Abstract:
An improved and simplified outboard motor construction wherein the cooling and exhaust systems for the engine are formed with a minimum number of components and sealing joints and incorporating a non-metallic cam shaft for reduced cost and weight without sacrifice of durability. The exhaust system includes an elongated expansion chamber formed in the drive shaft housing. In addition, the drive shaft housing has a cylindrical section that is journaled within a swivel bracket for its steering movement. The volume between the external portion of the drive shaft housing and the internal portion of the swivel bracket forms a second expansion chamber that is employed for the low speed above the water exhaust gas discharge. The flow of cooling the water to and from the engine is controlled so that the exhaust gas interchange area between the power head and the drive shaft housing will be well cooled, as will the oil reservoir for the engine and the oil returned to it.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a Continuation in Part of our co-pending application entitled: “Engine for Outboard Motor”, Ser. No. 09/111442, Filed Jul. 7, 1998, now U.S. Pat. No. 6,067,951, and assigned to the assignee hereof. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a camshaft construction for the engine of an outboard motor and more particularly to a non-metallic camshaft for a four-cycle, outboard motor engine. 
     Two-cycle internal combustion engines have been frequently used as the prime mover for an outboard motor. The reason for the use of two-cycle engines is because of their compact nature and their high specific output. These features are particularly important in an outboard motor due to the very compact nature of such a propulsion device. 
     However, with increasing concerns about environmental protection, there has been a growing interest in the application of four-cycle engines for many applications that previously utilized two-cycle engines because of their aforenoted advantages. One of the advantages of four-cycle engines over two-cycle engines is also a feature that gives some disadvantages in connection with outboard motor application. 
     With two-cycle engines, the lubricating oil for the engine is generally consumed during engine running. That is, although two-cycle engines may use direct lubricating systems, the oil used for lubrication nevertheless is consumed during engine operation and any residual amounts is discharged to the atmosphere. This obviously has some environmental problems. 
     Four-cycle engines, however, have greater complexity than two-cycle engines, and thus tend to be more expensive. Furthermore, the greater number of moving parts also gives rise to concerns of potential wear and service requirements. 
     One area where such additional components are required and where the components are subject to wear is the valve actuating mechanism for the engine. Unlike two-cycle engines, four-cycle engines generally have poppet-type valves that are operated through an operating mechanism that includes a camshaft. The camshaft either operates the valves directly or through intermediaries, such as push-rods or the like. In any event, the cam lobes are subject to wear. 
     In addition, the camshaft is driven by a timing drive at one-half crankshaft speed, and this requires the provision of some form of timing gear or sprocket on the camshaft. Furthermore, at times the camshaft may be utilized to operate other mechanisms, such as operating the plunger of a fuel pump. Thus, it has been the practice to employ hardened steel shafts for this purpose, and these not only add to the cost, but require high-cost components that are engaged by the camshaft so as to avoid their wear also. 
     It is, therefore, a principal object of this invention to provide an improved, low-cost, long-life camshaft for a four-cycle engine. 
     It is a further object of this invention to provide an engine design that accommodates the use of a nonmetallic camshaft that can be formed from a resinous plastic or the like. 
     SUMMARY OF THE INVENTION 
     This invention is adapted to be embodied in a four-stroke internal combustion engine having a crankcase chamber, a cylinder head and a cylinder block. A valve mechanism is contained in the cylinder head for operating valves associated with a cylinder bore formed in the cylinder block. A valve actuating mechanism including a camshaft driven from the engine crankshaft is employed for operating the valve mechanism. This camshaft is formed from a non-metallic material. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a side clevational view of an outboard motor constructed in accordance with an embodiment of this invention, shown attached to the transom of a watercraft, illustrated in cross-section, and at rest in a body of water in which the watercraft is operating. 
     FIG. 2 is a view looking in the same direction as FIG. 1, but shows certain components of the outboard motor broken away and in section. 
