new spark plug issues.

[rfoll]

This engine is way off what is usually considered here, but I have encountered something I can't explain. I have an 84 Evenrude 4 horsepower 2 cylinder 2 stroke outboard. I runs fine. I did a tuneup of new points and condenser because the spark was weak. It runs fine after. I decide to buy new plugs because I don't have spares and point ignition 2 strokes tend to foul when trolling at low speed for a long time. New Champion RL86C to replace old identical plugs. They won't spark, but the old ones spark good. I took them back and bought NGK BR6HS, theoretically same cross reference plug. They won't spark. I pulled a clean but used AC 45S plug out of my pile and it sparks even better. Why don't the new plugs fire? Would it be a problem using the AC 45S automotive plug in a 2 cycle engine? This is so odd I can't even make a wild guess as to what is going on here. Thanks, Rick.

[mag2555]

Plan old new but bad plugs!
You can run the 455s plugs as long as the tip length is close to the same and the heat range / color change over point on the ground strap after a heavy full throttle run is not too far past the bend point on the ground strap.

Heat range wise plugs have a 600 degree range between numbers where they will run as expected.

If you find the color change over point on the ground strap occurring half way down its lenght then you have the perfect 300/ 300 degree heat range for your motor.
As you mentioned though low speed with two cycle without fouling can call for having that color change over point more towards the body of the plug then centered from my experience with out board 2 cycle motors.

[rfoll]

Turns out, the NGK plugs are resistor, not good for a weak magneto system, but the question remains, why do the old plugs fire and the new ones don't?

[ProPower engines]

Turns out, the NGK plugs are resistor, not good for a weak magneto system, but the question remains, why do the old plugs fire and the new ones don't?

I have seen that before as well.
the resistor plugs are much more common today then years ago for 2 stroke engines.
The older ignitions did not fire as hot at low speeds compared to newer engine ign. systems
If you can find an older paper Bosch catalog they have a spec section which allows you to source a plug by your specific needs and create a part number for such a plug.
Its an elegant method of getting what you want not just whats on the shelf. You may have to order them as some numbers are not common and on the shelf but can be factory ordered.

But you still really need a non-resistor plug.
You may want to check the plug wires for any extra resistance since they don't last forever as they should be a solid core type wire.

[rfoll]

The question remains, why do the old plugs fire and the new ones don't? The new RL86C plug is the original equipment plug for the engine, and the new ones would not fire, while the old ones worked fine..

[ProPower engines]

Faulty construction of the new plugs.
Or too much resistance of the new plugs.

I would bet the plugs have too much resistance built into them as newer engines will be OK with the extra resistance but the older engines not so much.
Change out the plugs for a different brand but same heat range and try again to confirm or even just get another set of the same but from a different supplier.