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CARBURETOR MIXTURE ADJUSTMENT
• See Figure 76
Each individual carburetor body contains either an adjustable idle mixture screw or a fixed orifice, depending on the year and serial number of the powerhead. A fixed air bleed orifice, found immediately above the idle mixture screw or orifice, controls the intermediate mixture. A fixed orifice in the fuel float chamber controls the high-speed mixture. The single float fuel chamber maintains the proper fuel level in the carburetor bowl for all power settings.
• For models not equipped with any mixture screws, see a knowledgeable Evinrude/Johnson parts counterperson for information on alternate orifices to compensate for high·altitude operation.
On models equipped with an idle (low speed) mixture screw, the. carburetor idle mixture can be adjusted to compensate for changes In the fuel system due to carburetor rebuild or replacement or ~hang~s in operating conditions such as moving from sea-level to high-altitude. In order for the adjustments to occur properly the motor must mounted on a . launched boat that is operating and unrestrained. You'll need an assistant to safely navigate the craft while the adjustment is being made.
• If the engine is not operating under normal exhaust back·pressure (which occurs from the gearcase operating under normal conditions, submerged below the transom), mounted at a normal trim angle, with the correct propeller installed and the boat unrestrained, proper mixture adjustment will not occur.
This adjustment is NOT a periodic maintenance item and should not be touched unless all other attempts to resolve an idle speed operation problem have failed. The ignition and fuel system components must all be in good condition and operating properly. The carburetor linkage must be properly adjusted before attempting this procedure, for details please refer to Timing and Synchronization in the Maintenance and Tune-Up section.
1. With the engine top cover removed for access (and, on most models, the air intake silencer as well), make a matchmark between the carburetor body and idle mixture screw. . 2. Start the engine and allow it to idle until normal operating temperature is reached. . 3. Once warmed, shift the engine in forward and run at Idle speed for 3 minutes. 4. If the adjustment is too lean, the engine will sneeze and backfire; to correct this, note the reference mark, then rotate the idle mixture needle (low speed screw) 1/8 turn counterclockwise. Wait 15 seconds for the . engine speed to stabilize before turning the screw again. Turn the screw until you reach the highest steady engine speed. 5. If the adjustment is too rich, the engine will be rough an unsteady; to correct this note the reference mark, then rotate the idle mixture needle (low speed'screw) 1/8 turn clockwise. Wait 15 seconds for the engi~e speed to stabilize before turning the screw again. Turn the screw until you reach the highest steady engine speed.
6. Allow the engine to run at idle in gear for 3 minutes, then move to the next mixture screw. Repeat until all carburetor body screws have been adjusted. . 7. Run the engine at or near Wide Open Throttle (WOT) for 3 minutes and then reduce speed to idle, leaving the motor in gear. The motor should not stumble, spit or backfire. If any of these problems are found, repeat the adjustment procedure, making small adjustments only, until the engine operates normally.
