Here a spark plug, there a sparkplug...

[Auto Doc's]

I cannot explain the why. I can only say what was done and how it was fixed. The only explaination I can think of was the plug was partially fouled. Yes I don't understand why the timing was changed, only that it was. And that by changing the plug the issue was corrected. I replaced the plug with a Champion of the correct heat range, the same heat range as the NGK and what was recommended by Honda. Nothing was changed except the spark plug, same fuel, same ignition unit, same carburetor and air filter, even checked the flywheel key, was not sheared even slightly. It was more than one mower that did this and it was different owners. The first one to do this took quite a while to figure out as I blamed it on every thing except the spark plug.

I suggest you may have had a bad experience with counterfeit NGK plugs. They are "grey market" imported Amazon garbage mostly, but they find their way to small parts stores struggling to stay in business.

I have dealt with this in both small engine and automotive applications.

Most modern spark plugs are made with a resistor pellet inside the porcelain segment. The resistor has to be correct for the spark plug to work correctly.

One solution is to make sure you have genuine plugs or use non-resistor designs.

 

[bentrim]

These were usually OEM Honda installed plugs. As I said I could not explain the why, I can only say what happened and after replaceing the plugs with Champion had no more problems. They only reason I can put on the problem was there was a problem with the resistor in the plugs. By the way the Champions were a resistor plug.

If you haven't seen it here is some interesting information on Champion and AC plugs.

How Albert Champion(ed) Two Major Spark Plug Brands - Hagerty Media

Albert Champion was responsible for two spark plugs companies: Champion, of course, and AC. His story is a remarkable one.

www.hagerty.com

 

[Auto Doc's]

These were usually OEM Honda installed plugs. As I said I could not explain the why, I can only say what happened and after replaceing the plugs with Champion had no more problems. They only reason I can put on the problem was there was a problem with the resistor in the plugs. By the way the Champions were a resistor plug.

If you haven't seen it here is some interesting information on Champion and AC plugs.

How Albert Champion(ed) Two Major Spark Plug Brands - Hagerty Media

Albert Champion was responsible for two spark plugs companies: Champion, of course, and AC. His story is a remarkable one.

www.hagerty.com

Another issue to consider when buying used plugs (resistor type), even slightly used is risky due to an unknown history or any rough handling. Plugs are rather tough, but if they are dropped on a concrete floor hard or multiple times, there is a high risk that the resistor pellet has been fractured, and they would fail quickly.

Another factor that people often overlook is the "heat range". There are plug manufacturer charts that explain this pretty well.

I have run into incorrect heat range issues a few times over the years. That is usually identified by an engine that misfires and shuts down when hot. Once it cools it restarts until it gets hot again. It is easy to condemn a coil for this problem, so swapping the plug is usually the quickest solution to confirm the problem area.

 

[bentrim]

Another issue to consider when buying used plugs (resistor type), even slightly used is risky due to an unknown history or any rough handling. Plugs are rather tough, but if they are dropped on a concrete floor hard or multiple times, there is a high risk that the resistor pellet has been fractured, and they would fail quickly.

Another factor that people often overlook is the "heat range". There are plug manufacturer charts that explain this pretty well.

I have run into incorrect heat range issues a few times over the years. That is usually identified by an engine that misfires and shuts down when hot. Once it cools it restarts until it gets hot again. It is easy to condemn a coil for this problem, so swapping the plug is usually the quickest solution to confirm the problem area.

As cheap as spark plugs are I never use used sparkplugs unless it is a known good for testing only to be replaced by a new plug.
Yes I agree sometimes people don't respect the spark plug and careless handling especially when dropped on the floor result a damaged plug. I have seen insulators broken and the ground electrode damaged or closed by careless peole.
Also had a NEW plug without a ground electrode installed.

As to heat range Yes it can causes issues including as you explained or even engine damage from the wrong heat range. Usualy too cold fouls plugs and too hot may cause detonation and engine damage.

As for customers doing their own work I have had seen flat head engine that require a RJ19LM come in with a Rc12YC installed and vice versa.

 

[sgkent]

The timing and the spark voltage are affected by the plug gap, not how beefy the coil is. The electrons do not care what conductor is carrying them, other than some plugs and wires, rotors etc., that have resistors built into them to limit ignition current. The timing change by changing the gap is infinitesimal, but it is there. If the gap is too small, the firing voltage will be too low. If it is too wide, it may exceed the coil's output, or the di-electric inside the coil, wires etc.. If a starter rope pulls backwards, that is not the plug. In fact, if it snapped back, the flywheel / magneto could have been out of time, and the change moved the flywheel back to the correct spot, in which case it will be back. I have only seen one set of bad NGK plugs and that was in my Opel GT over some 50 years ago. I use NGK, Champion, and Denso. All are good brands in my humble opinion. In fact, I often just glass bead the old plug to clean it up, re-gap it, and reuse it the next time I need one. Almost everything I own that runs on gasoline starts on the first or second pull.

 

[bentrim]

The timing and the spark voltage are affected by the plug gap, not how beefy the coil is. The electrons do not care what conductor is carrying them, other than some plugs and wires, rotors etc., that have resistors built into them to limit ignition current. The timing change by changing the gap is infinitesimal, but it is there. If the gap is too small, the firing voltage will be too low. If it is too wide, it may exceed the coil's output, or the di-electric inside the coil, wires etc.. If a starter rope pulls backwards, that is not the plug. In fact, if it snapped back, the flywheel / magneto could have been out of time, and the change moved the flywheel back to the correct spot, in which case it will be back. I have only seen one set of bad NGK plugs and that was in my Opel GT over some 50 years ago. I use NGK, Champion, and Denso. All are good brands in my humble opinion. In fact, I often just glass bead the old plug to clean it up, re-gap it, and reuse it the next time I need one. Almost everything I own that runs on gasoline starts on the first or second pull.

I have no doubt that major spark plug manufacturers are making good products whether it is NGK, Champion, AC, Bosh, Denso or others. I know what happened and had a sore hand for a week. Also as stated before the first one was totally checked out, carb, coil, flywheel key, valve settings, fuel, etc. The only thing that fixed it was replacing the spark plug. The other techs in the shop did not believe me either --- until it happened to them. I wish I had saved some of those plugs so I could prove I wasn't daydreaming. My personal choice is Champion but I have seen them fail too, but not that way.