Title of Invention

DECOMPRESSOR-EQUIPPED CAM MECHANISM

Abstract

To provide a decompressor-equipped cam mechanism formed so that the width of a cylinder head does not become large. [Solving Means] In a decompressor-equipped cam mechanism 160, a centrifugal weight portion 163 is located close to a cam-driven sprocket 67, a shaft portion 162 extends parallel to a camshaft 66 and is supported to be capable of rotationally moving relative to the camshaft 66 and the cam-driven sprocket 67, and the shaft portion 162 is rotationally moved by turning and swinging the centrifugal weight portion 163 outward by centrifugal force caused by rotation of the cam-driven sprocket 67, whereby the decompression cam nose 164 is protruded from and retreated to the camshaft 66. The decompressor-equipped cam mechanism 160 is configured so that the cam-driven sprocket 67 has a weight opening portion 169 in a portion facing the centrifugal weight portion 163 of the decompression cam 161, and at least part of the centrifugal weight portion 163 is located in the weight opening portion 169.

Full Text

[Document Name] Description [Title of the Invention] DECOMPRESSOR-EQUIPPED CAM MECHANISM [Technical Field] [0001] The present invention relates to a decompressor-equipped cam mechanism in which the compression pressure in a combustion chamber is reduced at the time of the start of a reciprocating internal combustion engine to facilitate the start thereof. [Background Art] [0002] In reciprocating internal combustion engines, a gaseous mixture of air and fuel introduced into a combustion chamber by opening an intake valve is compressed by a cylinder and then burned, and the cylinder is reciprocated by the energy of this burning, thus obtaining power. However, there are cases where an engine is difficult to start if the pressure of the gaseous mixture in the combustion chamber is high at the time of the start of the engine. Accordingly, other than a cam which opens and closes an exhaust valve in ordinary operation, a decompressor is used which reduces the pressure in the combustion chamber by opening the exhaust valve by a small amount and exhausting part of the compressed gaseous mixture when the engine rotates at a speed less than a predetermined one, such as during starting the engine. As such a decompressor, a compressor has been known which is constituted as follows: a centrifugal weight portion is provided outside a cam sprocket which rotationally moves a cam; and the centrifugal weight portion is swung by centrifugal force caused by the rotational movement of the cam sprocket, whereby a decompression cam nose which opens and closes the exhaust valve is protruded and retreated (e.g., see Japanese Patent Application Laid-open No. 2003-254025). [Disclosure of the Invention] [Problems to be Solved by the Invention] [0004] However, the centrifugal weight portion requires a certain amount of weight and therefore becomes large. In the case of the above-described constitution, the width of a cylinder head of the engine becomes large by an amount corresponding to the thickness of the centrifugal weight portion. Accordingly, it has been an object to provide a decompressor in which the width of a cylinder head is not large even in the case where a centrifugal weight portion is located outside a cam sprocket. [0005] The present invention has been accomplished in light of the above-described problem. An object of the present invention is to provide a decompressor-equipped cam mechanism in which part of a centrifugal weight portion is located in an opening portion formed in a cam sprocket so that the width of a cylinder head does not become large. [Means for Solving the Problems] [0006] In order to solve the above-described problem, a decompressor-equipped cam mechanism according to the present invention includes: a camshaft having at least one cam (e.g., an exhaust cam 71 in an embodiment); a cam sprocket (e.g., a cam-driven sprocket 67 in the embodiment) coupled to the camshaft, and rotated and driven in conjunction with a crankshaft; and a decompression cam including a cylindrical shaft portion, a decompression cam nose formed on a peripheral surface side on one end side of the shaft portion, and a centrifugal weight portion extending in a direction perpendicular to an axis line of the shaft portion on other end side of the shaft portion. The centrifugal weight portion is located close to the cam sprocket, the shaft portion extends parallel to the camshaft and is supported to be capable of rotationally moving relative to the camshaft, and the shaft portion is rotationally moved by turning and swinging the centrifugal weight portion outward by centrifugal force caused by rotation of the cam sprocket, whereby the decompression