Johnson Service Bulletin SB-937 - Spark Plug Fouling, Bottom Cylinder

Issued January 14, 1963, this Johnson bulletin explains how a sagging crankcase-to-cylinder drain hose can cause oil accumulation and spark plug fouling in cylinder No. 4 on V-75 engines. The factory photograph illustrates the recommended hose clamp arrangement.

Johnson Motors Service Bulletin letterheadFactory figure 1 showing the V-75 drain hose secured to the by-pass cover
January 14, 1963
Bulletin No. SB-937 Trouble Shooting
SUBJECT: SPARK PLUG FOULING - BOTTOM CYLINDER. All V-75 engines.
Field reports indicate that some V-75 engines have developed a miss on #4 cylinder due to excessive oil on the spark plug. This problem has in some instances been traced to a sagging of the crankcase - to - cylinder drain hose which provides a low spot for the build up of sludge. Obstructing the passage in this hose prevents the oil bleed- off system from operating properly and allows oil to back up into the crankcase of the lower cylinder. The oil is then drawn from the crankcase into #4 cylinder and causes spark plug fouling.
Any engine showing this condition can easily be corrected by cleaning the inside of the hose, part #307139, and anchoring it to the by-pass cover as shown, using clamp #309041. The clamp will eliminate the low spot and allow normal operation of the bleed-off system.
FIGURE 1.
Other conditions that can contribute to spark plug fouling on #4 cylinder are: a ruptured fuel pump diaphragm, clogged bleed-off passage screens in leaf plate, bleed-off valve back strip installed backwards and a loose lower crankcase head.
Part #309041 clamp has recently been incorporated on all current production 75 h.p. engines as is shown in figure 1.
Sincerely,
JOHNSON MOTORS
Robert Compton Service Department
RC:eb
NOTE: Post on your V-75 index.

Parts reference

The factory identifies crankcase drain hose 307139 and retaining clamp 309041. The bulletin also lists other potential causes of No. 4 cylinder spark plug fouling.