Correct Spark Plug Size

[Dale51]

Hey Guys:

Don't forget, the heat range of the plug is more important than the brand. All the plug has to do is light the mixture. If a plug lights the fire, then that's all that is needed.

Remember, a hot plug is recommended when the engine is used very little, for a short time or may be pumping a little oil. A cooler plug is recommended for a clean burning engine that works hard for extended time periods. Sometimes you just have to experiment with different ranges to see what works best for you.

And if I don't get another chance, Merry Christmas and Happy Holidays to each of you.

This says it very well but also lets not for get that these tractors are running on a fuel that they are not built for (unleaded).
With out lead or some other retardment it burns hotter .
If you mix this with a plug that is to hot you will have preing.and that leads to holes in pistons and burnt valves and it will make the rings wear faster as it burns the oil of of the cyl. walls & we all know what happens when metal runs on metal with no lube.
Dale51

[Jim Becker]

That is all fine if you are talking about a high speed, high compression engine. The Cub engine was originally designed to run on 70 octane gas. Didn't take much lead to meet 70 octane.

[John *.?-!.* cub owner]

I'll add one more variable to all of this. I use a middle grade unleaded gasoline (Amoco Silver), not because of spark knock, but because I do seem to have less spark plug fouling with it. Though the fact that my cub runs slightly rich, may be a factor affecting that.

[47studebaker]

Hello everyone! In reading this thread I saw Jim Beckers comment about octane rating of 70. Is that a typo as my book says the cub is designed to run on 90 octane. Am I missing something?

[Jim Becker]

Not a typo. What you find in print probably depends on the age of the book. Early IH Owners Manuals just said something like "Use a good grade of clean gasoline." and they were usually more concerned about cleanliness than octane rating (read the filler cap).

I believe I found the 70 figure in an I&T Red Book from the early Cub years, although I can't put my hands on it right now.

Later Cubs had higher compression to take some advantage of the improved gasoline. Manuals reflect that change. My 154 manual says to use a minimum of 90R and indicates that is about 84M. A pump delivering that gasoline today will be labeled 87.