Testing spark plugs with a DMM - Page 1
Forum section: electronics / general

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2025, 08:46:18 pm
content: My bike developed a miss last week. It will clear after a short drive but was getting worse. Normally I use my finger to locate the problem by cold starting the motor and running it for a couple of seconds. I then touch each exhaust pipe and see if I can feel a difference. Done this for many years.There are lots of videos on YT where they show how to use a DMM to test a plug. They always measure the center electrode. Really I want to know the leakage current as the plug fowls. BM869s can measure conductance so when I pulled the bad plug I tried to measure it. It turns out to be low enough to measure with standard ohms.Showing fowled plug reading 6Meg and a used good plug reading 0.01nS. That's a night and day difference and seems like it may be possible to use this method rather than starting the engine.

author: Analog Kid
author_rank: Super Contributor
timestamp: August 23, 2025, 08:55:24 pm
content: I think the word you want is "fouled".Otherwise, interesting.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2025, 09:19:17 pm
content: Correct. Most of my posts typically have at least a couple of spelling and grammar errors. If they are really bad, I will sometimes correct them.

author: bdunham7
author_rank: Super Contributor
timestamp: August 23, 2025, 09:50:31 pm
content: There used to be "anti-fouling" spark plugs that had an internal gap (after the resistor if it had one, IIRC). IDK if those are a thing anymore, but if they are you're most likely to find them on something that is susceptible to plug fouling like an old bike or tractor.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 23, 2025, 11:05:16 pm
content: I don't think that you can really test spark plugs with an ohm meter and under static conditions. At least until they are thoroughly fouled. I road a motorcycle (a Kawasaki A7) with a 2 cycle engine back in the day and fouling was an everyday fact of life with a 2 cycle engine. Someone that I knew owned a shop that had a spark plug tester that you could screw a plug into a chamber with a glass window and see if it was sparking or not. The tester had an HV source that you could connect to the plug and you could see through the window if the plug was sparking and if it was sparking across the gap where it should be or if it was just arcing through the carbon fouling. You could also pressurize the chamber with different air pressures (from an external compressor) and very often a spark would fire ok until you started applying some air pressure and then the arc would go out completely or the arc would move and start arcing across the carbon fouling (which would not ignite the fuel/air mix). I used that tester A LOT (literally like every other day!) and I found that it took a lot more cleaning of a spark plug than I expected before it would work under pressure. My results on the motorcycle were similar. The tester also had a port that was part of a sand blaster so you could clean (sand blast) the plug and then test it. I quickly found that even clean looking plugs sometimes didn't work when you added some air pressure. And that quite often plugs would work at low pressures but not at high pressure. It was very common to have to clean and test and repeat 3 to 4 times in order to get the plugs to the point that that worked properly, and a significant number (~20%) of the plugs simply could not be cleaned enough to ever work satisfactorily.I've also testedallof the major brands of spark plugs on several of my motorcycles and several of my cars and I always found that the Autolite plugs were among the very best and the Champions were some of the worst!Later I boughtand leaned to usea CRT based ignition analyzer and with one of them you can clearly tell what's going on with a spark plug by looking at the firing voltage and what the voltage drops to after the fuel mixture ignites, etc etc. And a decent analyzer will let you see that on every cylinder simultaneously.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2025, 11:27:39 pm
content: Quote from: bdunham7 on August 23, 2025, 09:50:31 pmAdding the second gap will increase the breakdown voltage, and once it goes, it's at a higher current. Hard on the ignition. I would do this with my old bike by pulling back the cap to create the second gap and clear the plug. See clip of my vintage plug:

