Re: Spark Plug 1975 15hp?
I also have a 1975 15HP Johnson and have had a lot of trouble with the spark plug fouling. They did not redesign the head to help the problem until 1977.
Here is the plugs I use. UL81C in the right plug but I do not know the new Number.
Orignal Spark plugs was a Champion L7J, Later the plug changed to a RL82C and is now a Champion 874 or a L82C now a champ 811. The Autolite plug is a 4092 or 4093.
Plugs one heat range higher is Champion L86C.
Plug two heat ranges higher is a Champion L90C now a Champion 896 and crosses to a Autolite 425.
I use the Champion 896 which is the Champion L90C. This plug is two heat ranges higher and has no resistor but then I do a lot of trolling or useing the motor to slow our drift when offshore fishing with high winds.
NOTES:
1. All spark plugs are Champion brand unless otherwise specified.
2. When more than one plug number appears, the first plug listed is the prefered spark plug.
3. On ALL VRO/OMS equipped motors the "QL" supressor plug is recommended.
* This chart was assembled with the latest available Parts Catalogs, Service Manuals and Service Bulletins as of the
revision date. If any discrepencies are found, please notify the moderator.
Revised: 2/24/2005
Horsepower Year Spark Plug Number
(see notes 1 and 2) Gap (in.)
10, 15 Commercial 1994 - 2001 QL82C .030
14 1988 - 1989 QL77JC4 .030
14 Commercial 1981 - 1991 QL77JC4 .030
15 1974 - 1976 UL81C .030
15 1977 - 1992 QL77JC4 (or L78V Surface Gap) .030
9.9 & 15 Big Bore 1993 - 2001 QL82C .030
15 1954 - 1956 J6C .030
15 Commercial 1992 - 1993 QL77JC4 .030
15 (4-Stroke) 1995 - 2001 RA8HC .030
15 (4-Stroke w/Mikuni Carburetor) 2003 - 2004 NGK DCPR6E (see note 1) .031
15 (4-Stroke w/Keihin Carburetor) 2005 & newer NGK DCPR6E (see note 1) .031
15 (Kerosene) 1995 - 2001 UL77V Surface Gap
18 1957 - 1973 J4C .030
18 Jet (25hp powerhead) 1995 - 2001 QL77JC4 or L78V or QL82C .030
20 1966 - 1973 J4C .030
25 1952 - 1955 J4C .030
25 1969 - 1976 J4C .030
25 1977 QL77JC4 or QL82C .030
20, 25, 28, 30 1978 - 2001 QL77JC4 or L78V or QL82C .030
