I have read here often enough that laying plugs out on the head and watching them spark is not a valid test of whether they will work. Is there a valid test, other than just changing to different plugs to see if it makes a difference?
i think the issue is that it takes more fire to jump the gap under pressure so you may see a spark in the open atmosphere but nut under compression. philip
Make up a more or less permanent gap device out of a block of wood and either some wire or sheet metal that presents a pointed gap of exactly 1/4". Put a short piece of wire on one side of the gap with an alligator or means of connecting that to a ground near the end of a spark plug wire. Once that is connected then fasten another alligator clip or stud to the other side of the gap that is fully insulated from ground by more than 1/2" and that is then the "hot" side of the gap. Pull the spark plug wire off the plug and fasten it to the hot side alligator clip and then hand crank the engine (with switch on BAT)and make sure that when the timer stops on the coil that you are testing that you then get a full buzz and spark jumping across the 1/4" gap at then what is the spark plug wire end. This does not check the plug itself but checks the spark energy at that plug. If the spark fails to jump the 1/4" and continue to spark while the coil buzzes then you probably have no spark under normal compression either. This test becomes invalid if you have a plug gap wider than the normal 1/32" or have a high compression head or domed pistons that increase the compression substantially higher than the T typical 4 to 1.
Philip is correct Steve, under compression the plug may not fire. an In-line tester such as this works on late models
and they are cheap so you may be able to modify one to test your plugs under running conditions (alligator clips or something similar). I have also seen with this device where a plug will fire at idle but stop firing as the throttle is opened! Hope this helps
Back in the day, Champion made a plug tester that has a pressurized chamber for testing plugs. It has a model T coil inside and connects to an air compressor. I have one and it works great. I set the pressure at around 55 lbs to test T plugs. The difference in spark is eye opening.
Yes, I know why the "laying-out" test is invalid. I don't have the period tester, so John's answer is perfect for me.
I like your tester Steve-----need to make one as I don't understand Johns method without a picture.
Doing many two cycle engines with yard work usually I can identify a bad plug with an ohm meter. Continuity is lost in the ceramic insulator even when cleaned with a plug cleaner.
How important is it to have actually a 1/4" gap? I suspect that if the gap was 1/2" you may damage your coils? Sparking internally? So it may be worthwhile to construct something a little more permanent than just a couple nails in a piece of wood, that can become bent out of shape. Maybe a couple of bolts? Sounds like a good project. Even enhance it so that you do not need jumpers. I like it!
You can get a tester thingy at the local auto parts store for maybe $5, that consists of a plastic body with an electrode on one end connected to a large alligator clip so you can attach it to a head bolt or some other ground. The other end has a "Bolt" of sorts that screws in and out so you can set it to your desired gap (1/4" in this case). The "head" of the bolt is designed for a modern spark plug boot to snap onto. If you run that sort of wires, then you are already good to go. If you run regular T wires, you would have to find a way to attach them to the "Bolt". Another alligator clip would work, or a small clamp, duct tape, whatever.
Ooh. Just noticed the price on that link. That is WAY more than I paid. I'm pretty sure you can find that same thing for $5-10.
If you are at all handy, something like this would be rather easy to make. Once the idea is there, the rest is just getting to it. Mike
Be VERY careful with store bought testers since they assume the coil insulation can withstand 40,000 volts or so which is typical for a good regular automotive type coil. If the maker of the device uses any sort of series resistance in the device then it can destroy your T coil by presenting it with too wide of a gap to jump. 1/4" is the exact gap you want to use. Notice that one tester says you can adjust the gap to 40,000 Volts. At one atmosphere of pressure the typical gap measurement is 75,000 volts per inch of gap. That is affected by moisture and other environmental issues. Hence if you dial in 40,000 volts you will present your wood T coil with a gap in excess of 1/2" and you can kiss that coil good by in pretty short order. If you test at less than 1/4" in free air then you are not really checking the coil under a worst case scenario and if you test it at greater than 1/4" you risk harming it because it can arc internally in the coil. Before you start improving the device as I described - maybe you should just build it and try it as described. If you want to make a more permanent one then OK but make sure you don't assume that a modern device isn't gonna hurt your T coil - that is where the trouble will sneak in.
