Topic: 1947 Johnson PO-15 Question – Antique Outboard Motor Club,Inc

This topic has 3 replies, 4 voices, and was last updated 8 years, 10 months ago by garry-in-michigan.

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June 2, 2015 at 3:50 pm #1663
seaflash
Participant
My 1947 PO-15 manual states that Champion R-7 was the spark plug of choice back in the day. Recent cross reference of this out of date plug came up as Champion D8.

As can be seen below with the original Champion R-7, the center electrode and ground electrode are arranged as so:

The recently cross referenced Champion D8 center electrode and ground electrode are configured differently as pictured below:

I may be wrong (not the first time), however I believe I remember reading on this board a while back that the ground electrode on the D8 may be filed back (somewhat) thus exposing the center electrode. If this may be the case, then approximately how much of the ground electrode should be filed back?

Thank you in advance for any input.
June 2, 2015 at 9:40 pm #17457
Mumbles
Participant
On the older J series plugs, the ground electrode runs about halfway across the center electrode. This was done to help prevent carbon and other particles from jamming between the electrodes and shorting them out. Carbon is less of a problem now with TCW III oil so shortening the side electrode might not be necessary.
June 2, 2015 at 9:48 pm #17458
jerry-ahrens
Participant
US Member
On my PO 15, I filed the arm back to where the end of it stopped in the middle of the center electrode.. I’ve been running it this way for years.
June 4, 2015 at 3:32 pm #17594
garry-in-michigan
Participant
Lifetime Member
From a 1948 Champion spark plug booklet –

I always file my new sparkplugs to convert them to the old "J" type for two cycle engines. Be sure to leave a sharp edge, as this increases the efficiency. Some racers go so far as to adjust the plug gasket so the spark gap faces the incoming charge.
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