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My Harley-Davidson reprint K Model operator’s manual does not specify what spark plugs (or gap) these bikes should have. Rather, the owner is advised to go to their local Harley dealer. That may be good for business, but does not help the owner who wants to maintain his machine.
The spark plugs recommended by my local Harley dealer, who is an experienced KHK rider, was (of course) Harley’s own brand with the number designation “3-4.” Because the need for a spark plug can come at a time and place where a Harley parts counter is not available, it is necessary to find out just what sort of spark plugs will work on the Harley-Davidson Model KHK.
The next place (after the Harley dealer) to look is to see what plugs are in the bike now. A local mechanic had put Champion N5C (Autolite AU996 is similar) in the engine. This is a long reach plug with threads extending well into the combustion chamber. The ceramic center is below the lip of the threads. The center electrode has about the same exposure as the H-D 3-4. I like having the spark deep in the combustion chamber, but am uneasy about having exposed threads that might crud up and make the spark plugs difficult to remove.
Fortunately, Vintage Iron Riders has several K and KH model riders who are willing to share their knowledge about these bikes. Harvey is using the Harley number 7 plug; however, H-D no long has that plug.
Emerson uses the Champion J8H. The local auto parts stores tell me they cannot get this plug. The J8C plug used for flathead Briggs and Stratton lawnmower engines (also made in Milwaukee) is similar.
Mike S. runs the daylights out of his KH motorcycles. He uses NGK B7HS or B6HS plugs. The higher number is supposed to be the hotter plug; however, Mike does not see any difference in service or performance. If the NGK plugs are not available, Mike will use the Autolite 85 or Champion J12Y.
When I compared the Champion J12YC (the J12Y with a copper core) to the Harley 3-4, I found they were identical. In fact, they both had “876”) stamped on the base. Apparently, they are both made by Champion. The J12YC is made in Mexico and costs $2; whereas the Harley 3-4 is made in USA and costs $4.
Because I am running through spark plugs in an attempt to tune the Linkert carburetor on my KHK, guess which plug I have elected to use? When Mike Scarborough and the local Harley shop (and KHK) owner both agree on essentially the same plug, who am I to disagree? After a recent 300+ mile run this fall, the KHK has had no plug related problems with the J12YC plugs. Well, there is a soot problem that I will ask for guidance on in a later post.
By the way, the Harley 3-4 plugs come with a ~0.03+ gap; so, that must be the correct setting for that plug.
The spark plugs recommended by my local Harley dealer, who is an experienced KHK rider, was (of course) Harley’s own brand with the number designation “3-4.” Because the need for a spark plug can come at a time and place where a Harley parts counter is not available, it is necessary to find out just what sort of spark plugs will work on the Harley-Davidson Model KHK.
The next place (after the Harley dealer) to look is to see what plugs are in the bike now. A local mechanic had put Champion N5C (Autolite AU996 is similar) in the engine. This is a long reach plug with threads extending well into the combustion chamber. The ceramic center is below the lip of the threads. The center electrode has about the same exposure as the H-D 3-4. I like having the spark deep in the combustion chamber, but am uneasy about having exposed threads that might crud up and make the spark plugs difficult to remove.
Fortunately, Vintage Iron Riders has several K and KH model riders who are willing to share their knowledge about these bikes. Harvey is using the Harley number 7 plug; however, H-D no long has that plug.
Emerson uses the Champion J8H. The local auto parts stores tell me they cannot get this plug. The J8C plug used for flathead Briggs and Stratton lawnmower engines (also made in Milwaukee) is similar.
Mike S. runs the daylights out of his KH motorcycles. He uses NGK B7HS or B6HS plugs. The higher number is supposed to be the hotter plug; however, Mike does not see any difference in service or performance. If the NGK plugs are not available, Mike will use the Autolite 85 or Champion J12Y.
When I compared the Champion J12YC (the J12Y with a copper core) to the Harley 3-4, I found they were identical. In fact, they both had “876”) stamped on the base. Apparently, they are both made by Champion. The J12YC is made in Mexico and costs $2; whereas the Harley 3-4 is made in USA and costs $4.
Because I am running through spark plugs in an attempt to tune the Linkert carburetor on my KHK, guess which plug I have elected to use? When Mike Scarborough and the local Harley shop (and KHK) owner both agree on essentially the same plug, who am I to disagree? After a recent 300+ mile run this fall, the KHK has had no plug related problems with the J12YC plugs. Well, there is a soot problem that I will ask for guidance on in a later post.
By the way, the Harley 3-4 plugs come with a ~0.03+ gap; so, that must be the correct setting for that plug.
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