cam nose is protruded from and retreated to the camshaft. The cam sprocket has a space portion (e.g., a weight opening portion 163 in the embodiment) in a portion facing the centrifugal weight portion of the decompression cam. At least part of the centrifugal weight portion is located in the space portion. [0007] Here, the following is preferable: the space portion is formed to pass through the cam sprocket; and the centrifugal weight portion is inserted in the space portion from a surface of the cam sprocket which is opposite to the cam, and an end portion of the centrifugal weight portion is located in approximately the same plane as that of a surface of the cam sprocket on the cam side or located in the vicinity of the plane. [0008] Further, the center of gravity of the centrifugal weight portion is preferably located at a distance greater than half of a radius of the cam sprocket from the center of the cam sprocket. [0009] Moreover, the space portion of the cam sprocket preferably has a stopper portion which limits the swinging of the centrifugal weight portion. [0010] Furthermore, the following is preferable : the shaft portion of the decompression cam includes a first shaft portion and a second shaft portion with the decompression cam nose interposed therebetween, which first shaft portion extends toward the centrifugal weight portion and is supported by the camshaft, which second shaft portion extends toward the cam and is supported by the camshaft; and the shaft portion of the decompression cam includes a cutout portion formed in a peripheral surface of the second shaft portion in a direction in which the decompression cam nose is formed. [Effects of the Invention] [0011] By constituting the decompressor-equipped cam mechanism according to the present invention as described above, it is possible to reduce the width of the camshaft in the axis direction while ensuring the weight of the centrifugal weight portion which is necessary for the swinging of the decompression cam. Accordingly, the cylinder head can be made compact. Further, by forming the space portion so as to pass through the cam sprocket, the thickness of the centrifugal weight portion located in the space portion can be made approximately equal to that of the cam sprocket. This makes it easy to ensure the weight of the centrifugal weight portion. Further, by locating the center of gravity of the centrifugal weight portion at a distance greater than half of the radius of the cam sprocket from the center of the cam sprocket, the centrifugal weight portion can be caused to effectively function. Further, the provision of the stopper portion in the space portion eliminates a dedicated stopper member. Thus, the manufacturing cost of the decompressor-equipped cam mechanism according to the present invention can be reduced. [0012] Furthermore, since the shaft portion of the decompression cam includes the first and second shaft portions and the decompression cam is supported at the first and second shaft portions, both ends thereof are supported, and the runout of the shaft portion can be reduced. Further, by providing the cutout portion in the second shaft portion at the same time, a space for tilting the axis of the shaft portion is formed. Accordingly, pinching occurs between the shaft portion and a portion receiving the shaft portion, and it is possible to reduce unnecessary vibration in the centrifugal weight portion at rotation speeds equal to or less than a release rotation speed of the decompressor. [Best Mode for carrying out the Invention] [0013] Hereinafter, a preferred embodiment of the present invention will be described with reference to drawings. First, a scooter-type vehicle 1 on which a secondary air supply system according to the present invention is mounted will be described using Fig. 1. The scooter-type vehicle 1 has a vehicle body frame 2. The vehicle body frame 2 includes a head pipe 3 located in a front portion of the vehicle body frame 2 and extended in the vertical direction; a down frame 4 extended from the head pipe 3 downward; a pair of left and right side frames 5 extended from approximately the central portion of the down frame 4 downward, backward, and then diagonally upward to the back; and arcuate curved frames 7 attached to the side frames 5 with side plates 6 interposed therebetween. [0014] A steering shaft (not shown) is attached to the head pipe 3 to be capable of rotating. A front fork 8 is attached to the lower end of the steering shaft. Further, a front wheel 9 is movably supported at the lower end of the front fork 8. On the other hand, to the upper end of the steering shaft, a handle post 10 is attached. Furthermore, a handlebar 11 is attached to the upper end of the handle post 10. Thus, the