Johnson Service Bulletin SB-853 - Exhaust Gas Induction and Rough Idling

June 28, 1961

Bulletin No. SB-853 Trouble Shooting

SUBJECT: Engines inducting own exhaust gases through carburetor cause ... (a) RPM loss at full throttle, and/or (b) Rough idling (supplement to SB-838) 35 h.p. models RD thru RDSL-21 & 21B; 40 h.p. models RD thru RDSL-22 thru 23; 50 h.p. models V4 thru V4SL-10 thru 11B; 75 h.p. models V4S & V4SL-12 thru 13C; 75 h.p. models V4A & V4AL-13 thru 13F.

RPM LOSS AT FULL THROTTLE OPERATION: From time to time reports have been received that some engines would lose rpm while running on boats at full throttle with the shroud's front door closed. Upon opening the door, the engine would regain the specified full-throttle rpm until the door was again closed whereupon rpm loss returned.

FIG. 1 - Illustrating location of exhaust housing ring seal on V-75 engines. Typical of subject 35, 40 & 50 h.p. engines except on 35 & 40 h.p. models shift rod connector inspection is on starboard side and ring seal is located above inspection holes.

Cause and effect: While there are many reasons for engines losing rpm at full throttle, we have determined the foregoing situation is frequently the result of damaged exhaust housing ring seals, figure 1. When this seal is damaged in such a manner that it fails to completely seal the space between the outer exhaust tube and the exhaust housing covers, a path is provided up to the powerhead through which the engine's own exhaust gases can pass and ultimately be inducted through the carburetor. This induction of exhaust fumes (carbon monoxide) unbalances the fuel/air ratio which tends to starve the engine of the oxygen required for efficient combustion. As a result, engine speeds may occasionally drop as much as 2000 rpm depending upon the amount of exhaust fumes inducted; (only a very small amount is required to accomplish this). Upon opening the front door the fuel/air ratio again becomes balanced to the engine's requirements which in turn restores full-throttle rpm.

Remedy: Note that opening and closing the shroud's front door is an indication the engine's exhaust housing ring seal may be damaged. Whenever you receive customer complaints of rpm loss at full throttle, first check the complaint by alternately opening and closing the front door. If rpm are lost with the door closed but are regained so the engine runs at the specified rpm with the door open, check the condition of the exhaust housing ring seal before checking other possibilities such as ignition, carburetion, compression, etc. Should inspection reveal a ring seal damaged so as to allow the slightest amount of exhaust gases to

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pass by, replace the ring seal and be certain it does not become damaged in re-assembling the exhaust covers.

To eliminate the possibility of inducting exhaust gases leaking from other points on the engine check points 1 through 5 contained under "ROUGH IDLING" as follows below.

ROUGH IDLING: Rough idling is also occasionally caused by induction of exhaust fumes. This however occurs as a result of exhaust gases being expelled at the exhaust housing's inspection plate for the shift rod connector, and is evident only at idling rpm. Its effect on fuel charge combustion is the same as explained under "Cause and Effect" for rpm loss at full throttle operation, except it occurs at idling speeds. At higher rpm the exhaust gases by-pass the inspection plate and are expelled through the exhaust outlet in the normal manner. Always avoid the foregoing possibility by checking the following causes and remedies.

FIG. 2 - Illustrating RIGHT position of exhaust relief boot in exhaust housing cover on V-75 engines. Typical of subject 35, 40 & 50 h.p. engines. (See Fig. 3)

FIG. 3 - Illustrating WRONG position of exhaust relief boot in exhaust housing cover. (See Fig. 2)

Causes and remedies of rough idling: 1. No gaskets under shift rod connector inspection plates (INSTALL GASKETS). 2. Faulty inspection plate gaskets, (REPLACE). 3. Loose inspection Plates, (TIGHTEN). 4. Exhaust relief boot not securely clamped over exhaust relief outlet on the exhaust housing, (TIGHTEN OR REPLACE AS NECESSARY). Figure 1. 5. Free end of exhaust relief boot incorrectly positioned (cocked) in the rear exhaust housing cover, (SEAT FULLY IN EXHAUST HOUSING COVER'S TAPERED RECESS). Figures 2 and 3. 6. Damaged or faulty exhaust housing ring seal, (REPLACE). Figure 1.

If the engine continues to idle roughly after these points have been checked and/or corrected as circumstances dictate, refer to service Bulletin #SB-838 on the subject of "Trouble shooting rough idling", and proceed checking the various points contained therein.

JOHNSON MOTORS W. W. DeWees Service Department

WWD:eb

NOTE: Post on your RD (35 & 40 h.p.), V-50 & V-75 indexes.