How to test if a spark is strong enough?

[Andy 308GTB]

A friend is rebuilding a 50cc bike. When cranked there is a spark but I want to be able to confirm the spark is strong enough.
The gap is 1mm. We have found some literature that suggests if it is strong enough, it should be able to cross a 7mm gap.
I don't think opening the gap out to 7mm is feasible but we could give it a try.

Spark plug testers are ten a penny but most only show that there is a spark but not the strength of it. What methods are there to do this test?

Thanks

[steveo3002]

if you can see it in daylight id say its good enough,,nice sharp blue spark

[geeman237]

You need a tester like this. Yes, this is on the US Amazon, but you can see how it works. Basically there is a threaded adjuster so you can see how far the spark 'jumps'.
https://www.amazon.com/Tester-Adjustable-Ignition-...

[Andy 308GTB]

geeman237 said:

You need a tester like this. Yes, this is on the US Amazon, but you can see how it works. Basically there is a threaded adjuster so you can see how far the spark 'jumps'.
https://www.amazon.com/Tester-Adjustable-Ignition-...

Thanks that looks like it would do the job.

In the interim, in absence of such at tool, I've instructed my friend to put 2 nails 7mm apart in a block of wood. Then wire them to the plug lead and the earth and give it a go...

[paintman]

Is this an 'out of curiosity' thing or does the bike not start?

[hellorent]

The easiest way is to hold the spark plug attached to the ht lead about 5mm away from the engine block
with insulated pliers if the spark jumps that gap then yr ign side is ok.

[Boosted LS1]

Ask your friend to stand 7 mm away from you and ground him to earth. You should hear a yelp if all works well.

[spanner10]

Boosted LS1 said:

Ask your friend to stand 7 mm away from you and ground him to earth. You should hear a yelp if all works well.

This!!

[Andy 308GTB]

paintman said:

Is this an 'out of curiosity' thing or does the bike not start?

The bike doesn't start. It's an old 50cc Derbi two stroke that my friend bought as a pile of bits. He's already spent a load of time on it. He gave up about 6 months ago. I'm trying to help him now, we are going back to basics and working through it one step at a time.

The ignition is very similar to Piaggio and Vespa bikes of that era & uses a Hall Effect sensor. It looks like the orientation of everything is correct for the timing to be OK. Once we confirm that the spark is strong enough, we'll look at the carb - the first step will be to take the vacuum fuel feed out of the equation. The carb is a chinese copy...

[paintman]

Take it you've tried a squirt of easystart to see if it will fire/attempt to fire?

[Andy 308GTB]

paintman said:

Take it you've tried a squirt of easystart to see if it will fire/attempt to fire?

Yup. We gave that a good go earlier this week. Chummie should be testing the spark today.

Thanks

[GreenV8S]

Have you heated up the plugs until they're too hot to hold, and then quickly refitted and tried starting? That can help a great deal.

[Andy 308GTB]

GreenV8S said:

Have you heated up the plugs until they're too hot to hold, and then quickly refitted and tried starting? That can help a great deal.

Really? This sounds similar to the earlier suggestion about testing the spark plug by standing 7mm from Chummie...

[GreenV8S]

Andy 308GTB said:

Really?

Yes. You burn the electrodes clean in a gas ring and refit while they're hot.