     FIG. 3 is an enlarged side elevational view of the power head with portions broken away and shown in section. 
     FIG. 4 is a cross-sectional view taken through the engine of the power head taken along a plane perpendicular to the plane of FIG.  3  and passing through the center of the cylinder bore. 
     FIG. 5 is a cross-sectional view taken along the line  5 — 5  of FIG.  4  and shows the valve operating mechanism and the mechanism by which lubricant from the splash lubrication system is delivered to the valve chamber of the cylinder head. 
     FIG. 6 is a view of the valve chamber of the cylinder head looking in the direction of the arrow  6  in FIG.  4  and with the valve cover removed. 
     FIG. 7 is a cross-sectional view taken along the line  7 — 7  of FIG.  4  and shows the camshaft drive and decompression device. 
     FIG. 8 is a view looking in the direction of the line  8 — 8  in FIG.  7  and shows the decompression device in the starting mode. 
     FIG. 9 is a view, in part similar to FIG.  8  and shows the condition during normal engine running. 
     FIG. 10 is a view showing the cylinder block with the cylinder head removed. 
     FIG. 11 is a view showing the surface of the cylinder head which mates with the portion of the cylinder block shown in FIG.  10 . 
     FIG. 12 is a side elevational view looking in the same direction as FIG. 3, but showing only the outer peripheral configuration of the powering internal combustion engine. 
     FIG. 13 is a side elevational view of the engine looking from the side opposite to FIG.  12 . 
     FIG. 14 is an enlarged cross-sectional view showing one of the supports for the fuel tank. 
     FIG. 15 is a top plan view showing the support plate portion of the drive shaft housing for the engine in the power head. 
     FIG. 16 is a top plan view showing the configuration of a portion of the crankcase chamber forming member and specifically the oil reservoir therefore. 
     FIG. 17 is a cross-sectional view of this component. 
     FIG. 18 is a bottom plan view of this component. 
     FIG. 19 is a schematic view showing the flow of cooling water through the outboard motor and its return back to the body of water in which the watercraft is operating. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now in detail to the drawings and initially primarily to FIGS. 1 and 2, an outboard motor constructed in accordance with an embodiment of the invention is identified generally by the reference numeral  21 . The outboard motor  21  is shown as being attached to the transom of an associated watercraft The transom is shown only partially in cross-section and indicated by the reference numeral  22 . 
     The watercraft with which the transom  22  is associated and outboard motor  21  are designed so as to be operated in a body of water, indicated at  23  in FIG.  1 . The water level  23  illustrated in FIG. 1 is the water level when the watercraft is relatively stationary. The watercraft is of the planing type and as its speed increases, the degree of submersion of the outboard motor will be reduced, as is well known in this art. 
     The outboard motor  21  is comprised of a power head portion, indicated generally by the reference numeral  24 . The power head portion  24  includes a four cycle, internal combustion engine, which appears partially in cross-section in FIG.  2  and which is identified by the reference numeral  25 . The power head is completed primarily by a protective cowling that is comprised of a lower tray portion  26  and an upper main cowling portion  27 . 
     The outboard motor  21  includes a swivel bracket, indicated generally by the reference numeral  28 . This swivel bracket  28  is generally a tubular member which supports a drive shaft housing and lower unit assembly, indicated generally by the reference numeral  29 , in a manner to be described. This unit assembly  29  is mounted, in a manner to be described, in the swivel bracket  28  so that it rotatably journals the drive shaft housing and lower unit  29  and thus the outboard motor  21  for steering about a vertically extending axis. 
     The swivel bracket  28  is, in turn, connected by means of a pivot pin  31  to a clamping bracket  32 . This pivotal connection permits tilt and trim adjustment of the outboard motor  21  about the pivot pin  31  relative to the hull transom  22 . A trim pin arrangement  33  permits selective setting of the trim angle. 
