CN213627719U - Motorcycle engine starts decompression structure - Google Patents

Abstract

The utility model provides a motorcycle engine starts decompression structure belongs to motorcycle technical field. It solves the problem that the light weight is not enough in the starting pressure reducing structure of the existing engine. This motorcycle engine starts decompression structure includes camshaft, sprocket and relief pressure valve assembly, is equipped with fastener and sprocket on the sprocket and links firmly on the camshaft through the fastener, and relief pressure valve assembly is including setting up the flail piece on the sprocket side, the one end of flail piece is articulated mutually with the sprocket and the flail piece can radially throw away along the sprocket under the effect of centrifugal force, be connected with the elastic component that resets on the flail piece, the flail piece supports after resetting under the effect of elastic component that resets and leans on the fastener, and the edge of sprocket is equipped with spacing arch, and the hinged end of flail piece leans on and prescribes a limit to throw away the stroke that the piece thrown away with spacing protruding counterbalance after throwing away. The utility model discloses make the sprocket assembly realize the lightweight design, still have compact structure's advantage simultaneously.

Description

Motorcycle engine starts decompression structure

Technical Field

The utility model belongs to the technical field of the motorcycle, a motorcycle engine starts decompression structure is related to.

Background

The motorcycle is a very common vehicle, and for motorcycles, especially some high-end motorcycles which seek acceleration performance, lightweight design is a very important index.

When the motorcycle engine is started, new gas needs to be sucked for compression, great compression resistance is generated near the compression end point, and at the moment, if decompression is not performed, the starting force is increased, and starting is very difficult, so that decompression needs to be performed by adopting a decompression structure to ensure that the engine is started smoothly.

The decompression structure is usually arranged on a camshaft, for example, an engine starting automatic decompression mechanism (application number: 200720188746.5; grant number: CN201144719Y) disclosed in Chinese patent document comprises a camshaft, a driven sprocket and an exhaust cam are arranged on the camshaft, two same centrifugal blocks are symmetrically arranged in a sinking platform of the driven sprocket through a pin shaft center and can rotate around the pin shaft, a mandril is driven to rotate by the two centrifugal blocks through a decompression disc, the other end of the mandril extends into a camshaft core, a radial pin hole is arranged on one side of a base circle of the exhaust cam, a jacking pin is arranged in the pin hole, a compression spring is sleeved on the jacking pin, one end of the jacking pin is always in contact with the mandril through the compression spring, and the jacking pin can be higher or lower than a base circle of the exhaust cam by different lift ranges when the mandril rotates with a. The pressure reducing mechanism has the following disadvantages: firstly, this decompression mechanism is equipped with the step on driven sprocket, comes to prescribe a limit to the throw-out stroke of centrifugal piece, under the prerequisite of guaranteeing driven sprocket structural strength, this limit structure's setting leads to sprocket edge gear part thickness increase, leads to the sprocket quality to increase, can't satisfy sprocket assembly's lightweight design requirement. In addition, when the centrifugal block is reset, the shifting pin on the centrifugal block is matched with the clamping groove on the pressure reducing disc to overcome the elasticity of the return torsion spring to realize the positioning of the centrifugal block, and due to the factors such as fit clearance error and abrasion, the shifting pin can be separated from the clamping groove or the shifting pin is blocked, so that the pressure reduction is invalid.

At present, in order to achieve the lightweight design requirement of the sprocket assembly, the conventional mode is realized by reducing the mass of each component of the sprocket assembly, such as a decompression disc and a sprocket. Taking a chain wheel as an example, the thickness of the chain wheel is reduced on the basis of the same structural strength mainly through material improvement at present, or lightening holes are formed in the chain wheel, and the lightweight design requirement of the chain wheel is met.

Disclosure of Invention

The utility model aims at having the above-mentioned problem to current technique, provided a motorcycle engine starts decompression structure, the utility model provides a current engine starts decompression structure has the not enough problem of lightweight.

