OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Engines Questions On Briggs & Stratton Engines Briggs and Stratton problems 550EX O9P702-0127-H5

Often wondered what is actually happening when a spark plug goes "bad".

I have plugs which are are 50 + years old still working perfectly.

Last week I was using a 20 year old brushcutter with it's original NGK plug and it refused to start after a fuel refill. I took out the plug, put the body on a good earth and couldn't get a spark across the gap, nuthing, yet when the body was held 1/4 inch away from an earth an excellent spark jumped from the body to earth.

Cleaned the insulation around the central electrode, brightened up the end of electrode and the bridge metal bit with a thin file but couldn't get a spark across the gap.

Seemed that the plug's insulation had broken down.

Why ?

Detonation subjects the combustion chamber to adverse mechanical shock pressures. The
effect of this mechanical shock can damage spark plug electrodes or crack the insulator
core. Detonation in an engine will be audible .
When an engine has experienced light to medium detonation, there can be little or no
additional evidence. Prolonged heavy detonation which can lead to spark plug damage
becomes apparent upon cylinder removal or via borescope inspection. Detonation damage
usually shows in the edges of pistons and on the cylinder head between the spark plug
ports and valves. The cylinder head and piston should be inspected for a “sand-blasted”
look. A lack of deposits or a clean head and piston face can also indicate that detonation
has occurred,

carbon buildup within the cylinder itself or improperly functioning spark plugs can cause pre ignition.

In spark-ignition internal combustion engines, knocking (also knock, detonation, spark knock, pinging ,pre ignition ) occurs when combustion of some of the air/fuel mixture in the cylinder does not result from propagation of the flame front ignited by the spark plug, but when one or more pockets of air/fuel mixture explode ...


You can even drop a spark plug and damage the insulator but you can't see the damage.

Thanks for your reply Max but apologies - I didn't word my question properly.

"Seemed that the plug's insulation had broken down."

What I meant was the insulation had stopped insulating. There was no sign of any of the ceramic insulation material being damaged, but obviously there was high voltage going through the insulation material. The material around the central electrode exposed to combustion looked 100 % intact with normal deposits after years of use and I cleaned that carefully. The material outside the cylinder head also looked perfect, no cracks, clean and shiny.

When testing the plug by putting the body on bare metal there was no spark across the electrode gap and when testing by separating the plug body a few mm away from the metal earth there was a very obvious spark jumping across to earth.

The most common cause of plug failure is carbon fouling.

You may need to sand blast the plug and check it for spark.

Carbon deposits are conductive, and as they accumulate along the insulator nose they reduce the insulation resistance of the spark plug.

You may not always see Insulator Damage: Heat can cause the insulator (the ceramic part of the spark plug) to crack or degrade. This can compromise the spark plug's ability to maintain the necessary electrical isolation and can lead to misfires and breakdown.

There is no real way to see if the insulator is bad without disassembling the plug. Yes deep inside the metal the plug ceramic insulator can be cracked. I have done this just to prove to myself what I was thinking was happening on some engine where the plug would fire outside the compression zone but not when under compression load.

Of course this will destroy the plug. Just easier to test with a known good plug. Even new plug can be bad right of the box. And just accept the results especially under shop conditions where time wasted is not good for the bottom line. When shop rates are around $65 or higher a $3 plug can cost a lot more.

But during quite times learning what was the problem does help in future troubleshooting. Here the 10mm NGK CMR6 plug has such a high failure rate that I replace them with Bosch plugs which rarely see a failure with.

I only ever use NGK or Bosch plugs and yes that was kind of part of the problem.

Another guy tested the magneto and said it was on they way out...I have to trust him on that.

The real problem was the throttle cable which had become frayed, stretched and nearly broken inside the outer protective layer.

It must have just been hanging on just enough, to cause intermittent trouble, and stretching so in the end it broke and was not engaging the choke or the throttle control at all.

I couldn't get an original cable, and had to settle for another generic one, (it's control arm works opposite to the original Masport one) which was quite a fiddly fit and I had to cut quite a lot off the cable and inner wire to fit my mower.

I remembered a tip which was to first remove the inner wire and cut the outside cable cover before fitting the inner wire back on and shortening to suit.

The z bend tool I also bought with the cable came in real handy but it took me a few goes to perfect the z bend technique.

After installing the new magneto, spark plug and throttle cable the mower still wouldn't start UNTIL I realised:

1. the new plug had no visible gap straight out of the box (never happened before and I didn't notice when I installed it) ..maybe I bumped it or something.
2. the plug connection, as part of the new B&S magneto, had a much longer boot and required pushing much deeper onto the plug cap.

After that it fired up and after a few start stops, kept firing first pull.

Maybe I could have got away with only a new throttle cable

Thanks to everyone for their input and hopefully I get a few more trouble free years out of it now, rather than posting again with more troubles

It's more a learning journey for me rather than paying a hundred dollars an hour at a mower shop