     The drive shaft housing and lower unit  29  includes a lower housing portion  34  to which is fixed a lower unit housing  35  that contains a conventional bevel gear reversing transmission, indicated generally by the reference numeral  36 . This bevel gear transmission  36  can selectively be coupled to a propeller shaft  37  that is journaled in the lower unit  35  in any suitable fashion. The control for this transmission  36  will be described later, but any known system may be employed. A propeller  38  is affixed to the propeller shaft  37  for propelling the watercraft in a well known manner. 
     The steering support for the outboard motor  21  will now be described in more detail by particular reference to FIGS. 2 and 3. It may be seen in FIGS. 2 and 3 that the drive shaft housing and lower unit  29  is a unitary construction which may be formed from a lightweight material, such as an aluminum alloy or the like. This includes an upper supporting plate portion  39  which is integrally connected to a generally tubular portion  41  that depends downwardly from the powerhead  24  to the lower unit portion  35 . A drive shaft  42 , which is driven in a manner to be described by the engine  25 , extends through this tubular portion  41  and has a bevel gear affixed to its lower end which forms a portion of the bevel gear reversing transmission  36 . 
     The swivel bracket  28  is of a longitudinally split, two-piece construction and has a generally vertically extending cylindrical portion  43  that embraces the drive shaft housing cylindrical portion  41 , but is radially spaced outwardly therefrom so as to define an expansion chamber area  44  therebetween, for a purpose which will be described. 
     This two-piece outer construction defines an upper shoulder  45  and a lower shoulder  46  which extend radially inwardly toward the drive shaft housing tubular portion  41 . Split elastic supporting members  47  are interposed between these shoulders  45  and  46  and a downwardly facing shoulder  48  of the upper support plate portion  39  of the drive shaft housing and a lower, upwardly facing shoulder  49  formed at the upper end of the lower drive shaft housing portion  34 . 
     These elastic supporting members  47  are split so as to be inserted around the drive shaft housing cylindrical portion  41  at the upper and lower ends thereof. Split nylon bushings  51  and  52  are placed between the upper and lower ends of these members  47  and the drive shaft housing shoulder  48  and  49 , respectively. 
     The elastic members  47  have face portions  53  that are engaged with the respective bushings  51  and  52 . A plurality of lightening holes  54  are formed in the hub portion of the elastic members  47  so as to provide lightening and to increase their resilience. 
     When the swivel housing  48  is placed together in embracing relationship around these nylon bushings and the elastic members  47 , there will be provided an effective journaling of the drive shaft housing  29  in the swivel bracket  28  with gas tight seals formed at opposite ends of the expansion chamber  44  for a purpose which will be described. 
     A tiller  55  (FIG. 1) is affixed suitably to the tray member  26  of the protective cowling of the powerhead  24  for steering of the outboard motor  21  about the vertically extending axis formed by the swivel bracket  28 . In addition, a steering lug  56  may be connected to an upper portion of the drive shaft housing tubular portion  41  for connection to a remote steering mechanism for steering of the outboard motor  21  from a remote location. The swivel bracket  28  and specifically its housing member  43  is provided with a slot so as to accommodate this steering motion. 
     The construction associated with the powerhead  24  will now be described by particular reference to FIGS. 2 through 18. Referring first to the engine  25 , its internal construction is shown best in FIGS. 2 through 9 and will be described by principle reference to those figures. The engine  25  is comprised of an engine body having three main portions. These comprise a cylinder block portion  57 , a cylinder head portion  58 , and a oil reservoir forming portion  59 . These portions are connected together in a manner which will be described. 
     The cylinder block  57  defines, in this embodiment, a single horizontally extending cylinder bore  61 . One end of this cylinder bore is closed by an upper crankcase chamber  62 , that is formed primarily by the lower or forward end of the cylinder block member  57  and which is completed by an oil reservoir forming portion  63  of the oil pan forming member  59 . This oil pan forming member  59  is affixed to the lower face of the cylinder block  57  in closing relationship to the cylinder block upper crankcase chamber  62 . 
