Strange problem

Hi My lawnmower has a B&s Intek Edge 1200 series engine that wont start.
I have tried everything, it is not the fuel side, that checks out ok.
I have replaced the spark plug, tested the kill swith, reset the coil gap, even reset the valve lash, and it still will not start, but the strange thing is it will start and run ok if I leave the spark plug tester connected.
The spark plug tester is the type that has a plastic tube with leads to connect it, and a light that flashes inside it when the plug sparks.
I have never seen anything like this before, it is a mystery to me, can anyone help please.

grover said:

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The spark plug tester is the type that has a plastic tube with leads to connect it, and a light that flashes inside it when the plug sparks.
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. You are using the wrong style tester to test for a weak coil. You need to have tester that you can test for a good 1/4 inch of spark travel.

Or you have the wrong plug ( non resistor ) or wrong plug gap ( to small ) or the plug got wet & the spark was tracking down the side of the plug.
The tester is adding some resistance to the line causing a higher voltage to arrive at the plug

bertsmobile1 said:

Or you have the wrong plug ( non resistor ) or wrong plug gap ( to small ) or the plug got wet & the spark was tracking down the side of the plug.
The tester is adding some resistance to the line causing a higher voltage to arrive at the plug

Click to expand...

Hi I cant thank you enough, you have solved my problem. It was driving me crazy. It was that the plug gap was to small.
I had put a new spark plug resistor type, Champion RC12YC in, and the B&S spec was 0.020, which I set it at, which gave me the problem.
I changed it to the Champion recomended gap of 0.030, and it fired up first pull.
Who would have thought that 10 thou would have made such a difference !!
Thanks again
Best Regards grover

reynoldston said:

. You are using the wrong style tester to test for a weak coil. You need to have tester that you can test for a good 1/4 inch of spark travel.

Click to expand...

Hi I wasnt testing for a weak coil, the problem was that the engine would not run unless the spark plug tester was connected, this was my strange problem.
It has been solved by bertsmobile1 who offered several sugestions regarding the spark plug, one of which was
that the sparkplug gap was to small. I had set it to the B&S spec of 0.020, I changed it to the Champion size of 0.030, and it started first pull. ( without the plug tester connected ) I didnt think that 10 thou would make all that difference but it did.
Thanks for trying to help.
Best Regards grover

Sounds as though your "spark plug tester" is actually a Gunson's "Colortune" plug; designed to tune carburetors by being able to see the correct mixture colour (bunsen blue) inside the combustion chamber whilst the engine is running.

grover said:

Hi I cant thank you enough, you have solved my problem. It was driving me crazy. It was that the plug gap was to small.
I had put a new spark plug resistor type, Champion RC12YC in, and the B&S spec was 0.020, which I set it at, which gave me the problem.
I changed it to the Champion recomended gap of 0.030, and it fired up first pull.
Who would have thought that 10 thou would have made such a difference !!
Thanks again
Best Regards grover

Click to expand...

Why do spark plugs have to be so important? LOL Excellent diagnosis!:smile:

The plug tester adds a small amount of resistance to the HT Circuit which is reality is not a circuit because it is broken at the end by the gap in the spark plug
When there is sufficient electricial pressure ( called voltage ) the circuit closes by means of a spark.
For every different circuit you need a different break at the end or plug resistance if you like.
With a little effort every engine could be designed to use the same spark plug but that would require co-operation so it aint going to happen.
So if the engine ran with the tester but not without it, it needs more resistance in the HT circuit.
When a coil is weak you get almost the opposite the tester will flash when the plug is outside the engine but not flash when the plug is inside the engine where the compressed gasses increase the electrical resistance at the plug gap.