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speed it will most often experience.

   A. Carburetor Sizing
  The importance of correctly sizing the primary side of your marine carburetor cannot be over rated. The primary must be capable of passing enough fuel and air to satisfy engine load requirements during most cruise mode operations. If you use your boat in a manner different than that anticipated by the manufacturer, a different sized primary carburetor may be required. Racing and ski tournament boats are two examples of load application which may require larger sized primary and secondary carburetors. However. in most applications, the carburetor which is supplied by the engine/ boat builder will suffice for a variety of engine/vessel usage. It is well to bring your vessel to its best cruising speed, have an observer open the hatch and indicate the primary throttle position, for all cruising should be done on the primary only. Once this speed/ throttle position is noted. open the helm throttle until the secondaries just begin to open, noting this engine speed and its resulting vessel speed as maximum throttle for best part throttle fuel consumption.

B. Main Metering Jet Selection vs. Manifold Vacuum and RPM
   Once a prudent cruise speed has been chosen for a particular vessel/engine combination, best main jet selection begins. Best manifold vacuum at cruise will be our indicator as it was in our idle set procedure The best main jet for part throttle (secondary throttle closed) will be that main jet which gives the highest manifold vacuum at a given engine speed (RPM) It follows that when best power is made, the throttle will be further closed. and the highest manifold vacuum will be recorded for a given vessel speed. When selecting best jet, care should be exercised not to go too lean. Engine temperature is a good indicator. If temperature begins to climb, use 20^o F maximum, as a rule of thumb go no leaner in jet size. The richer end of the jet selection is harder to find, but best vacuum will be found in a 2 or 3 jet range, without serious implications to the engine.

   C. Power Valve Selection
Simply put, the best cruise performance should be with the power valve closed. This will change depending on the load and speed of the vessel. Only in special cases, as in pulling skiers or other towing, will it be necessary for the primary power valve to be selected with a number above the cruise intake vacuum. In towing skiers, watch engine temperatures. If. after long distance pulling, the engine temperature begins to climb, the added fuel through the power valve will help cool the engine as well as deliver a bit more power. For example, a 351 Ford engine in an 18 foot ski boot would cruise pulling one skier at a 9" hg manifold vacuum. If this resulted in an  increase of 20^o F  in engine operating temperature from the same boat without a skier. the power valve selected should be 9.5" hg. This will allow added fuel through the power valve and would lower the engine temperature to the accepted range.

   D.  Flame Arrestor Size Selection
There is an old adage in marine calibration and it goes like this: "All marine engines must have a Coast Guard approved flame arrestor, and some arrestors are not properly suited for performance marine engine use." Tests have shown that flame arrestors have more to do with top vessel speed than some propellers. The flame arrestor is supposed to limit flame propagation in the event of a backfire, but some flame arrestors also seriously limit the amount of air available to the engine. The result is lower vessel speed and lower engine operating speeds. Engineering tests on a 305 Chevrolet engine revealed an increase of 300 RPM at wide open throttle by changing to a larger, less restrictive flame arrestor. Suffice it to say. in the case of approved flame arrestors. "bigger is better.

   E. Vessel Factors
Vessel factors affecting best cruise performance, in order of importance, are:
   1. Vessel trim
The attitude of the vessel while underway is directly related to the amount of fuel consumed and horsepower required. By changing the trim of a boat by means of hydraulic trim tabs in the case of an inboard engine, or by tilting the outdrive in an  I/O vessel and observing engine speed. one can select the best trim for most economical cruising.
   2. Hull condition
Marine growth on underwater hulls is probably one of the most overlooked reasons for reasons for ass in cruise performance A growth of algae in fresh water or a case of barnacles in the ocean can slow a boat down by 5 mile per hour without ever showing itself. The wise boater keeps his hull clean and free from underwater growths and realizes his effort by increased performance and decreased fuel consumption.