front wheel 9 can be steered by handling the handlebar 11. Further, an upper stay 12 and a lower stay 13 are attached to the head pipe 3 so as to extend from the head pipe 3 forward. A front basket 14 is attached to the front ends of the upper and lower stays 12 and 13. [0015] A fuel tank 15 is attached between the pair of side frames 5 in lower portions of the pair of side frames 5. The front end of a power unit 16 is attached to the rear ends of the curved frames 7 located behind the fuel tank 15 to be capable of swinging vertically, and the rear end of the power unit 16 is attached to the rear end of one side frame 5 with a rear cushion unit 17 interposed therebetween. Here, the fuel tank 15 is provided with a fuel pump 18 having an overturning sensor inside, and is constituted so that fuel can be supplied from a fuel inlet (cap) 19 provided in an upper portion of the fuel tank 15. Further, the power unit 16 includes an engine 50, a transmission 90, a throttle body 21, an air cleaner 22, an exhaust pipe 23 extended from the engine 50 backward, and muffler 24 connected to the rear end of the exhaust pipe 23. Further, a rear wheel 25 is attached to the rear end of the power unit 16. The rear wheel 25 is driven by the power unit 16. Thus, the scooter-type vehicle 1 is constituted so that it can obtain power to run. [0016] A luggage box 26 and a grab rail 20 are attached to a rear portion of the side frame 5. A front portion of the luggage box 26 is supported through a front-side plate 28 by a front-side cross pipe 27 attached to the side frames 5, and a rear portion of the luggage box 26 is supported by a gate-shaped frame 29 attached to the side frames 5. Further, the grab rail 20 is supported by a rear stay 30 extending from the side frames 5 upward. Note that helmets 31 and 32 are shown in a state where they are contained in the luggage box 26. Further, a rear storage box 33 is attached to a rear portion of the grab rail 20. [0017] In the scooter-type vehicle 1 constituted as described above, an upper front portion of the vehicle body is covered with a front cover 34, and left and right front side covers 35 are attached to lower side portions of the front cover 34. Turn signal lamps (not shown) are attached to the front side covers 35. A front inner cover 36 is attached to a rear portion of the front cover 34. Side portions of a step floor 37, on which a driver and a passenger put their foot, are covered with side covers and an under cover 38. Further, a seat 39 is attached to an upper portion of the luggage box 26, and a region under the seat 39 is surrounded by a body cover and body side covers 40. Furthermore, a handlebar cover 42 covering the handlebar 11 is attached to an upper portion of the front cover 34, and a head lamp 43 is attached to the handlebar cover 42. Further, a front fender 44 is attached so as to cover an upper portion of the front wheel 9, and a rear fender 45 is attached so as to cover an upper portion of the rear wheel 25. [0018] Incidentally, a tail lamp 46 is placed under the grab rail 20, and a battery 41 is contained between the front cover 34 and the front inner cover 36. [0019] Next, details of the power unit 16 will be described using Figs. 2 to 5. Incidentally, in this embodiment, the direction of arrow F in Fig. 2 is the forward direction, and the direction of arrow U in Fig. 3 is the upward direction. The engine 50, which is a component of the power unit 16, includes a cylinder head cover 51, a cylinder head 52, a cylinder block 53, and a crankcase 54. A piston 56 is placed in a cylinder chamber 55a, which is formed to be surrounded by a cylinder sleeve 55 inserted in the cylinder block 53, so as to be capable of sliding upward and downward. The piston 56 is connected through a connecting rod 57 to a crankshaft 58 which is held in the crankcase 54 so as to be capable of rotating. An intake port 60 and an exhaust port 61 communicate with a combustion chamber 59 formed by the cylinder block 53 (cylinder sleeve 55), the cylinder head 52, and the piston 56 through an inlet and an outlet, respectively. Further, one ends of a mushroom-shaped intake valve 62 and a mushroom-shaped exhaust valve 63 are attached to a timing gear to be supported by a retainer, and the other ends thereof are spring-biased by valve springs 64 and 65, which are supported by the cylinder head 52, in directions in which the inlet and the outlet are always closed, respectively. [0020] Furthermore, a camshaft 66 for performing the acts of opening and closing the intake valve 62 and the exhaust valve 63 is placed in the cylinder head 52 so as to be capable of rotating. A timing chain 69 is wound on a cam-driven sprocket 67 which the camshaft 66 has and a cam-drive sprocket 68 provided