HOW TO POWER TUNE YOUR BOAT

       
    

INTRODUCTION
  Boat Power Tuning requires two very essential tools a good tachometer to read engine speed and a manifold vacuum gauge which will be an indicator of engine load. The vacuum gauge should read from O inches of Mercury to about 30 inches of Mercury. Propeller selection is varied, but both pitch and diameter changes can be made to improve overall speed and performance.

   HOW ARE PROPELLERS SELECTED?
  The boat manufacturer has a wide selection of propellers to choose from and does so to optimize Fewer (ski boats) or speed (cruisers). The first basic to remember is that propellers are listed by their diameter and pitch, in that order. So, a 16 X 18 propeller is 16 inches in diameter and has a pitch of 18 inches which means the distance the propeller would travel through a solid in one revolution. However, water is not a solid and propeller slip which is roughly 30% will effectively lower the pitch. Propellers are also right hand (clockwise rotation) or left hand (counter-clockwise rotation). More will be said about propellers in the body of this presentation; but for now, your propeller effectively transfers engine power to the water to move the boat.

   I. SETTING BEST IDLE
  Setting the best idle on your marine engine is a very essential step in power tuning your boat. Proper idle speed is very important for transmission/outdrive life, as well as dependable engine operation for docking maneuvers Check your manufacturer's specification for idle speed recommendations  and do not exceed these speeds. Engines which are new have greater frictional losses. and, therefore, require higher "green" engine speeds. These engines will require resetting of the idle speed as the engine is broken in. Your boat will handle best with the slower of these two idle speeds.
  Engine speed at idle depends on two factors: idle air adjustment (idle speed) and idle fuel adjustment, with a good engine speed tachometer, an engine vacuum gauge, and with the engine warmed up to proper operating temperature. Here is the procedure to follow:

   A. First, set engine speed (RPM on tach) to  manufacturer's specification.
Example: 600 RPM.

   B. Then observe vacuum gauge with engine idling  in neutral as idle fuel is set richer and /or leaner. To  assure an initial idle fuel balance, with engine off,  carefully turn each idle adjusting fuel screw in until  it seats in the carburetor  casting before setting  the  speed of Part A. Do not use a large screwdriver or  excessive force since the seat may be damaged  if the screw is turned in too hard. After establishing  that each needle is seated, back out  (counter clockwise) the screws one full turn each.  (This will be the initial set point at which the speed  is set with the idle speed screw.)

   C. Balance each idle fuel screw until the highest  engine vacuum is realized. It is highly likely that  when this point is reached, the engine speed will be  higher than that which was set in Step A, The speed  screw should be reset until it reads true on the tach.  and the idle fuel setting should again be repeated  for best idle mixture at the new speed setting.

   This "best idle speed" will give you control during docking maneuvers without adding undue stress to your transmission or outdrive. Depending on the propeller you have selected, you may find, especially on ski boats with high pitch props, that the boat will perform better while docking with an even slower than recommended idle speed. In any case, select a best idle speed for your hull and prop, and set the best fuel idle mixture to the procedure outlined above.

   II. TUNING FOR BEST ACCELERATION
   How your boat accelerates from idle (no wake speed) to cruise depends on a great variety of factors. some of which are:
-engine size
-boat weight and loading
-wetted hull condition (marine growth)
-water conditions
-fuel octane
-propeller condition
-ignition system
  Assuming that the above list has been carefully addressed, the carburetor can be custom tuned to deliver best performance during accelerations by:

   A. Accelerator Pump Calibration
The function of the accelerator pump is to deliver fuel to complement the idle and main system operation during transition operation. The start of fuel delivery from the pump is as critical as the amount of fuel delivery from the pump. With the engine off, have someone operate the helm throttle, while observing the pump discharge nozzles in the primary venturi. The slightest movement on the throttle linkage should result in a delivery of fuel from the discharge nozzle (pump "tip in"). If there is a lag, accelerations will be compromised. By adjusting the pump override screw, the pump can be set to deliver fuel as the throttle is opened. One caution here is to check the travel left at the pump lever at wide open throttle, where there should still be some pump diaphragm travel (approximately .020).