The purpose of the utility model can be realized by the following technical proposal: the utility model provides a motorcycle engine starts decompression structure, includes camshaft, sprocket and relief pressure valve assembly, be equipped with the fastener on the sprocket and the sprocket links firmly on the camshaft through the fastener, relief pressure valve assembly is including setting up the flail piece on the sprocket side, the one end of flail piece is articulated mutually with the sprocket and the flail piece can be along the radial throw-out of sprocket under the effect of centrifugal force, be connected with the elastic component that resets on the flail piece, its characterized in that, the flail piece supports after resetting under the effect of the elastic component that resets and leans on the fastener, the edge of sprocket is equipped with spacing arch, the hinged end of flail piece leans on and prescribes a limit to throw the stroke that the piece throwed away after throwing away.

The chain wheel is fixedly connected to the camshaft through a fastener, so that the chain wheel is convenient to install and good in stability. When the engine starts, the camshaft begins to rotate, can make the flail piece throw away because of the effect of centrifugal force when the camshaft reachs certain rotational speed, and at this moment, the spacing arch of the edge of sprocket leans on with the hinged end counterbalance of throwing the piece to realize throwing the restriction that the piece was thrown away the stroke the most. Get rid of the piece at the in-process that outwards throws away, because the hinged end is close to the pin joint, consequently for its one end of free swing, the distance of the outside swing of hinged end is shorter, and utilizes hinged end and spacing arch to lean on, and the distance that can design between the piece is thrown to spacing arch and when being in the reset state is less, and the diameter requirement to the sprocket is lower. At this moment, because spacing arch sets up the edge at the sprocket again for on the piece of throwing away satisfies the required design's of stroke basis, can do for a short time with the diameter of sprocket to the at utmost, thereby realize the reduction of sprocket quality. And, spacing protruding a little portion that is protruding on the sprocket side only, therefore spacing bellied setting has also satisfied the lightweight design requirement of sprocket.

When the engine stalls, the flail piece resets and then supports and lean on the fastener owing to the effect of the elastic component that resets for flail piece after resetting has obtained the location, and the flail piece can swing at the angle within range who sets for all the time like this, makes decompression structure reliable operation. At this moment, the fastener has played the dual function of connecting fixedly and restriction flail piece and has reset the stroke, and such design has saved additionally to set up relevant structure and has supplied to flail piece spacing when reseing, consequently makes sprocket assembly structure obtain simplifying, or has reduced spare part assembly quantity to make sprocket assembly realize the lightweight design.

In the motorcycle engine starting pressure reducing structure, the number of the throwing blocks and the number of the fasteners are two, the two throwing blocks form central symmetry about the center of the chain wheel, each throwing block is connected with the reset elastic piece, and the two throwing blocks are abutted against the two fasteners in a one-to-one correspondence under the action of the reset elastic pieces. The centrifugal block is provided with two centrosymmetric centrifugal blocks to form a pair of equal couple, when the engine is started, the centrifugal blocks which are symmetrically arranged can reduce the eccentric force of the centrifugal blocks in the centrifugal process, reduce the eccentric wear of parts of the pressure reducing valve, and improve the reliability and stability of the work of the pressure reducing structure. The setting of two fasteners has satisfied the demand that the sprocket was stably installed, leans on with two flail blocks respectively through two fasteners simultaneously for sprocket assembly structure obtains simplifying, the structure is more compact, makes sprocket assembly realize lightweight design.

In the motorcycle engine starting pressure reducing structure, the middle part of the chain wheel is provided with the mounting hole, and the fastening piece is positioned between the inner side edge of the corresponding throwing block and the edge of the mounting hole. The design of mounting hole is convenient for the installation of sprocket fixed. Carry out rational design through the position to the fastener for the fastener is located and corresponds between the border of the inboard side of getting rid of the piece and mounting hole, guarantees to get rid of the piece after reseing, and the inside wall of getting rid of the piece can support and lean on the fastener, thereby realizes the spacing requirement when getting rid of the piece and reseing and realizes sprocket assembly's lightweight design.