for the crankshaft 58. Accordingly, the camshaft 66 rotates in accordance with the rotation of the crankshaft 58, and cams 70 and 71 formed in the camshaft 66 push down the intake valve 62 and the exhaust valve 63 via rocker arms 72 and 73, whereby the inlet and the outlet are opened or closed. Further, an injector 74 is provided for the intake port 60. Fuel is converted into fine particles by the injector 74 to be injected in the intake port 60. Furthermore, an ignition plug 75 is attached to the cylinder head 52. [0021] Note that a cam chain tensioner 84 for adjusting the tension of the timing chain 69 to adjust the timings of opening and closing the intake valve 62 and the exhaust valve 63 using the cams 70 and 71 is attached to a side portion of the cylinder block 53. [0022] In the engine 50 constituted as described above, a gaseous mixture of air cleaned by the air cleaner 22 and fuel injected by the injector 74 is supplied from the intake port 60 to the combustion chamber 59, compressed by the piston 56, then ignited by the ignition plug 75 to burn, converted into energy which rotates the crankshaft 58 through the piston 56, and thereafter exhausted as exhaust gas from the exhaust port 61 to the outside. [0023] The crankshaft 58 includes a right crankshaft half 58a and a left crankshaft half 58b. The connecting rod 57 is connected to the crankshaft 58 with a crankpin 76 interposed therebetween, which crankpin 76 is provided to connect the two crankshaft halves 58a and 58b. Further, the crankcase 54 containing the crankshaft 58 includes a right case half 54a and a left case half 54b. One end (journal portion of the right crankshaft half 58a) of the crankshaft 58 is supported by the right case half 54a with a bearing 77 interposed therebetween, and the other end (journal portion of the left crankshaft half 58b) is supported by the left case half 54b with a bearing 78 interposed therebetween. Further, an ACG 79 and a cooling fan 85 are placed at a tip portion of the journal portion of the right crankshaft half 58a. [0024] A left-side portion of the left case half 54b extends backward to constitute part of a transmission case 80. A transmission 90 is contained in a transmission chamber 82 surrounded by a transmission cover 81 attached to a left side surface of the left case half 54b. The transmission 90 includes a drive pulley 91 which is attached to a tip portion of the journal portion of the left crankshaft half 58b extending in the transmission chamber 82 and rotates together with the left crankshaft half 58b; a countershaft 92 which is located behind the transmission chamber 82, extended parallel to the crankshaft 58, and attached so as to be capable of rotating; a driven pulley 93 which is attached to approximately a central portion of the countershaft 92 so as to be capable of rotating relative to the countershaft 92; a clutch 94 which is attached to the left end of the countershaft 92 and locks or releases the driven pulley 93 and the countershaft 92; and a transmission belt (not shown) which is wound on the drive pulley 91 and the driven pulley 93 and transmits the rotation of the drive pulley 91 to the driven pulley 93. [0025] The drive pulley 91 includes a fixed pulley half 91a which is attached to the crankshaft 58 so as to be capable of rotating together with the crankshaft 58, and a movable pulley half 91b which is capable of moving relative to the fixed pulley half 91a in the axis direction and rotating together with the crankshaft 58. The transmission belt is held between the fixed pulley half 91a and the movable pulley half 91b. On the other hand, the driven pulley 93 includes a fixed pulley half 93a which is attached to the countershaft 92 so as to be capable of rotating relative the countershaft 92, and a movable pulley half 93b which is capable of moving relative to the fixed pulley half 93a in the axis direction and rotating relative to the countershaft 92. The transmission belt is held between the fixed pulley half 93a and the movable pulley half 93b. Accordingly, by variably setting the pulley widths of the drive pulley 91 and the driven pulley 93, the wound radiuses with which the transmission belt is wound on the pulleys 91 and 93 can be continuously changed to control the transmission gear ratio steplessly (continuously). [0026] An idle shaft 95 and a rear-wheel shaft 96 to which the rear wheel 25 is attached are attached parallel to the countershaft 92 so as to be capable of rotating. The rotational driving force of the countershaft 92 is transmitted to the rear wheel 25 through a gear array attached to the countershaft 92, the idle shaft 95, and the rear-wheel shaft 96. Thus, the output of the engine 50 is transmitted to the crankshaft 58, changed in speed by the