-side-down,
whereas something like a Lawn-Mower is always operating close to level with the ground, and
can depend on Gravity for it's operating requirements.

The Carburetor will not operate correctly when these Rubber-Diaphragms get stiff and
harden from age, and exposure to Gasoline, especially OLD-Gasoline.
After around ~1-year, a small adjustment to the Carburetor-Jets "may" get things working reasonably well.
But, after 2-years, THEY MUST BE REPLACED, or they WILL NOT function correctly.
( Just because the Engine may still crank and run, does not mean it is working correctly ).
Fuel stabilizers, such as "Sta-Bil", do help, but will not completely prevent this problem from occurring.

Arborists, ( Tree-Surgeons ),
expect that their Chainsaws start and perform perfectly,
every time they yank that cord,
or they're seriously pissed-off.

It's not difficult to learn how to replace these Rubber-Diaphragms in the Carburetor,
but learning how to adjust a 2-Stroke-Carburetor, after any changes have been made to it,
is an acquired art-form that few people possess, even supposed ""professionals"",
and also relies heavily on having an accurate "Oil to Gasoline Ratio" ( Pre-Mix ), in the Fuel-Tank.

Therefore, if You are not particularly handy with Engines,
and the Bill for repairs at your local Lawn-Mower-Shop is likely to be close to ~$100.oo,
it may be cost-effective to simply purchase a new Machine every 2-years.
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As for brands, I swear by Briggs and Stratton, any Tecumseh I have owned, I have had problems.

 

To rule out the presence or not of ignition spark, some monitoring gadget would be nice to have

Either everyone disagrees with you or they don't want to say that they don't have an idea how to help you - and they don't want to admit it.

I like your idea, I just don't have a solution to propose.

 

There are lots of things like this about which connect between the plug lead and the plug.
https://www.amazon.co.uk/Silverline...2319&s=diy&sprefix=spark+tester,diy,68&sr=1-1

 

Take any old spark plug and attach a big alligator clip (or rare earth magnet), so it can be clipped (or stuck) onto the engine?
Magnetos with points must be less common now, but I've revived several engines in the past by cleaning the points and setting the gap. The points are conveniently located next to the magneto coil, under the flywheel. Electronic magneto ignitions are probably more reliable, but can't be fixed as cheaply.

 

Take any old spark plug and attach a big alligator clip (or rare earth magnet), so it can be clipped (or stuck) onto the engine?

Most engines these days are aluminum cast. True, you could stick it to a bolt, but we used to just hold the plug against the engine and someone yank the cord. See a spark? Iggy is good. Bad spark plug - replace. They're cheap enough. And the engine will run better. You don't even need an old spark plug, just pull the one out of the engine head. You should inspect it anyway. Make sure it's not fouled. AND with the plug removed the engine will spin a lot easier and coast a lot longer, giving plenty of time to see the spark - if there is one.

If the machine starts then quits I know it's not ignition related.

Used the same principal with car engines. If it wouldn't start squirt a few drops of fuel down the throat of the throttle body or carburetor. If the engine fired then quit - you knew the ignition and timing were all working good. And you know the problem is a fuel related one. If the engine doesn't start then you suspect either the ignition or timing. Make sure there's a spark. If there is then you may have a bad (skipped tooth) timing chain or belt. A broken belt is easy to find - if you have a distributor. If it's not turning when you crank the engine - the timing belt is broken.

Back to lawn equipment - why do you want to build something when there are so many other - easier ways to tell if there's spark.

 

As for brands, I swear by Briggs and Stratton

You can't kill a Briggs and Scrap-Iron.

I just don't have a solution to propose.

A old AM portable radio, tuned off any station, gives a satisfying click on an ignition system

Sounds like a solution. I lean toward the more simple approach. Diagnose the problem before you start building space ships. If it's ignition - pull the wire off, expose (or insert a bolt), expose the wire and put it near by the engine. Even a screwdriver stuck in the plug end will work. Just don't expand the spark plug clamping ring.

I've revived several engines in the past by cleaning the points and setting the gap.

A neighbor bought a new mower. It ran for 30 minutes then it could not be restarted. Try as he might - it wouldn't start. He put it in the garbage pile at the curb one afternoon. I asked why. I then asked if he minded me taking it. I did as my dad taught me - pull the plug wire, hold it close by the engine and pull the cord. No spark! So I took the flywheel off and checked the points. The gap had closed down so much that the points practically didn't open. I loosened the clamp holding the condenser and point, adjusted the gap (with dad's help), put it back together and had a running mower. For 30 minutes. Opened it up and saw the gap had closed up again. The clamp holding the condenser was not holding it in place. So I pinched the clamp slightly. That mower ran for years after that.

 

The clamp holding the condenser was not holding it in place. So I pinched the clamp slightly. That mower ran for years after that.

Similar thing happened to me, someone had replaced the condenser with a capacitor !!

 

we used to just hold the plug against the engine and someone yank the cord. See a spark? Iggy is good.

