Thread: Spark Test
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Old 06-22-2019, 03:43 PM
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ndutton ndutton is offline
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Spark Test

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.
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Neil
1977 Catalina 30
San Pedro, California
prior boats 1987 Westsail 32, 1970 Catalina 22
Had my hands in a few others
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