transmission 90, and then transmitted to the rear wheel 25. [0027] Incidentally, when the above-described power unit 16 is operated, it is necessary to cool each part because heat is generated. Accordingly, for the cylinder head 52, a water jacket 122 through which cooling water is passed is formed, and the cylinder head 52 is cooled using the cooling water; for the cylinder block 53, a plurality of cooling fins 86 protruding from a side portion outward are formed, and heat generated in the cylinder block 53 is released from the cooling fins 86; and the transmission case 80 is cooled by providing in a front portion of the transmission case 80 a cooling-wind guide passage 83 which extends forward and through which outside air is taken in the transmission chamber 82. [0028] Next, a decompressor-equipped cam mechanism 160 used in the scooter-type vehicle 1 will be described using Figs. 6A to 7. The decompressor-equipped cam mechanism 160 includes the camshaft 66, the cam-driven sprocket 67, and a decompression cam 161. The camshaft 66 are supported by and attached to the cylinder head 52 so as to be capable of rotating in such a manner that the both ends of the camshaft 66 are supported via bearings 87 and 88, and the camshaft 66 has the intake and exhaust cams 70 and 71, and the like. The cam-driven sprocket 67 and the decompression cam 161 are attached to the camshaft 66. [0029] In the camshaft 66, the intake cam 70, the exhaust cam 71, and a guide portion 66b are formed so as to be protruded from the outer peripheral surface of a cylindrical shaft portion 66a and be arranged in this order. The guide portion 66b is formed in an end portion on the end surface 66c side to which the cam-driven sprocket 67 is attached. Note that cam noses 70a and 71a which push up the intake and exhaust rocker arms 72 and 73 in order to push down the intake and exhaust valves 62 and 63 are respectively formed in the intake and exhaust cams 70 and 71. [0030] In the camshaft 66, a decompression-maintaining hole 170 is formed which has an opening portion 170a on the end surface 66c side and extends inside the camshaft 66. The decompression-maintaining hole 170 is formed at a position in the exhaust cam 71 which is deviated from the axis line of the shaft portion 66a to the end portion side opposite to the cam nose 71a to be parallel to this axis line. Further, the decompression-maintaining hole 170 is open to the outside on the outer peripheral surface of the shaft portion 66a between the guide portion 66b and the exhaust cam 71 to form an opening portion 170b. Moreover, the decompression-maintaining hole 170 reaches the position where the exhaust cam 71 is formed. A groove portion 71b is formed in a portion opposite to the cam nose 71a of the exhaust cam 71 across the shaft portion 66a. An opening portion of the groove portion 71b is formed to be continuous and integral with the cam opening portion 170b. [0031] Next, the decompression cam 161 attached to the decompression-maintaining hole 170 will be described. In the decompression cam 161, a centrifugal weight portion 163 which extends in a direction perpendicular to the axis line of a cylindrical shaft portion 162 is formed at one end of the shaft portion 162, and a decompression can nose 164 and a decompression groove 165 are formed in the vicinity of the other end of the shaft portion 162. Incidentally, the shaft portion 162 includes a first shaft portion 162a formed between the centrifugal weight portion 163 and the decompression cam nose 164, a decompression portion 162b in which the decompression cam nose 164 and the decompression groove 165 are formed, and a second shaft portion 162c formed at the above-described other end. The peripheral surface of the second shaft portion 162c in the same direction as the direction in which the decompression cam nose is formed is cut out to form a cutout portion 162d. Incidentally, the decompression cam nose 164 and the decompression groove 165 formed in the decompression portion 162b are formed at positions opposite to each other across the axis line of the shaft portion 162. Further, the centrifugal weight portion 163 is formed in a sector shape which spreads from the shaft portion 162 as center when viewed in a side view, and a protruding portion 166 which protrudes outward is formed on the surface of the centrifugal weight portion 163 on the side where the shaft portion 162 extends. [0032] On the other hand, a weight opening portion 169 which spreads in a segment shape outward from a position slightly deviated from the center toward the outside is formed in the disk-shaped cam-driven sprocket 67 attached to the camshaft 66. The weight opening portion 169 passes through the cam-driven sprocket 67 in the thickness