engine has spark, but no spark when sparkplug is installed. yamaha moto 4 1991

[heath]

i just bought a yamaha moto 4, i have sprayed 'start ya barstard' into the carb and on the spark plug and it doesnt even fire! although when i have the spark plug on the outside of the motor it shows spark, the engine also leaks oil from the starter,

[Mech]

It needs a better spark to jump under compression than it does at atmospheric pressure.

A new spark plug might fix it, or the coil or plug lead/cap might be defective and letting the spark jump out of the engine, but once in and under compression the spark might find it easier to jump to earth through some bad insulation or an internal short in the coil.

Most coils they say to test by making sure they will jump a spark six mill which is 1/4 inch. If it will spark that far then it's internal insulation is good, but the cap or wire could still short.

Edited November 9, 2024 by Mech

[Gwbarm]

I have had plugs do that before, i learned firing issues replace plug first and go from there.

[Mech]

Yup, plugs are cheap and it's good to have a spare known good one sitting on the shelf for testing purposes if it's not needed.

[zajavajunkie]

best advice i got from an old mechanic "compression kills spark"

[Mech]

I'm an old mechanic, and I agree.

Now days we mostly just toss used plugs, but years ago we cleaned and tested them, and the test was to use a spark machine which let you adjust the air pressure the plug was under while it was sparking. Lots of plugs that started out sparking fine would stop sparking at around 80 Lb  pressure.. which is way less than engine compression.

[spock58]

I'm curious - how do you know it isn't sparking under compression? That would be pretty hard to test for...

On the other hand, doing a basic compression test might give an idea of the shape this engine is in.

[Mech]

Good point Spock. Always good to confirm the simple things first.

If you aren't sure if it's got compression Heath then a simple and rough check is to poke your finger in the plug hole and give the engine a crank. If it blows your finger out it probably has enough to at least fire, but if you can keep your finger in there sealing the compression, it's not enough.