     A crankshaft  64  is rotatably journaled within the crankcase chamber  62  by means of an upper main bearing  65  that is carried in an upper end face of the cylinder block member  59 . In addition, a lower main bearing  66  is carried by the crankcase forming member  59  and journals the lower end of the crankshaft  64 . This is in proximity a splined coupling  67  between the crankshaft  64  and the upper end of the drive shaft  42 . 
     The cylinder head  28  is affixed to the crankcase forming member  59  and the cylinder block  57  by means of a plurality of threaded fasteners  68 . Thus, the opposite end of the cylinder bore  61  is closed by the cylinder head member  58 . 
     A piston  69  is supported for reciprocation in the cylinder bore  61 . A connecting rod  71  connects the piston  69  to a throw of the crankshaft  64  upon which the connecting rod  71  is journaled in a well known manner. 
     The surface of the cylinder head member  59  that faces the cylinder bore  61  and which closes it is formed with a recess  72  that forms the combustion chamber of the engine with the piston  69  and the cylinder bore  61 . A fuel air charge is delivered to this combustion chamber by an induction system which will now be described, again primarily referring to FIGS. 3 and 12 through  14 . 
     Air for combustion by the engine  25  is admitted to the interior of the protective cowling in a manner which will be described by principle reference first to FIG.  3 . First, it should be noted that the tray portion  29  of the protective cowling is affixed to the upper support plate portion  39  of the drive shaft housing  29  by threaded fasteners  73 . The lower area of the tray  26  is provided with an air inlet slot  74  so that atmospheric air may be drawn into the interior of the protective cowling in the air manner shown by the arrows  75  in this figure. 
     The air flows through the interior of the protective cowling and excess air is discharged through an upwardly facing opening  76  formed in the main cowling member  27 . The main cowling member  27  is provided with a cover plate  77  that extends across the opening  76  so as to block direct water entry thereto, but which also has slotted openings for exit of the air back to the atmosphere as shown by the arrows  75 . Thus, there is provided water separation while permitting adequate air flow for engine combustion and some cooling. 
     This air is then delivered to a carburetor  78  which may be of any known type. If desired, an air silencer may be affixed to the inlet of the carburetor  78  for silencing the intake air. The carburetor  78  receives fuel from a fuel tank  79  in a manner which will be described shortly. 
     The carburetor  78  delivers the formed charge of fuel and air to an intake manifold  81  which communicates with an intake passage  82  formed in the cylinder head  58 . This intake passage  82  terminates at an intake valve seat which is valve by an intake valve  83 . The intake valve  83  is urged to a closed position by a coil compression spring assembly  84  that acts against a keeper retainer assembly fixed to the stem of the intake valve  83  in a well known manner. The intake valve  83  is opened and by a valve actuating mechanism which includes a rocker arm  85  that is pivotally supported in the cylinder head  58 . The valve mechanism described is contained in a valve chamber that is closed by a valve cover  86 . The way in which the rocker arm  85  is operated will be described later by principle reference to FIGS. 4-9. 
     The charge which has been admitted to the combustion chamber recess  72  will be compressed when the piston  69  moves upwardly and then fired at an appropriate time by an ignition system including a spark plug  87 . The burnt charge is exhausted through an exhaust valve seat which is valved by a poppet type exhaust valve  88 . Like the intake valve  83 , the exhaust valve  88  is suitably supported in the valve chamber of cylinder head  58  and is urged to its closed position by a coil compression spring  89 . A rocker arm  91  is associated with the exhaust valve  88  for operating it in a known manner. As has been noted the way in which the rocker arm  85  is operated will be described later by principle reference to FIGS. 4-9. 