direction. In the weight opening portion 169, a stopper portion 169a is formed on one end side of the portion corresponding to the chord of the segment, and a spring-biased portion 169b is formed on the other end side. Further, the cam-driven sprocket 67 is attached to the end surface 66c of the camshaft 66 by inserting bolts 172 into mounting holes 173, 173 which pass through the cam-driven sprocket 67 in the thickness direction. At this time, the cam-driven sprocket 67 is attached to the camshaft 66 so that the opening portion 170a of the decompression-maintaining hole 170 formed in the camshaft 66 is at a position where the opening portion 170a can be seen from the outside through the weight opening portion 169. [0033] The decompression cam 161 is attached by inserting the shaft portion 162 into the decompression-maintaining hole 170 from the opening portion 170a formed in the end surface 66c of the camshaft 66 in the state where the cam-driven sprocket 67 is attached to the camshaft 66; the first shaft portion 162a is supported by the decompression-maintaining hole 170 formed in the guide portion 66b so as to be capable of rotating; and the second shaft portion 162c is held by the groove portion 71b so as to be capable of rotating. At this time, the decompression cam nose 164 and the decompression groove 165 are at the position of the cam opening portion 170b. Further, the protruding portion 166 of the centrifugal weight portion 163 is located in the weight opening portion 169 of the cam-driven sprocket 67, and an end portion of the protruding portion 166 is located in approximately the same plane as that of the surface of the cam-driven sprocket 67 on the cam 71 side or located in the vicinity of the foregoing plane in the weight opening portion 169. With the above-described constitution, the decompression cam 161 can rotationally move about the shaft portion 162, and the centrifugal weight portion 163 swings along the surface of the cam-driven sprocket 67. Along with this, the protruding portion 166 always swings in the weight opening portion 169. The movement of the decompression cam 161 in the axis direction of the camshaft 66 is limited by a limiting member 171 attached to the camshaft 66 with the bolts 172 together with the cam-driven sprocket 67. Thus, the decompression cam 161 is constructed so that it is not detached from the decompression-maintaining hole 170. That is, the bolts 172 can also be used for coupling the cam-driven sprocket 67 and the limiting member 171. [0034] Incidentally, a cylindrical spring-mounting portion 167 extending along the axis line of the shaft portion 162 is formed in the portion of the centrifugal weight portion 163 to which the shaft portion 162 is connected, and a return spring 168 is wound on the spring-mounting portion 167. The return spring 168 is attached to the decompression cam 161 in the direction in which the centrifugal weight portion 163 approaches the camshaft 66, i.e., the direction in which the protruding portion 166 biases the spring-biased portion 169b of the weight opening portion 169 (position L in Fig. 6A). Thus, in the case where the decompression cam 161 is at position L, the decompression cam nose 164 is located in the cam opening portion 170b, and the decompression groove 165 is located in the decompression-maintaining hole 170. [0035] On the other hand, when the engine 50 starts and the cam-driven sprocket 67 starts rotating, centrifugal force acts on the centrifugal weight portion 163 according to the rotation speed of the cam-driven sprocket 67, and the centrifugal weight portion 163 is turned outward around the shaft portion 162 as center to swing. At this time, since the decompression cam 161 swings against the biasing force of the return spring 168, the centrifugal weight portion 163 is not turned outward until the cam sprocket 67 reaches a predetermined rotation speed. When the centrifugal weight portion 163 is turned outward by centrifugal force, the protruding portion 166 comes into contact with the stopper portion 169a of the weight opening portion 169, and the centrifugal weight portion 163 does not swing any longer (position H in Fig. 6B). Thus, in the case where the decompression cam 161 is at position H, the decompression groove 165 is located in the cam opening portion 170b, and the decompression cam nose 164 is located in the decompression-maintaining hole 170 (state shown in Fig. 6A). Incidentally, a groove 170c is formed in the portion of the decompression-maintaining hole 170 which is opposite to the decompression cam nose 164 so that the decompression-maintaining hole 170 does not interfere with the decompression cam nose 164. [0036] In an end portion of the exhaust rocker arm 73 provided for the decompressor-equipped