Spark Test

[ndutton]

Perhaps the most fundamental test we can perform on a non-running engine determines if the issue is fuel or spark related. Of the two, spark is much easier to test provided you're prepared. A simple test is to remove a plug or coil wire and hold it close to the engine block or head and watch for a spark as the engine is cranked but here are a few other ways that once in place may be even simpler still:

The Test Pen
We have mentioned it on the forum before, a cool test tool available on Amazon, eBay, etc. that does not require anything to be unplugged. Place the tip of the test pen on a spark plug wire and watch for the test light inside the pen barrel while cranking. This is a yes or no test, it does not offer any spark quality information.

The Spare Spark Plug
The purpose of the following is to have a testing means readily available without digging through lockers and spare parts boxes.

For quite some time Tom Thatcher (Thatch on the forum) has kept a spare spark plug laying loose on the engine's alternator/lifting eye bracket for a handy spark test. He found the plug picked up sufficient ground for a test just by sitting there. His test method was to pull a plug wire, put it on the test plug and crank. Recently he decided to make it a little more permanent (elegant?) and improve the grounding so he came up with a magnetic base spare plug mount that can be positioned anywhere on the engine.

As he and I tend to do, we each came up with our own spare plug mount systems and then compared notes afterward. Here is my version. It utilizes the lifting eye hole that we aren't using anyway.

Test Protocol
Remove a plug wire and place it on the spare test plug and crank the engine. Observe for spark (yes or no) and quality (blue = good, yellow = not so good). No spark = ignition problem, yes spark = fuel problem.

[thatch]

Thanks Neil, It' obvious that any number of versions of this device can be made. I guess, ironically, that both Neil and I found the nice flat section of the lifting ring/alternator bracket, to be the ideal spot to place this gadget.
Tom

[ndutton]

I guess, ironically, that both Neil and I found the nice flat section of the lifting ring/alternator bracket, to be the ideal spot to place this gadget

We may be victims of our own boats (Catalina 30's) with zero clearance for anything higher than spark plug #1.

[roadnsky]

Man... the creative brain power between you two is amazing.

I'll never forget spending an evening with both of you.
I was mentally exhausted just listening to the back-n-forth conversation!

[thatch]

Jerry, I'd love to recreate that evening again sometime.
Tom

[Dave Neptune]

Great way to mount. Don't like seeing them on good running well maintained A-4's. I do like seeing them on engines with issues for diagnostics.

I too have used a spark plug for checking spark however it can be difficult to see the "jump of the spark". I recommend removing the ground lug from the spark plug. First this gives a larger gap to jump and it is far easier to see!!l You will also be able to "audibly" hear the "snap" as well as see the spark. The bluer the better. A spark with a yellowish flash is not good.

FYI, the hi-voltage ignition (the spark) needs to jump across a gap which is no big deal but when you add compression to the "chamber" this compression also offers up a great deal of resistance which is why you need a blue snappy spark!

Dave Neptune

[ndutton]

Good info on removing the ground ear Dave. About having a spare plug on a good running engine, it's the same as having a fuel pressure gauge. It rides along harmlessly until it's needed and then you're glad you have it.

[GregH]

Tried looking online but maybe not seeing it it.. what is the ground lug/ear and how does one remove it?

[ndutton]

Dave was referring to the little curved piece on the bottom of the spark plug. Wire cutters or a small grinder will do the trick.

[lat 64]

I made one for my lawn mower. just a bent wire permanently fix on the cyl. head.
Every engine should have one!

[CajunSpike]

Ditto on the spark color. Trying to get the motor running when I first got the boat. Checked the spark and saw a yellow spark. Though it may be enough but my experiences with beetle motors told me it should be blue. Eventually determined points were badly adjusted too open, so was not enough time to build up power before spark...resulting in low voltage yellow spark. This was the final key to getting the motor started and running.