  After adjusting the pump "tip in", take your boat out to an area where accelerations can be conducted in a safe manner, and accelerator pump capacity testing can be done. Pump Cam Kit Number 20-12 provides a wide range of accelerator pump cams which have cam ramp angles to vary fuel delivery versus throttle angle. Using the chart provided with the Kit, note which pump cam comes standard with your carburetor. With the help of a stop watch and a "third eye" on the tach, run the boat from idle to 2,000 RPM and check the elapsed time. Try cams with capacities above and below the standard cam and record the resulting elapsed times. The best idle to 2,000 RPM speed times will indicate the best cam for your vessel.
   If, while running the above test, any bogs or hesitations occur, go no lower (or higher) on the pump cam selection. Your boat has very simply told you that it needs more (or less) fuel than last cam selected.

   B. Correct Secondary Operation
Once you have tailored your accelerator pump to your vessel, the higher speed accelerations are most affected by secondary throttle opening. A secondary diaphragm "motor'' opens the secondary throttle according to a calibrated diaphragm spring The vacuum opens the valve, and the spring closes it.
  A variety of these springs are available in Kit Number 20-13. The explanations contained in the spring kit rate the springs by "relative load". Simply put, the lighter the load, the earlier the secondary throttle opens. If your boat runs at 4,400 RPM at wide open throttle, a good rule of thumb would be to have your secondaries calibrated to be wide open at about 3,800 RPM engine speed. Of course, the secondary throttle will begin to open much sooner, about 2,900 RPM, and this opening will improve vessel acceleration and will be audible with the engine hatch open. If the secondaries open too soon (lower RPM), a bog or hesitation will result. If they open too late (higher RPM), fuel distribution to the engine suffers. It has been our development experience that springs heavier in load than the plain spring are not required in marine engines.
  With stop watch in hand, and a safe course for acceleration, time your speeds from 2,000 RPM to wide open throttle for best secondary diaphragm spring selection.

  Holley has developed a very handy Kit Number 20-59 which facilitates the removal of the cover and the replacement of the diaphragm spring, without removing the choke assembly. So, should conditions exist where continued change of that spring is indicated, try one of these kits for a real time saver.

   C. Main Metering Jet Selection
Your Holley carburetor has been developed to supply fuel for correct engine operation, and should not require main metering jet changes for most acceleration calibrations. If, however, operating conditions or engine displacement to hull length combinations require fuel delivery changes, the changes should not result in main jet changes of more than four sizes up or down from the standard main let supplied.

   II Power Valve Timing and Power Results

   By definition. a power valve is a valve which opens at a given manifold vacuum and delivers fuel to match power requirements at or near maximum engine speeds. This valve is closed at high manifold vacuum and opens at a set lower vacuum. Fuel consumption when the valve is opened is from 25 to 40% higher than that at part throttle (valve closed). It is very important that at all cruise speeds, the power valve be closed for maximum fuel economy. Using the vacuum gauge from the idle set section, record the manifold vacuums and engine speeds at cruise speeds. A typical small block Chevrolet might cruise at 2,800 RPM at an 8" manifold vacuum. In this case. the power valve should be selected below on 8" vacuum range. Example: 5.5" Hg opening.
During accelerations, the power valve should open according to the load of the engine. A light throttle operation will be clean and economical without the power valve opening. The acceleration would be made from 17" hg to 10" hg manifold vacuums. Deeper throttle accelerations requiring more power from the engine will be made below 8" manifold vacuum and, consequently, require the power valve to be open. Holley's single stage power valves are available in l" hg vacuum increments from 2.5" hg to 10.5" hg ranges. Again, a good rule of thumb is to select a power valve number (opening vacuum) which is at least 2" hg lower than your vessel's cruising manifold vacuum.

   III. TUNING FOR BEST CRUISE PERFORMANCE
   (SECONDARY THROTTLES CLOSED)
   A boat engine is much different than the engine in the family car. A vehicle rolling down the highway at 55 miles per hour needs perhaps 15 to 18 horsepower to maintain vehicle speed. A boat, on plane, cruising at 30 miles requires 10 times that horsepower. The marine engine works much harder than the family car engine and therefore, should be tuned for best performance at the

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