author: joeqsmith
author_rank: Super Contributor
timestamp: August 23, 2025, 11:53:48 pm
content: Quote from: Stray Electron on August 23, 2025, 11:05:16 pmThat plug I would say was on the verge of being completely fouled. It would clear on it's own but it was going down hill fast. It was easy enough to find with my finger test. Once pulled it was easy to see it was the culprit. Once replaced, engine runs smooth. Now could I reliably detect which plug is starting to go be measuring the leakage, verdict is still out. Post is in the dodgy area for a reason.Quote from: Stray Electron on August 23, 2025, 11:05:16 pmI had an old Kaw triple 2-stroke that used gap-less plugs. It was also very hard on plugs. I bet with modern oils, it would do much better. I cleaned the plugs with a torch.Quote from: Stray Electron on August 23, 2025, 11:05:16 pmThat jig I show in that video allows me to insert all 4 plugs. Very similar setup. In this case though, it's not warranted. I used it to test the other ignition componentsand experiment with the gap. Plus this bike uses smaller thread size.Quote from: Stray Electron on August 23, 2025, 11:05:16 pmYou could have used that system in this thread to provide some data.https://www.eevblog.com/forum/rf-microwave/ferrite-bead-on-spark-plug-wire/

author: Analog Kid
author_rank: Super Contributor
timestamp: August 24, 2025, 01:50:48 am
content: I'm no engine expert, but it's hard for me to imagine a 2-stroke engine thatwouldn'tfoul its plugs.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 24, 2025, 03:08:13 am
content: Quote from: Analog Kid on August 24, 2025, 01:50:48 amThere are 2-stroke compression ignition engines. Look up Detroit Diesel 2-stroke orhttps://en.wikipedia.org/wiki/Hugo_G%C3%BCldner. When I was a child, friends and I would play with toys that used 2-strokes that were also compression ignition. Most used a glow plug but I had one with an adjustable compression ratio.Gasoline wise, Japan produced one of the first HCCI motors using a modified Honda CR 250, I think. They brought it to the USA to compete against the 4-strokes in the Baja, I think. Look up homogeneous charge compression ignition.That said, with modern oils and 50:1 mix ratios I don't have a problem with plugs.

author: d-el
author_rank: Regular Contributor
timestamp: August 25, 2025, 02:28:51 pm
content: Quote from: joeqsmith on August 23, 2025, 08:46:18 pmThe most interesting thing is that to check using this method you don’t need to unscrew the spark plug from the engine.But there is a more well-known test of spark plugs with a hand-held piezoelectric generator. The force of the generator handle indicates the spark plug breakdown point.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2025, 05:07:40 pm
content: Quote from: d-el on August 25, 2025, 02:28:51 pmNot only no need to unscrew the plug, but wouldn't have to start the engine. I just have no idea if you could reliably measure conductance to detect when a plug is starting to fail.What does this hand-held piezoelectric generator you mention look like. Provide some more details and a link.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 25, 2025, 05:31:12 pm
content: Quote from: joeqsmith on August 23, 2025, 11:53:48 pmI owned a Kawasaki Mach III (3 cylinder 500cc) a few years later. It was basically an A-7 with a third cylinder added and it was fast as all hell! I didn't keep it very long before I went overseas. I don't know what the top speed on it was but I pushed the A-7 to well over 120 on a couple of occasions.I only read the first few posts of that thread and I don't think that I ever commented on it. But IMO you want the high frequencies and the sharp pulses in an ignition system so I don't think that ferrite beads are a good idea. But they might be good for attenuating some of the RF emissions. In fact, IIRC the Mach III came with a solid state ignition system and the sharp ignition pulses was one of the advertising claims for it.

author: joeqsmith
author_rank: Super Contributor
timestamp: August 25, 2025, 06:15:11 pm
content: Quote from: Stray Electron on August 25, 2025, 05:31:12 pmThat bike was a death trap!!! Mine had a distributor and the plug wires went inside the engine cases. I bought it with a dead ignition. I think the dealer wanted more for a new ignition than I paid for the bike. It seems they showed the SCR driver in the owners manual. I replicated their circuit and used that up until I gave the bike to a friend a few years later. I added a set of expansion chambers. It was loud, fast and steered like a cow. My cheap, hard rubber tires sure didn't help.Guy in the thread was making claims of the ferrite delaying the spark. They had no way to measure it so I showed a few examples using a scope. Guessing they were loosing power was all and advancing the timing to try to make up for the lost performance.