In the motorcycle engine starting pressure reducing structure, the throwing block comprises a hinged part and a strip-shaped swing arm part, the throwing block is hinged with the chain wheel through the hinged part, one end of the swing arm part is connected to the side surface of the hinged part, which is back to the chain wheel, and the bottom surface of the swing arm part, which faces the chain wheel, is higher than the limiting bulge, the throwing block abuts against the fastening piece through the inner side edge of the swing arm part, and the hinged part of the throwing block abuts against the limiting bulge after the throwing block is thrown out. In order to satisfy the minor diameter designing requirement of sprocket, when getting rid of the piece and being in reset state, spacing arch and get rid of the distance between the piece and be than nearer, this moment, it is the form that articulated portion and swing arm portion combine to get rid of the piece design, make swing arm portion connect on articulated portion side of the sprocket dorsad, thereby raise swing arm portion and make its bottom surface be higher than spacing arch, avoid getting rid of the piece and outwards throwing away the in-process pendulum arm portion that forms with spacing arch and interfere, guarantee that spacing arch can support with articulated portion and lean on.

In the motorcycle engine starting pressure reducing structure, the number of the limiting protrusions is two, the limiting protrusions and the chain wheel are integrally formed, the limiting protrusions are strip-shaped, the length direction of the limiting protrusions is arranged along the circumferential direction of the chain wheel, and the two limiting protrusions are arranged on the outer sides of the hinging portions of the two throwing blocks in a one-to-one correspondence mode. Spacing arch and flail piece integrated into one piece for the sprocket is made conveniently, and moreover, because flail the piece and lean on when leaning on at spacing arch, the camshaft all is in high-speed pivoted state usually, and spacing arch can bear great pressure effect this moment, and spacing arch and flail piece integrated into one piece make spacing protruding structural strength high, can bear great pressure effect.

In the motorcycle engine starting pressure reducing structure, the side surface of the chain wheel, which is back to the camshaft, is the front side surface of the chain wheel, the two throwing blocks are both arranged on the front side surface of the chain wheel, and the throwing direction of the throwing blocks is anticlockwise in a view angle of looking at the front side surface of the chain wheel. The swing block and the clockwise swing are convenient in the resetting process of the swing block due to the design, and the tightening direction of the fastener is also the clockwise direction, so that the action force for driving the fastener to be reversely loosened is not brought to the fastener at the moment that the swing block is contacted with the fastener, the fastener is always in a tightening state, and the stability of the chain wheel installation is further ensured.

In the motorcycle engine starting pressure reducing structure, the fastening piece is a cylindrical head bolt, the head of the fastening piece is abutted against the front side face of the chain wheel, the swing arm part is provided with a suspension section which extends out of the hinge part and is suspended relative to the front side face of the chain wheel, the suspension section is arc-shaped, and the inner concave surface of the suspension section is abutted against the head of the corresponding fastening piece. The suspension section is the arc, and the fastener adopts the cylinder head bolt for the suspension section has great area of contact when the fastener supports and leans on, promotes positioning effect, avoids because the less condition of bringing of area of contact wearing and tearing even deformation of spare part. In the installation process of the bolt, in order to increase the contact area, a gasket needs to be arranged between the head of the bolt and the front side face of the chain wheel, or a flange bolt with a turned edge integrally formed at the head is adopted. At the moment, the suspension section which is suspended relative to the front side face of the chain wheel is arranged on the swing arm part, so that the suspension section avoids the turning edge of the gasket or the flange bolt, and the suspension section can abut against the cylindrical surface of the cylindrical head bolt.

In the motorcycle engine starting pressure reducing structure, the hinge part is plate-shaped, the hinge part and the swing arm part are both arranged in parallel with the side surface of the chain wheel, and the hinge part and the swing arm part are of an integrated structure. Such design makes the flail block make conveniently, and structural strength is high. Meanwhile, the hinged part and the swing arm part are arranged in parallel with the side face of the chain wheel, so that the whole structure of the chain wheel assembly is compact, and the arrangement and installation of the chain wheel assembly are facilitated.

