From:Dave Hennessey Electronic:dave -A-
Subject:RE: Spark plugs 1959 165ST Date:Mon May 30 19:09:33 2011
Response to:15902

If the spark plug looks right (tan to golden brown color), it probably is right.

The usual HD sources don't specify which plug to use. I looked at, and they recommended a Champion H8C or RH8C for 165, 175 and Topper. I believe this is approximately the same as a Harley #3.

I tried to compare the center electrode insulator lengths between a H8C and some old take-apart Harley #3 (marked Harley #3 on the base, but Champion #3 on the outer insulator), but the #3's are old and crudded up, so I couldn't get accurate readings.

BTW - Harley, LOWER numbers are hotter. Champion, higher numbers are hotter.

My opinion is that it really doesn't matter much, unless you're doing a lot of driving. I ride mine sporadically, and a long ride is about 20-30 miles. Most rides are like 5 miles. I've been running an H10C in my 1955 Hummer, and that's a little hotter than a #3. But next time I go to the store, I'm gonna pick up an H8C.

Note: Pick a H8C over a RH8C, you don;t need the resistor since you don't have a radio to cause interference with. If you're at an real old-timey store, pick a H8 over an H8C, but those plain H8's are hard to find.

How To Read Spark Plugs

Put a new plug in, and drive your bike for about 15 minutes at normal speeds - the kind of driving you normally do. Take out the plug. If it has tan to rusty-brown dry deposits, it's OK, put it back in.

If the spark plug is glassy or glazed-over looking, it's over-heated - try a colder plug.

If the plug has WET black deposits - it's from over-oiling - either bad rings or too much oil in the gas. Or you need a hotter plug.

If the plug has DRY black deposits, it's from too much gas - the mixture's way too rich (or you forgot to open the choke). Adjust the carburator.

PS If you have antique machinery, you probably foul plugs from bad rings, etc., and nothing cleans up a spark plug like a made-for-the-purpose spark plug sandblaster.

Check out

Mine's 50 years old, but it looks just like the one on Amazon. $20, and you can make your spark plugs like new anytime you feel like it!


Question. I am currently using the #4, however I was wondering if this is the correct plug. I did lean out the carb recently to compensate for the higher altitude I moved from sealevel to 5000' , I checked the plug and the color seems to be ok. #2, #3, #4 My understanding is #4 will be a colder plug ?
Thanks any info would be great. Jeff