There are usually a couple of the Champion spark plug testers on ebay. There is also a fellow who restores them and sells parts. I bought mine 20+ years ago and when the coil died, I had Ron rebuild the it and I rewired it. It a handy tool to have around the shop.
John,. If you used the actual spark plug under suspicion could you open the gap to 1/4 inch and see how it performs unpressurized? You would be testing the coil and plug at the same time. If the plug's insulation is up to par it should spark at the electrode. Only problem is I'm not sure if you open the gap to 1/4 inch some internal component maybe less than 1/4 inch
This works great for me. It is an electric fence tester that can be purchased at any farm supply store. Been using one for the last 20 years.
If no lights appear- you have no fire.
If one to three light glow- you have weak spark.
If 4 lights glow- you have good spark.
If all 5 lights glow- keep your dog-gone hands away from the top of the spark plug!
For the past week I've been busy with other things, but this morning I looked into this subject a little more. It seems we're talking about two different tests here. John's ¼" gap test, and all the "spark testers" advertised online, tell you whether the coil and the plug wire are delivering sufficient juice to the spark plug. Steve Blancard's old Champion tester is the other kind, which tests whether the actual plug itself fires under compression. I happen to have a Champion plug tester I picked up at an auction, but it's a later vintage and doesn't take Model T plugs. Studying it and the pictures I see online, it appears that all you need is a coil, a closed chamber to hold about 55 pounds of air pressure, and a window to see the spark. Is that correct? If that's the case, it shouldn't be terribly difficult to make a Model T plug tester.
Steve - that about it. Since I took the above photos I added a pressure gauge to mine. I put a plug in it and connect to my compressor line. Then using a regulator in the line, I adjust pressure until the gauge shows around 55 lbs, then press the button the side that fires the coil. Its a model T type coil with slightly different terminals on it. Inside the housing is a step down transformer (I think that's what it is) that takes the voltage from 120 to something less. There is a pushbutton on the side to close the circuit. I think I paid about $30 for mine at a swap meet many years ago. It really is a great tool.
Nothing too it! We had an AC rig in Dad's shop; when I was a teenager I cleaned, gapped, and tested a lot of fouled outboard motor plugs. The AC tester had the window in the bottom and you observed the spark in a mirror, that way if the glass blew out under pressure at least the pieces were headed toward the floor and not your eyes. It also had an adjustable pressure regulator so you could know at exactly what pressure it broke down, and you could tell the borderline plugs from the completely dead ones.
Many years ago when I was big into antique motorcycles I had a website related to Splitdorf generators and magnetos. I no longer do that work and forgotten most of it, but a friend still hosts my old website for reference because it contained a lot of good info.
Anyway, here is a page that has more of a discussion on the Champion spark plug tester among several topics.
For anyone interested in the full site with other topics, here is a link:
Here is a current photo of my Champion tester. The gauge is a little oversize, but works well with older eyes.
I am in the same boat as Paul Vitgo states above (6/21/17 @ 10:28). I can't visualize John Regan's method. If anyone has one, or make's one using his design, would you post a picture or two, or more?
As I mentioned yesterday, John's test determines whether enough juice is getting to the plug. It's a spark tester, not a plug tester.
As I understand John Regan's post, the plug has been taken out of the equation and the result would only indicate whether the coil is hot enough to fire the plug under compression, assuming you have a good plug which was the question in the first place. Am I missing something?
Steve and I were typing at the same time, sorry.
Yes its a spark tester but if that spark is thus proven good THEN it is a plug tester when you hook the proven good spark wire to the plug.