cam mechanism 160 constituted as described above, a push portion 73a is formed which is formed so as to protrude from a side portion of the exhaust rocker arm 73 downward and which is pushed upward by the cam nose 71a of the exhaust cam 71. The push portion 73a is located in the decompression-maintaining hole 170 from the cam opening portion 170b. Further, the push portion 73a is constituted so as to be located in the decompression groove 165 formed in the decompression cam 161 when the decompression cam 161 is at position H. [0037] Thus, the decompression cam nose 164 is located in the cam opening portion 170b when the rotation speed of the crankshaft 58 of the engine 50 is low at the time of the start of the engine 50, i.e., when the decompression cam 161 is at position L. Accordingly, immediately before a compressed gaseous mixture is ignited in the combustion chamber 59, the push portion 73a of the exhaust rocker arm 73 is pushed up by the decompression cam nose 164 to open the exhaust valve 63 by a small amount, and the pressure of the gaseous mixture in the combustion chamber 59 decreases. On the other hand, when the rotation speed of the crankshaft 58 becomes a predetermined value or more after the engine 50 starts operating, the centrifugal weight portion 163 of the decompression cam 161 is turned outward and swings to move to position H. Then, the shaft portion 162 rotationally moves, and the decompression groove 165 is located in the cam opening portion 170b. Accordingly, the push portion 73a is located in the decompression groove 165. Consequently, the exhaust valve 63 is not opened immediately before ignition, and the engine 50 is normally operated. [0038] Incidentally, since the decompression cam 161 has the return spring 168, the decompression cam 161 returns to position L when the rotation speed of the crankshaft 58 becomes a predetermined value or less. [0039] By constituting the decompressor-equipped cam mechanism 160 as described above, part (protruding portion 166) of the centrifugal weight portion 163 which is a component of the decompression cam 161 is located in the weight opening portion 169 formed in the cam-driven sprocket 67. This makes it possible to ensure a weight necessary for the swinging of the decompression cam 161 and narrow the width of the camshaft 66 in the axis direction. Accordingly, the cylinder head 52 can be made compact. Incidentally, by constituting the weight opening portion 169 so as to pass through the cam-driven sprocket 67 in the thickness direction, the thickness of the protruding portion 166 can be made approximately equal to that of the driven sprocket 67. Thus, the weight of the centrifugal weight portion 163 is easily ensured. [0040] Incidentally, by constituting the decompressor-equipped cam mechanism 160 so that the center of gravity of the centrifugal weight portion 163 is located at a distance greater than half of the radius of the cam-driven sprocket 67 from the center of the cam-driven sprocket 67, the centrifugal weight portion 163 can be caused to effectively function. Further, by bringing the protruding portion 166 into contact with the stopper portion 169a of the weight opening portion 169 to limit the rotational movement when the centrifugal weight portion 163 is turned outward to swing, the necessity of a dedicated stopper member is eliminated. This reduces the manufacturing cost of the decompressor-equipped cam mechanism 160. [0041] Moreover, when the decompression cam 161 is attached to the camshaft 66, the first shaft portion 162a of the shaft portion 162 is held in the decompression-maintaining hole 170 formed in the guide portion 66b so as to be capable of rotationally moving, and the second shaft portion 162c is held in the groove portion 71b so as to be capable of rotationally moving. Accordingly, both ends of the shaft portion 162 are supported, and the runout of the shaft portion 162 can be reduced. Further, when the rotation speed of the cam-driven sprocket 67 is low and the exhaust rocker arm 73 is pushed up by the decompression cam nose 164 (the rotation speed of the cam-driven sprocket 67 is equal to or less than a release rotation speed of the decompressor), if the decompression cam 161 vibrates, the second shaft portion 162c is not supported by the groove portion 71b and the shaft portion 162 can tilt with respect to the axis line direction of the decompression-maintaining hole 170 by a small angle as schematically shown in Fig. 8, because the cutout portion 162d is formed in the second shaft portion 162c. Accordingly, the first shaft portion 162a comes into contact with end portions (positions X and Y in Fig. 8) of the decompression-maintaining hole 170 in the guide portion 66b, whereby unnecessary