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MG Midget and Sprite Technical - How to test a spark plug?

I treated my Sprite to a new set of spark plugs (NGK BP6ES) when I rebuilt the engine. All went well through running in and at the first competition.

The carburettors were set quite rich so last week I adjusted and rebalanced them. As soon as I set off on a test run there was an intermittent misfire. One piston sticking maybe? It was very inconsistent but ran best above 2500 revs with only the occasional single beat missing.

I couldn't think why it should be the ignition as I hadn't touched anything there apart from removing and replacing the plugs to do a compression test.

I swapped the plugs for the old ones (not that old and they'd been fine before the rebuild, they just have fresh paint all over them!). When going to swap the leads over, I found that the bottom distributor cap clip wasn't fully home, so, after clipping it properly, set off on a test run - it went perfectly; problem solved.

On replacing the new plugs, the misfire returned so I did the weekend's hill climb on the old plugs. It ran perfectly.

I can't see any damage or marks on the new plugs and the working ends all look very similar. The resistance between the top and the centre electrode = 0 ohms on all of them. I couldn't detect any leakage through the ceramic insulator. Are there any other checks to do to identify the dodgy one apart from swapping them in and out one at a time?
C Mee

Did you clean all of the new plugs before you put them back in?

Did you look at the plugs with a good magnifying glass?

If the meter says they're good then they must be, a meter won't lie or not detect issues! surely not!

Did you check the meter was registering, and consistently correctly, not that a meter would ever be wrong! must be user, surely. Meter certificated and tested annually, not that it needs it.

If you've got some nice cheap HT leads (that pass a meter test of course) you can perhaps raise the plug boots up the leads so that much more of each plug is exposed, except the connection. Then you can go old school and water spray on each plug individually one at a time as the engine runs until you hear the one with the misfire, or do it in a darken area and look for the sparks.

But take care not to get water on the HT lead or connection to plug or you could get spurious results.

But really if you leave three of your old good plugs in you'll only be putting in the new plugs one at a time for a maximum of four times to find the faulty one - if it is one of the new plugs that is at fault.

When you find the faulty plug test it on you meter again and decide how useful the tool was in this instance, I'm sure you could get a figure on the display if you swing its leads about.
Nigel Atkins

I had this problem a couple of years ago. I just swapped the plugs until the faulty one was found. they were NGK's as well. The faulty one did not appear to have an obvious issue. I took the four plugs back to the supplier and they gave me a new set!!
Bob Beaumont

Maybe there's a slight crack in the plug between electrode and outer metal body and it shorts under HT voltage, like a faulty rotor arm or distributor cap. A multimeter won't show that.
Bill Bretherton

Hi Colin.
As Bill remarked quite rightly a multimeter won't tell you anything useful about the plug.
Your plan to swap them out one at a time until the fault returns is as good as you can get without specialised kit and should reveal the culprit.
However...
Many garages still have plug testers. I saw one only yesterday with a big "Champion" logo on it. I imagine a little local place would oblige if you ask nicely, which is less hassle and more certain as you may not be able to provoke the misfire yourself, depending upon other variable conditions.
Just a thought...
Cheers.
Greybeard

Thanks for the advice.

It looks as though I'll be swapping in and out but I'll take a closer look first with the jewllers loupe that turned up when I was investigating dusty boxes the other day!

Grey, thanks for your suggestion, my local (8 miles away) village garage just might have a spark plug tester.
C Mee

Found it.

I haven't had time to do anything with the Sprite until today. I've just spent an hour or so swapping plugs in and out and zooming, or spluttering, off up the road. It turned out to be the first plug that I tested but had to do the rest just to make sure.

I'll hang on to the dodgy one to see if my local garage can test it the next time I visit.

It's probably a bit obsessive of me, but I always try to make sure plugs stay with the same cylinder in case they are trying to tell me something. To maintain my effort to be more organised, I knocked up a spark plug holder out of a scrap of pine. I thought it might be useful to take spares with me to events and decided I needed a more presentable one, hence the MkII version in oak. Apologies for the paint splattered plugs, I wasn't going to take the shiny new ones out yet again just for a photo! These are the pre-rebuild plugs which are not that old and are working fine, so no point in chucking them away yet.

Thank you for all the advice.



C Mee

Well done Colin.

Very nice spark plug holder block but tatty spark plugs - 20 points off!

Did the jeweller's loupe and your eyes let you down same as the meter did?

Now you understand my distrust of "they all look good" and "I've tested with a meter" (and for HT leads and especially car batteries).

I like the idea that you double-check, double or treble-check all testing equipment, the testers and the tester (particular if it's me).

I'd be interesting to hear what your local garage find with the plug, other that it's faulty.
Nigel Atkins

I once saw someone identify a misfiring plug on an 8 cylinder engine using an infra red thermometer. With 8 cylinders swapping plugs would have taken forever and one intermittent misfire whilst 7 were working ok is very hard to detect. Three garages had failed to identify the problem. Then this young lass started the car and with an infra red digital thermometer quickly picked up that 1 cylinder was running at a lower temperature than the others. Replaced that plug and problem cured! Nice bit of lateral thinking. It took her less than 10 minutes.
GuyW

Alternative thinking indeed and much more effective than traditional methods.
I bought one of those digital thermometers a while ago (before they went up in price because of Covid) to check the temperature of an air cooled VW engine (if the shrouding isn't in place or properly affixed, they can overheat especially the rear cylinders on a type 4 engine).
Very useful too for checking central heating radiators, damp in walls and of course human temperatures. And now spark plugs.
Jeremy MkIII

The vehicle in question was a 5.2l Chaemera. It had an intermittent misfire that the other garage had been unable to sort out. We called in at the Dreadnaught tvr garage in Callendar. The young woman sorted it. She really new her stuff. This may sound sexist from an old, white, middle class male, but I only mean it as a compliment that was well deserved.
GuyW

I thought it might be a Chim you were referring to, they also get the HT lead to "heatshield" or "heatshield" to plug lift with heat and vibration so those need eliminating too but that easy done by pushing it back on so down to plug.

Personally I'd always prefer to see a woman mechanic over any Billy-big-bollocks or rip-off or con-man. As with many decent people in the car trade they don't stick it long and set up for themselves or change trade.

If the 5.2 Chim is bright yellow it might be one of the series of Chims a mate had many years back. Best engine he said was the 450 (4.6 to everyone else) as TVR had to do so little to it and best handling Chim he said was the last lot with 16" wheels all round, he said 16" were finally used at the front to clear the stock of them.

Been to Hexham Horseless a few time, the manager/part owner was a Nigel, and tight as a drum.
Nigel Atkins

Nigel, it was a silver one, the 4th of a series my son owned, going up it size. He preferred the later 5.2l one for overall driveability and performance.
GuyW

Doesn't sound like the same one as the yellow one as it was a bit of an animal and the mate could drive and well used to all sorts of cars.

Some how he persuade tight Nigel to let him take the new demo T350i up to Bute for a Rides event and we laughed at the gap on the rear window, I suggested you could post a letter through it and he less kindly said a parcel. I then was challenged to work out how you got petrol into the tank, I worked out what was the filler but not how to open it. I never went in it but apparently it was a fast car, some friends later had a T350c, never got it that either, obviously too fragile for me.
Nigel Atkins

If you have a strobe you can quickly try it on each plug and you will see which one is misbehaving.

Jan T
J Targosz

This thread was discussed between 09/08/2021 and 24/08/2021

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