     When opened, the exhaust gases can exit the combustion chamber through an exhaust passage  92  that is formed in the cylinder head  86 . As seen best in FIGS. 3 and 11, the exhaust passage  92  extends through a lower face of the cylinder head  58 . There it communicates with an exhaust system formed in initial part by the crankcase forming member  59 . This exhaust system will be described later. 
     The fuel supply system for supplying the fuel to the carburetor  78  from the fuel tank  79  and for permitting filling and charging of the fuel tank  79  will be now described by principle reference to FIGS. 3 and 12 through  14 . First, it will be seen that the fuel tank  79  has a filler neck portion  93  which extends upwardly toward an opening in the main cowling member  27 . A sealing gasket  94  provides a seal between the fill neck  93  and the cowling member  27 . 
     A fill cap  95  is threadedly connected to the upper end of the fill neck  93  externally of the protective cowling member  27 . This fuel cap  95  also has an air vent valve  96 . 
     The fuel tank  79  has a pair of spaced apart boss sections  97  formed on its opposite sides which are juxtaposed to respective lugs  98  formed on the cylinder block member  57 . Elastic grommets  99  (FIG. 14) are interposed between the lugs  97  and  98  and threaded fasteners  101  that mount the fuel tank  79  to the cylinder block  57 . 
     In addition, a recoil starter cover  102  also has lugs  103  that are affixed to the cylinder block  97  by the same threaded fasteners  101 . This recoil starter has assembly  102  has a pull handle  104  that is accessible from the exterior of the protective cowling member  27  for pull starting of the engine  25  in a well known member. In addition, a fly wheel magneto (not shown) may be also associated with the pull starter for generating electrical power for firing the spark plugs  87 . A decompression device, to be described later, functions to assist in pull starting. 
     Continuing to refer to the fuel supply system, the fuel tank  79  has a discharge port  105  that communicates with a first supply conduit  106 . This conduit  106  is connected to a combined shut off, drain valve  107  which, in turn, communicates with a supply line  108 . This supply line  108  extends to an engine driven fuel pump  109 . The drive for this fuel pump  109  will be described later. The fuel pump  109  will deliver fuel under pressure to the carburetor  78  through a supply conduit  111 . 
     Since the fuel tank  79  is mounted within the protective cowling, it will have a relatively small volume. Therefore, an external source of fuel may also be provided for supplying fuel to the engine. This external supply includes a quick disconnect coupling  112  that is mounted on the tray  26  as best seen in FIG.  3 . This coupling  112  includes a quick disconnect shut off valve  113  and a locating pin  114  so as to cooperate with a female coupling that can be connected to a remote fuel tank in a well known manner. 
     This assembly coupling and valve assembly is further mounted on a mounting boss  115  of the crankcase forming member  59  by means of a mounting bracket  116  and threaded fastener  117 . A conduit  118  connects the quick disconnect coupling  112  with the shut off and drain valve  107  and, accordingly, with the tank  79 . 
     The valve operating and lubricating system will now be described by primary reference to FIGS. 3-9. A camshaft  119  is rotatably journaled within the crankcase chamber  62  by suitable bearings formed at its opposite ends. In accordance with the invention, the camshaft  119  is formed primarily from a non-metallic material such as a suitable resinous plastic having relatively high strength and wear resistance. 
     The journaling structure for the camshaft  119  is shown in FIGS. 5 and 7 with the camshaft ends being indicated at  121  and  122 . The upper end  121  is journaled for rotation in the cylinder block member  58 . The lower end  122  is journaled for rotation in an appropriate bearing formed in the upper end of the oil pan forming member  59  which bearing appears in FIGS. 7,  16  and  18 , and is identified by the reference numeral  123  therein. 
     The camshaft  119  is driven at one-half crankshaft speed by a timing mechanism which appears in FIGS. 4 and 7. This includes a drive gear  124  that is fixed for rotation with the crankshaft  64  and a driven gear  125  that is formed integrally with the camshaft  119  and from the same material as previously noted. This driven gear  125  is formed at the lower end of the camshaft adjacent the bearing portion  122 . 