   3. Propeller sizing and condition
Propellers, as stated in the introduction. are the means by which horsepower is transmitted from the engine to the water. Volumes have been written on propeller selection, and it is not the intent of this guide to teach this subject. The load of the propeller increases as a cubic relation to speed. and is the limiting factor of final engine speed. Pitch and diameter can be varied to increase vessel speed, but normally. the propeller selected by the manufacturer covers the greatest range of uses by a particular hull design. Perhaps the condition of the propeller itself is more noteworthy to the boater. A small dent. a dulled edge caused by shallow water sand, or a knicked blade caused by floating debris can seriously affect propeller performance. These imperfections can be so small as to cause no vibration, the usual tip that underwater gear is faulty, yet can increase propeller slip by 10 to 15% A stainless steel propeller will, because it is much harder, last longer than an aluminum propeller and may be a boater's answer to damage. It will also because it is stronger, stay sharper. longer and keep propeller slip to a minimum.

IV. TUNING FOR WIDE OPEN THROTTLE
PERFORMANCE (SECONDARY THROTTLES OPEN)
   This section deals with the helm throttle completely open and the Vessel on water calm and unobstructed to allow the boat to reach its wide open speed. Typically. this speed is somewhere between 4,000 and 5,200 RPM. Great care must be exercised during this testing to stay clear of traffic, and to watch for underwater hazards.

   A. Carburetor Sizing
Your marine engine is basically a pump. which pumps air and fuel into the combustion chamber to be burned to produce power. Once the wide open throttle speed for a particular vessel is measured. a simple formula can help determine the required air flow of the carburetor.

Air flow in CFM = WOT RPM x Engine displacement in³ ft³
                                 1728 x 2    REV              in³

For a 454 CID marine engine which revs to 4,400 RPM:
               Airflow =        4400x454         =578 CFM
                                      1728 x 2
                   (assuming 100% volumertric efficiency)
  Which simply says: This 454 engine used 578 CFM at a WOT cruise speed of 4,400 RPM assuming no pumping loses." For proper operation, a carburetor which delivers 600 CFM would be adequate for this engine: but, in practice, larger carburetors are used. It may be desirable to use a smaller carburetor or larger carburetor depending on whether part throttle or wide open throttle operation is most often used. Other factors which affect carburetor size selection are engine compartment temperature (causes air density changes) and exhaust system back pressure (causes intake charge dilution).

   B. Secondary Diaphragm Operation
Your Holley marine carburetor has diaphragm operated secondary throttles. which means as indicated by engine speed and load, the air flow through the primary venturi actually opens the secondary at a predetermined point. As the secondary carburetor throttle begins to open, you will notice an increase in engine speed until the plates are wide open, and then the vessel will stabilize at its wide open throttle speed. The secondary opening point can be altered by changing the spring under the cover to either a lighter (earlier opening) or heavier (delayed opening) spring. The Spring Kit 20-13 will provide a spring range for almost any opening point. The spring is accessed by removing the secondary diaphragm housing, then removing top cover screws and the diaphragm spring is under that cover above the rubber diaphragm. As mentioned earlier, Holley makes a kit for a quick change of that spring without having to remove the diaphragm housing from the carburetor body (Kit No, 20-59). In the case of dual engine or multiple engine, to insure each engine goes to wide open secondary when the helm throttle is advanced, the secondary diaphragm housings must be all tuned to the same signal vacuum.
  Manufacturing tolerances in the engine and carburetor manufacture cause sufficient differences that will be eliminated by connecting the vacuum diaphragms together. Holley Part Number 20-28 provides the means to connect each diaphragm with its counterpart on the other engine, to insure each assembly is responding to the same vacuum (balanced). Once connected with a vacuum hose, the engines will respond in unison to the helm throttle wide open position.

  C. Power Valve Selection and Main Metering Jet Control
As discussed earlier in Section III. with the best main jet selected for cruise performance the power valve is now open to provide fuel for wide open power. The timing (opening point) of the power valve should be numerically greater than the manifold vacuum reading of the engine at wide open cruise. To check this, simply connect the vacuum gauge used in "Setting Best Idle" and read the wide open throttle vacuum.
Since the marine engine is really "putting out" at wide open, additional fuel calibration should not be attempted without consulting your dealer. Since the real horsepower output of your engine is near its rated maximum, fuel changes at that point could do more harm than good.   

   D. Flame Arrestor Sizing
Once you have recorded the manifold vacuum at wide open throttle, your flame arrestor can be rated. The importance of the flame suppression during a

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