This. You can see the spark in daylight, and it's a more sure-fire solution than any neon or other thing, because you get to see the actual spark in the actual spark plug, and have confidence that what you see happening outside the engine is happening inside the engine too, when you reinstall.

But why this caveat?

Most engines these days are aluminum cast. True, you could stick it to a bolt

Why does it matter if the block is aluminum? Why the bolt? I test spark against my aluminum block, not sure why it would be a problem.

 

Why does it matter if the block is aluminum?

Take any old spark plug and attach a big alligator clip (or rare earth magnet), so it can be clipped (or stuck) onto the engine

Using a magnet to hold the spark plug to the engine is impractical if you have an aluminum block.

 

Thanks.

There are lots of things like this about which connect between the plug lead and the plug.
https://www.amazon.co.uk/Silverline...2319&s=diy&sprefix=spark+tester,diy,68&sr=1-1

Yes, I have that identical one and cannot see sheeeet if not in darkness. Lots? Show more.

Take any old spark plug and attach a big alligator clip (or rare earth magnet), so it can be clipped (or stuck) onto the engine?

Yes, same sheeeet, seeing a 1mm spark at a distance when pulling the rope on daylight.

Dear gentlemen, thanks for the "repairing an engine" counseling; this is about detecting the spark from a point of view behind the engine a metre+ away while pulling the start cord under the sun and cranking noise. So far, I have to test the AM radio way or build a detuned AM receiver in a headphone.

 

Why does it matter if the block is aluminum? Why the bolt?

Why aluminum matters - see Pan's comment below. As to why the bolt - many spark plug wires come with a molded boot on the end to prevent accidentally touching the spark plug while the motor is in operation. I've felt the bite of that spark, and so did my cousin's dog when he put his foot down through the bars of a mini-bike with the unprotected spark plug sticking upwards. Sticking something inside the boot - not so big that it deforms the ring - effectively brings the electrode out to where it can be held in close proximity (but not touching) to the engine block. I've used screwdrivers in the past too. Also used old spark plugs - provided they were in decent condition when they were retired from service.

Once I had such a weak spark I thought I didn't have one at all. So - brave as I was - I held the ignition wire in my hand with the same hand resting against the block. OH YEAH - there was spark for sure. But not enough to jump the gap of a spark plug. As a kid I reasoned that I could bend the ground electrode closer to the center electrode and get a spark. It worked. Poorly, but it worked.

Using a magnet to hold the spark plug to the engine is impractical if you have an aluminum block.

 

My first recommendation to check if the ignition is generating an adequate pulse is to use a cheap timing light. The really cheap ones, that may only be available used, have a neon flash tube in series and leads long enough so that you can have it where you can see it. One of the cables connects to the top of the spark plug, the other to the end of the high voltage lead that was connected to the spark plug. Then give the rope a pull, and if the tube flashes, you have spark, although the plug may be fouled. The easier type of timing light uses a magnetic pickup that clamps around the plug wire. But they usually need 12 volts DC power, so that is a bit more bother. But timing lights work very well for verifying spark or not. AND they can help detect when the flywheel key has broken and the timing is off too much. That can happen when the blade of a rotary mower hits something hard.

OR try by squirting a bit of starting fluid onto the air cleaner, then give it a pull. If the engine fires and runs for a few seconds then the spark is OK , and it is a fuel issue.
What was not mentioned at all is that the alcohol in gasoline damages the fake rubber membrane in the cheaper carburetors after a while, At least that is whatdestroys carburetors here in Michigan.

 

I have that identical one and cannot see sheeeet if not in darkness.

Yes, same sheeeet, seeing a 1mm spark at a distance when pulling the rope on daylight.

1mm spark is rather short. I'm thinking you may not have sufficient ignition. But then again you don't detail how you're testing. If you're gutsy enough, take the wire in hand and make a tight fist. Then place THAT SAME FIST against the engine block, remove the spark plug from the machine entirely, and gently pull the start cord. If the ignition is strong you'll be cursing my name. If not - I think you may have bad ignition.

So far, I have to test the AM radio way or build a detuned AM receiver in a headphone.

Yeas, I get it - you want to detect ignition from a safe location. But I don't see a need for building stuff. For the number of times I've wondered about my yard machines ignition systems - it would be an unnecessary expense, time and effort situation. I haven't checked for ignition on any of my machines in well over 30 years. And 30 is just a guess. It could be significantly longer.

One safety caveat: Don't take the spark from one hand to the other. That could be more unpleasant, and potentially dangerous. So that's why I said to take the wire in hand and put that same hand against the machine. Don't go up and die on us. Otherwise, the spark will be unpleasant but not harmful if done right.

 

this is about detecting the spark from a point of view behind the engine a metre+ away while pulling the start cord under the sun and cranking noise.

Do you have a younger sister? That's how I tested lawnmower engine ignitions back in my very young days. Told her to hold the plug wire while I pulled the starter cord.

You must be a very highly trained school taught engineer. If you can't seem to figure out that lawn equipment is portable and you could take it into the shed or garage to see the spark.

 

Do you have a younger sister? That's how I tested lawnmower engine ignitions back in my very young days.

That was mean.