In the motorcycle engine starting pressure reducing structure, the end part of the camshaft is fixedly sleeved with the connecting seat, the connecting seat is provided with a flange surface attached to the rear side surface of the chain wheel, and the fastening piece penetrates through the chain wheel and is in threaded connection with the connecting seat. Through such design for the sprocket installation is stable, thereby guarantees this decompression structure job stabilization nature and reliability.

In the motorcycle engine starting pressure reducing structure, the camshaft is provided with an exhaust cam, a central hole is axially arranged in the camshaft, a pin hole communicated with the central hole is radially formed in the exhaust cam, the pressure reducing valve assembly further comprises a pressure reducing pin arranged in the pin hole and a pressure reducing valve rod arranged in the central hole and connected with the pressure reducing pin, one end of the pressure reducing valve rod is provided with a linkage disc extending into the mounting hole of the chain wheel, two shifting pins are arranged on the linkage disc, notch grooves are formed in hinged parts of the two throwing blocks, and the two shifting pins are correspondingly inserted into the notch grooves of the two throwing blocks one by one. When the engine stops working, the working end of the pressure reducing pin, which is matched with the exhaust valve, is higher than the cam base circle of the exhaust cam, and the throwing block drives the pressure reducing valve rod to rotate after throwing out, so that the pressure reducing pin retracts into the pin hole, and the working end of the pressure reducing pin is lower than the cam base circle of the exhaust cam.

Compared with the prior art, the motorcycle engine starting pressure reducing structure has the following advantages:

1. this pressure reduction structure is equipped with spacing arch on the sprocket and prescribes a limit to the formation of throwing away of throwing the piece, and utilizes the fastener to supply to throw the piece and support when reseing and lean on for the sprocket subassembly structure obtains simplifying, has perhaps reduced spare part assembly quantity, thereby makes the sprocket subassembly realize lightweight design, still has compact structure's advantage simultaneously.

2. In this decompression structure, get rid of the piece and obtained injecing along the radial swing amplitude of camshaft for get rid of the piece and swing on the orbit of design all the time, job stabilization and running accuracy are high, have avoided the possibility that the decompression became invalid.

3. This decompression structure is provided with two centrosymmetric and gets rid of the piece, forms a pair of couple that equals, and when the engine started, the piece of getting rid of symmetrical arrangement can reduce the piece and get rid of the eccentric force of the in-process that throws away, reduces the eccentric wear phenomenon of relief pressure valve spare part, promotes the reliability and the stability of decompression structure work.

Drawings

Fig. 1 is a schematic view of the three-dimensional structure of the present invention in a decompression state.

Fig. 2 is a side view of the present invention in a depressurized state.

Fig. 3 is a cross-sectional view a-a of fig. 2.

Fig. 4 is an exploded view of the present invention.

Fig. 5 is a schematic view of the three-dimensional structure of the middle flail block of the present invention.

Fig. 6 is a side view of the present invention with the pressure relief function closed.

Fig. 7 is a cross-sectional view a-a of fig. 6.

Fig. 8 is a cross-sectional rotated view of B-B in fig. 6.

In the figure, 1, a camshaft; 11. a connecting seat; 111. a flange face; 12. a central bore; 13. an exhaust cam; 14. a pin hole; 2. a sprocket; 21. a limiting bulge; 22. a front side; 23. mounting holes; 3. a fastener; 4. throwing blocks; 41. a hinge portion; 411. a notch groove; 42. a swing arm part; 421. a suspended section; 5. a restoring elastic member; 6. a pressure reducing valve stem; 61. a linkage disk; 7. a pin shaft; 8. a decompression pin; 9. and (4) pulling a pin.

Detailed Description

The following are specific embodiments of the present invention and the accompanying drawings are used to further describe the technical solution of the present invention, but the present invention is not limited to these embodiments.

