outboard motor parts

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The fuel system on an outboard can be a scary thing to observe by the average person, simply because it looks so different than what they may be used to. Let me assure you though, it is actually VERY simple in design until you get to fuel injection applications. Fuel injection troubleshooting requires special tools to diagnose, so this section will not mention anything other than changing fuel filters on a regular basis when it comes to that subject.
The fuel system contains much the same items as an inboard/outboard such as fuel tank, fuel hose and squeeze bulb Figure 1, fuel pump Figure 2, and carburetor Figure 3. The fuel pump and carburetors however, are very much different in design and operation.
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Fuel Pump

The fuel pump of an outboard does not operate off of a mechanical source as that of an inboard. An internal diaphragm operates from the pulse vacuum of a piston's up and down motion. This is usually the job of the bottom cylinder on most models. A check valve Figure 4 located on the side or front of the powerhead is in charge of controlling the pulse to the fuel pump via a rubber hose. Problems with a fuel pump can include a worn or punctured diaphragm, a bad check valve, or even low compression of the cylinder of the engine that supplies the vacuum. The squeeze bulb is required to be pumped up to a firm state by hand prior to starting motor. If squeeze bulb will not pump up to a firm condition, it most likely needs to be replaced, as the inner check ball is probably bypassing fuel. If you suspect the motor is not getting fuel (assuming that squeeze bulb is in proper condition and operation), first check to see that all hose connections are tight and not leaking air into the system as it attempts to draw fuel from the fuel tank. If all looks ok, next run a quick compression check on the the cylinder that controls the fuel pump. A low or 0 compression reading can show cause for a "starving for fuel" condition. If compression checks ok, its probably a sure bet that the fuel pump diaphragm is worn or punctured. Remove pump and inspect diaphragm closely.
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Carburetor

Although a few models have been manufactured using an accelerator pump, most outboard carbs are simply vacuum operated in design Figure 5. At idle, a carb is in the low speed circuit of operation, and an orifice (jet) is used to meter fuel flow as required. As the throttle is advanced, two things happen. First, the throttle linkage contains an arm which advances the ignition timing during movement at a slight instant before the butterfly of the carb opens. This is commonly referred to as the "idle timing". If the ignition timing was not advanced in this manner, the motor would cough and die immediately upon acceleration. Secondly, as the throttle is advanced, the high circuit of the carb (controlled by vacuum condition) takes over automatically, and the fuel is routed through the high speed orifices (jets) to supply fuel/air mixture on demand. Most smaller outboard carbs have a fuel/air idle mixture adjustment, and most larger outboards are metered by an orifice and idle air/fuel mixture cannot be adjusted, nor does it require to be unless operating at high altitudes. In that case, the orifices would have to be changed accordingly. The main problem usually associated with outboard carbs is the varnishing of oil from prolonged storage. A carb overhaul is probably the most common repair performed by repair shops. This can be avoided by performing proper storage procedures. (See "winterizing section")

All outboards require a choke when cold starting. Without a choke in proper operating condition, it is just about impossible to get one to start. Chokes can come in two different designs, either a solenoid operated flapper type Figure 5, or a solenoid operated injection type Figure 5. The flapper type choke should be regularly inspected for proper adjustment. By engaging the keyswitch with key in "on" position, you should distinctly hear the flappers operate with a "click" sound. If they do not, remove the air box from the carb/s and check to see that solenoid is getting power when key of ignition switch is depressed, and that the plunger is not sticky or missing.


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The solenoid injection type choke Figure 6 requires no maintenance other than checking regularly for leaking fuel. With this type of choke, you still hear a "click" when activated, but the "click" is VERY faint in volume. You have to listen pretty close to hear it. If you do not hear the solenoid "click", check to make sure that proper voltage is being supplied. The keyswitch should be depressed for about 15 seconds before cranking motor for proper operation and starting during cold start conditions. If starter motor ever fails, the manual valve can be opened for easy rope start choke needs.

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Fuel System Tips

  • Use only 93 octane or better fuel for best overall performance and internal carbon buildup prevention
  • Check squeeze bulb and fuel line for proper operation and good condition
  • Make sure vent is open if using a portable fuel tank
  • Make sure choke is in good operating condition
  • Winterize boat and motor following proper procedure to prevent carbs from becoming clogged with gunk
If you should have any questions about your fuel system that may not be answered in this section, feel free to email us. We will gladly help you in any way that we can.