Small Engine Spark Plug Tester

I couldn’t find a small spark plug tester so modified a standard automotive plug tester. Took less than 1/2hr to figure it out and works spot on. I left one side unmodified so I can test both sizes. Works a treat. Alot quicker than using a multi meter and only taking a matter of seconds. I fitted an internal 9v battery instead of using the power supply. Not sure how many tests I’ll get out of one battery. If it doesn’t last, there is enough room to fit a second battery. Be able to run in series and use a buck converter if needed. For now, it works well.

All that is required to be purchased is the spark plug tester and a 9v battery and connector.

Tools: screw driver, drill and die grinder. If adding the batter, soldering equipment.

Unscrew the back of the tester and unscrew the PCB. Be carful with the two earth wires going to the mounts. Unscrew one of the plug holder rings. The difference between a standard plug and a small engine plug is about 15mm. The difference between moving the plug holder from external to internal is very close. So by simply moving the holder, you can test the different size.

The hole through the case needs to be enlarge to allow the plug to pass through. The plug must sit on the holder to get a good connection. I used a 20mm drill bit to carefully drill the hole. A die grinder would also do the trick. I used the original mounting screw holes, so only remove necessary material or you will need to secure the holder another way eg epoxy.

Position the holder Internally. I needed to grind away a small recess in the holder as it fouled on the tester case. Screw the holder into position or glue if you chose to go that route. The plug is wider than the screws, so once the holder is fastened in position, die grind enough material off each screw to give clearance.

Screw it all back together and start testing.

I used a power supply to determine if 9v would run the circuit and it worked fine. Normally a 12v input but as long as a spark is created, that’s all we need.

Take a battery connector and solder to input terminals. If you are not sure which is the + and -, the Center of the plug should be positive and the surround the negative.

Apply double sided tape to the battery and place in position.

Test.

Refit the pcb and cover.

Keep handy on the bench or in the toolbox.