vibration of the decompression cam 161 can be reduced. [Brief Description of the Drawings] [0042] [Fig. 1] Fig. 1 is a side view of a scooter-type vehicle to which a decompressor-equipped cam mechanism according to the present invention is attached. [Fig. 2] Fig. 2 is a plan view of a power unit. [Fig. 3] Fig. 3 is a left side view of the power unit. [Fig. 4] Fig. 4 is a right side view of the power unit. [Fig. 5] Fig. 5 is a cross-sectional view of a cylinder head. [Fig. 6] Figs. 6A and 6B are views showing the decompressor-equipped cam mechanism according to the present invention. Fig. 6A is a side view from a cam-driven sprocket side. Fig. 6B is a side cross-sectional view including a cylinder head. [Fig. 7] Fig. 7 is a front view of a cam sprocket. [Fig. 8] Fig. 8 is an explanatory diagram showing the operation of a decompression cam during vibration. [Description of Reference Numerals] [0043] 58 crankshaft 66 camshaft 67 cam-driven sprocket (cam sprocket) 71 exhaust cam (cam) 160 decompressor-equipped cam mechanism 161 decompression cam 162 shaft portion 162a first shaft portion 162c second shaft portion 162d cutout portion 163 centrifugal weight portion 164 decompression can nose 169 weight opening portion (opening portion) 169a stopper portion [Document Name] Claims [Claim 1] A decompressorequipped cam mechanism comprising: a camshaft having at least one cam; a cam sprocket coupled to the camshaft, and rotated and driven in conjunction with a crankshaft; and a decompression cam including a cylindrical shaft portion, a decompression cam nose formed on a peripheral surface side on one end side of the shaft portion, and a centrifugal weight portion extending in a direction perpendicular to an axis line of the shaft portion on other end side of the shaft portion, wherein the centrifugal weight portion is located close to the cam-driven sprocket, the shaft portion extends parallel to the camshaft and is supported to be capable of rotationally moving relative to the camshaft, and the shaft portion is rotationally moved by turning and swinging the centrifugal weight portion outward by centrifugal force caused by rotation of the cam sprocket, whereby the decompression cam nose is protruded from and retreated to the camshaft, the cam sprocket has a space portion in a portion facing the centrifugal weight portion of the decompression cam, and at least part of the centrifugal weight portion is located in the space portion. [Claim 2] The decompressor-equipped cam mechanism according to claim 1, wherein the space portion is formed to pass through the cam sprocket, and the centrifugal weight portion is inserted in the space portion from a surface of the cam sprocket which is opposite to the cam, and an end portion of the centrifugal weight portion is located in approximately the same plane as that of a surface of the cam sprocket on the cam side or located in the vicinity of the plane. [Claim 3] The decompressor-equipped cam mechanism according to any one of claims 1 and 2, wherein the center of gravity of the centrifugal weight portion is located at a distance greater than half of a radius of the cam sprocket from the center of the cam sprocket. [Claim 4] The decompressor-equipped cam mechanism according to any one of claims 1 to 3, wherein the space portion of the cam sprocket has a stopper portion which limits the swinging of the centrifugal weight portion. [Claim 5] The decompressor-equipped cam mechanism according to any one of claims 1 to 4, wherein the shaft portion of the decompression cam includes a first shaft portion and a second shaft portion with the decompression cam nose interposed therebetween, the first shaft portion extending toward the centrifugal weight portion and being supported by the camshaft, the second shaft portion extending toward the cam and being supported by the camshaft, and the shaft portion of the decompression cam includes a cutout portion formed in a peripheral surface of the second shaft portion in a direction in which the decompression cam nose is formed.

Documents:

1297-CHE-2005 CORRESPONDENCE OTHERS 05-01-2012.pdf

1297-CHE-2005 EXAMINATION REPORT REPLY RECEIVED 19-10-2011.pdf

1297-CHE-2005 FORM-3 19-10-2011.pdf

1297-CHE-2005 OTHER PATENT DOCUMENT 19-10-2011.pdf

1297-CHE-2005 POWER OF ATTORNEY 19-10-2011.pdf

1297-CHE-2005 AMENDED CLAIMS 19-10-2011.pdf

1297-CHE-2005 CORRESPONDENCE OTHERS.pdf

1297-CHE-2005 CORRESPONDENCE PO.pdf

1297-CHE-2005 FORM-18.pdf

1297-che-2005-abstract image.jpg

1297-che-2005-abstract.pdf

1297-che-2005-claims.pdf

1297-che-2005-correspondnece-others.pdf

1297-che-2005-description(complete).pdf

1297-che-2005-drawings.pdf

1297-che-2005-form 1.pdf

1297-che-2005-form 3.pdf

1297-che-2005-form 5.pdf

1297-che-2005-other document.pdf