Spark Plug for 71 'rude 18?

[Oregon Jon]

OK, I was told by an old OMC mechanic that the best plugs to use for my motor were Champion J8C's. Some of the other threads say they are too hot for outboards in general or that I should stick with the OEM recommendation of J4's. I don't want to cause any damage. I fired up with the J8's in place and it seemed to run great in the test tank. Any advice is appreciated.

 

[Bindernut]

Re: Spark Plug for 71 'rude 18?

J4C is the factory recommended plug and would be the safest to use for general use. If you aren't running it wide-open...just putzing around the lake... you might be able to go one step hotter with a J6C to keep[ fouling down. I don't think I'd got up to the J8C, that could be a little on the warm side for long runs.

 

[Oregon Jon]

Re: Spark Plug for 71 'rude 18?

Thanks for the reply. Can someone please educate me here? What are the advantages / disadvantages of running hotter or colder plugs in a two stroke?:redface:

 

[JB]

Re: Spark Plug for 71 'rude 18?

Same as for a 4 stroke, Jon.

The operating temp of the plug affects ignition and flame propogation. A too-hot plug may cause preignition.

 

[Oregon Jon]

Re: Spark Plug for 71 'rude 18?

OK, I'll change out for a cooler plug then. Thanks for the lesson.