     The camshaft  119  is provided with a pair of cam lobes  126  and  127  for operating the intake valve  83  and exhaust valve  88 , respectively through their respective rocker arms  85  and  91 . A pair of tappets  128  are slidably supported within the cylinder block member  85  and cooperate with respective push rods  129 . Each push rod  129  is associated with a respective one of the rocker arms  85  and  91  for operating it in a manner well known in the art. 
     It should be noted also that the fuel pump  109  has a plunger that is be driven off of a further lobe  130  formed integrally on the camshaft  119 . Because the camshaft and its lobes  126 ,  127  and  130  are formed from a plastic material the tappets  128  and the plunger of the fuel pump  109  may be formed from relatively low cost materials without fear of premature wear. 
     An oil slinger gear, indicated by the reference numeral  131 , (FIG. 4) is mounted for rotation in an area proximate to the oil level in the oil reservoir  63  on a mounting bracket  132 . This oil slinger gear  131  is in mesh with the camshaft drive gear  123  but rotates about a transverse axis relative to it. Oil will be thrown by the gear  131  into the crankcase chamber  62  and in contact with not only the crankshaft  64 , camshaft  119 , and their bearings but also in a direction indicated by the arrow  133 . 
     This flow direction is, as best shown in FIG. 5, toward an opening  134  formed in the wall in which the tappets  128  are slidably supported. This opening  134  opens into the valve chamber, indicated generally by the reference numeral  135  in which the valve actuating mechanism comprised of the rocker arms  85  and  91  are contained. It should be noted that the lower surface of the cylinder head is formed with an enlarged opening  136  that is disposed above the cylinder block opening  133  and through which the slung oil may easily pass. 
     This oil will collect at a low portion in the valve chamber  135  where it can flow through a return passage  137  formed in the lower cylinder head surface, as also seen in FIG.  11 . This oil return passageway communicates with a return passageway  138  that is formed in the cylinder block  57  and which communicates with the crankcase chamber  62 . This returned oil may then fall into the oil reservoir  63  to be recirculated. An arrangement, to be described, is also provided for ensuring cooling of this returned oil. 
     It has been noted that the exhaust gases from the cylinder head exhaust port  92  are discharged to the atmosphere through an exhaust system. That exhaust system will now be described by primary reference to FIGS. 2,  3 ,  6 ,  10 ,  11  and  15  through  18 . Initial reference will be made to FIGS. 6 and 10 and  15  through  18 , which describe the structure by which the exhaust gases are collected from the cylinder head exhaust passage  92  and are delivered to an elongated expansion chamber  139  that is formed in major part in the tubular portion  41  of the drive shaft housing and lower unit outer housing  29 . 
     It has already been noted that the cylinder head assembly  58  is detachably connected to the crankcase forming member  59 . This crankcase forming member  59  is formed with an exhaust collector passage  141  in one side thereof, as best seen in FIGS. 3 and 6. This exhaust collector passage  141  has an inlet portion that communicates with the discharge end of the cylinder head exhaust passage  92  and then curves downwardly. This is disposed to one side of the oil reservoir portion  63  of this member  59 . The member  59  has an upper surface  142  that is affixed in sealing relationship with a downwardly facing surface of the cylinder block  57  and particularly the portion that forms the upper crankcase chamber  61 . 
     It should be noted that oil is maintained in the reservoir  63 . The aforenoted splash type lubricating system delivers this oil to the various components of the engine  25  as already noted. The crankcase chamber forming member  59  also has a cylindrical center boss  143  in which the bearing  66  is supported. 
     It will be seen that the lower face  144  of the crankcase forming member  59  is formed with a pair of rib-like portions  145  and  146  that define a path for the exhaust gases. These rib-like portions  145  and  146  cooperate with respective rib-like portions  147  and  148  formed in the upper portion of the supporting plate section  39  of the drive shaft housing  29  as best seen in FIG.  15 . 
     These cooperating rib-like portions  145  and  148  and  146  and  147  define an exhaust passageway  149  so that the exhaust gases will flow as shown by the arrow  151  in FIG. 15 toward the expansion chamber opening  139  formed by the drive shaft housing cylindrical portion  41 . 
     After flowing through the aforenoted relatively restricted path, the exhaust gases can expand in the expansion chamber volume  139  to provide a silencing effect. The exhaust gases then are discharged to the atmosphere through a path which is shown best in FIG.  2 . 
     It should be noted that the lower unit housing  35  also is provided with an expansion chamber portion  152  in which a further expansion of the exhaust gases may take place. The lower unit  35  is provided with an under water exhaust gas discharge  153  from which these exhaust gases may exit. This occurs when the watercraft is in a planing condition and this discharge  153  is relatively shallowly submerged. 
     However, when operating at idle or when the watercraft is stationary and the engine running as shown in FIG. 1, this discharge opening  153  will be deeply submerged. Also, the pressure of the exhaust gases will be relatively low. Thus, there is provided a low speed exhaust gas discharge path that is less restricted under this condition but which will also provide added silencing. This system is shown best in FIG.  2 . 
     As may be seen in this figure, the tubular portion  41  of the drive shaft housing  29  is provided with a restricted exhaust gas discharge opening  154 . This opening  154  is positioned proximately to the lower steering support of the drive shaft housing  29  provided by the elastic member  47 . From this opening  154 , the exhaust gases may pass into the aforenoted expansion chamber  44  formed in the area between the swivel bracket portion  43  and the cylindrical portion  41  of the drive shaft housing  29 . Thus, a further expansion will occur that will assist in the silencing. 
     An upper portion of the swivel bracket  28  is provided with an above the water exhaust gas discharge opening  155  through which these exhaust gases may pass to the atmosphere. Thus, even when operating at low speeds, there will be an effective discharge of the exhaust gases and silencing of them. However, when traveling at high speeds, the size of the discharge openings  154  and  155  will restrict any substantial flow of exhaust gases from this low speed path. 
     It has been noted that the engine  25  is water cooled. That water cooling system will now be described by principle reference to FIGS. 1 through 4,  7  and  12  through  16 . Also, the following description will explain how the water cooling system cooperates with the lubricating system including the oil reservoir  63  and the exhaust system so as to assist in maintaining the engine and its fluids at the correct temperature and also so as to assist in the exhaust silencing. 
     First, it should be noted that the lower unit housing portion  35  is provided with a gill-like opening  156  (FIG. 1) through which water may be drawn by a water pump  157  (FIG. 2) that is driven off of the drive shaft  42  in a well-known manner. This water under pressure is then pumped upwardly through a water delivery tube  158  that passes through the drive shaft housing cylindrical portion  41 . 
     As shown schematically in FIG.  19  and in actual construction in FIG. 15, this coolant is then delivered to a cooling jacket portion  159  that is formed in the upper surface of the drive shaft housing supporting plate portion  39 . The conduit  158  has a discharge fitting  161  that communicates with this portion  159 . It should be noted that the portion  159  is formed by the rib  147  that defines the exhaust gas passage  149  and the upper surface  142  of this drive shaft housing portion  39 . 
     Flow of water through the portion  159  also communicates with a water supply path  161  (FIG. 15) formed by the lower portion of the crankcase forming member  59 . This oil pan forming member water passage  161 , in turn, communicates with a slotted passage  162  that extends upwardly and which communicates with an inlet opening formed in a cylinder block cooling jacket portion which is shown best in FIG.  3  and which is identified by the reference numeral  163 . Thus, water can flow from this member directly into the cylinder block cooling jacket  163  and also into a communicating cooling jacket of the cylinder head  58 . This water path to the cylinder head cooling jacket is through slotted passages  164  formed in the lower face of the cylinder head (FIG.  11 ). 
     As seen in FIGS. 4 and 12, a thermostat housing and thermostat assembly  165 , which is shown schematically in FIG. 19, permits the discharge of coolant from the cylinder block and cylinder head cooling jackets back to a discharge passageway formed in the crankcase forming member  59  and supporting plate portion  39  of the drive shaft housing  28 . This includes an external return conduit  166 . 
     This return conduit  165  communicates with a water return passageway  167  formed in the drive shaft housing support plate portion  39  and which is closed by a cooperating passage portion  168  formed in the lower surface of the oil pan forming member  59 . This return water path, indicated by the arrows  169  flows along the opposite side of the exhaust passage  149  and thus further assists in the cooling of the exhaust gases. 
     This water is then dumped into the expansion chamber area  139  of the drive shaft housing cylindrical portion  41  for discharge back to the body of water in which the watercraft is operating through the under water exhaust gas discharge  133 . This water will drain through this path under all running conditions since back pressure is not a problem with respect to the water discharge. 
     It should be apparent that the cooling water flows around the oil reservoir  63  and thus provides good cooling of it. In addition, the lubricating oil that is returned to the oil reservoir  63  through the cylinder head and cylinder block drain passages  137  and  138  will also be cooled. This is because these passages are formed in proximity to the cooling water inlet opening  162  into the cylinder head and cylinder block as best seen in FIGS. 10 and 11. Thus, this oil will be cooled by the water when it is first admitted to the engine cooling jackets and is at its lowest temperature. Thus, the oil temperature will be kept quite low. 
     It has been noted that there is provided a decompression device for assisting in engine starting. This decompression device is associated with the camshaft  119  and appears best in FIGS. 7-9. As may be seen in these figures, and particularly in FIG. 7, adjacent the exhaust cam lobe  127 , there is provided a cross-drilled passageway  171  that extends through the camshaft. A sliding decompression plunger  172  is received in this passageway, and has its tip end disposed adjacent the heel of the exhaust cam lobe  127  in a position to contact the tappet  128  of the exhaust valve  88 . 
     A centrifugal-type mechanism, indicated generally by the reference numeral  173 , is associated with and supported by the driven timing gear  125  of the camshaft. This includes an arcuate-shaped centrifugal element  174  that is supported on a pivot pin  175  which is staked to the timing gear  125 . A hairpin-type spring  176  maintains this centrifugal member  174  in the position shown in FIG. 8 when the engine is not running or  5  being pull started. Under this condition, the plunger  172  is extended and will contact the exhaust valve tappet  128  during the compression stroke and relieve compression so as to facilitate starting. 
     However, once the engine is started, the centrifugal force on the member  174 , acting in the direction of the arrow in FIG. 9, will cause it to pivot to the position shown in FIG. 9, overcoming the action of the hairpin spring  176 . This will permit the plunger  172  to be withdrawn by centrifugal force so as to no longer affect the operation of the exhaust valve during the compression stroke so as to maintain normal engine running. 
     Again, because of the fact that the camshaft  119  is made from plastic, wear of these elements will also be reduced. 
     The mechanism for shifting the transmission  36  will finally be described by reference to FIGS. 2 and 3. A shift lever  181  is pivotally supported on the supporting plate portion  39  of the drive shaft housing  29 . This lever  181  is operated by a suitable, externally positioned shift lever. A shift link  182  is pivotally connected to an arm of the shift lever  181 . This shift link  182  depends into the drive shaft housing portion  34  and lower unit  35  to operate a shift cam (not shown) that operates the dog clutches of the transmission  36  in a well known manner. 
     Thus, it should be readily apparent from the foregoing description that the described system provides a very effective and low cost camshaft which reduces wear and accordingly the cost of the associated components. Of course, the foregoing description is that of a preferred embodiment of the invention and various changes and modifications may be made without departing from the spirit and scope of the invention, as defined by the appended claims.