Example one

As shown in fig. 1 to 3, the motorcycle engine starting decompression structure comprises a camshaft 1, a sprocket 2 and a decompression valve assembly, wherein the sprocket 2 is provided with a fastener 3 for fixing the sprocket 2 on the camshaft 1, and the camshaft 1 is provided with an exhaust cam 13. The middle part of the chain wheel 2 is provided with a mounting hole 23, a central hole 12 is arranged in the camshaft 1 along the axial direction, a pin hole 14 communicated with the central hole 12 is arranged on the exhaust cam 13 along the radial direction, the pressure reducing valve component comprises a throwing block 4, the pressure reducing pin 8 arranged in the pin hole 14 and the pressure reducing valve rod 6 arranged in the center hole 12 and connected with the pressure reducing pin 8, the side surface of the chain wheel 2, which is back to the camshaft 1, is the front side surface 22 of the chain wheel 2, the throwing block 4 is arranged on the front side surface 22 of the chain wheel 2, one end of the throwing block 4 is hinged with the chain wheel 2 through the pin shaft 7, the throwing block 4 can be thrown out along the radial direction of the chain wheel 2 under the action of centrifugal force, the throwing block 4 is connected with the reset elastic part 5, the throwing block 4 is reset under the action of the reset elastic part 5 and then is abutted against the fastening part 3, the edge of the chain wheel 2 is provided with the limiting protrusion 21, and the hinged end of the throwing block 4 after the throwing block 4 is thrown out is abutted against the limiting protrusion 21 so.

As shown in fig. 1 and 2, the limiting projection 21 is integrally formed with the sprocket 2, and the limiting projection 21 is in the shape of a bar and has a length direction arranged along the circumferential direction of the sprocket 2. Get rid of piece 4, fastener 3 and spacing protruding 21's quantity is two, two get rid of pieces 4 all set up on the leading flank 22 of sprocket 2, two get rid of pieces 4, two fasteners 3 and two spacing protruding 21 are equallyd divide and do not form central symmetry about the center of sprocket 2, fastener 3 is located to correspond between the border of the inboard side of getting rid of piece 4 and mounting hole 23, all be connected with the elastic component 5 that resets on every gets rid of piece 4, in this embodiment, the elastic component 5 that resets is the torsional spring of cover on round pin axle 7, two get rid of 4 one-to-ones of piece support and lean on two fasteners 3 under the effect of elastic component 5 that resets. Set up two centrosymmetric and get rid of piece 4, form a pair of couple that equals, when the engine starts, the piece 4 that gets rid of symmetrical arrangement can reduce the piece 4 eccentric power of getting rid of the in-process, reduces the eccentric wear phenomenon of relief pressure valve spare part, promotes reliability and the stability of decompression structure work.

As shown in fig. 3 and 4, the end of the camshaft 1 is fixedly sleeved with a connecting seat 11, the connecting seat 11 has a flange surface 111 abutting against the rear side surface of the sprocket 2, and the fastening member 3 passes through the sprocket 2 and is screwed with the connecting seat 11. Through such design for sprocket 2 installation is stable, thereby guarantees this decompression structural work's stability and reliability. One end of the pressure reducing valve rod 6 is provided with a linkage disc 61 extending into the mounting hole 23 of the chain wheel 2, two shifting pins 9 are arranged on the linkage disc 61, the hinge parts 41 of the two throwing blocks 4 are provided with notch grooves 411, and the two shifting pins 9 are correspondingly inserted into the notch grooves 411 of the two throwing blocks 4 one by one.

As shown in fig. 4 and 5, the fastening member 3 is a cylindrical bolt, the head of the fastening member 3 abuts against the front side surface 22 of the chain wheel 2, the swinging block 4 comprises a hinge portion 41 and a strip-shaped swinging arm portion 42, the swinging block 4 is hinged with the chain wheel 2 through the hinge portion 41, and the two limiting protrusions 21 are correspondingly arranged on the outer sides of the hinge portions 41 of the two swinging blocks 4. The hinge portion 41 is plate-shaped, the hinge portion 41 and the swing arm portion 42 are both provided in parallel with the side surface of the sprocket 2, and the hinge portion 41 and the swing arm portion 42 are integrally structured. The swing arm 42 has a suspending section 421 extending out of the hinge portion 41 and suspending in the air relative to the front side 22 of the sprocket 2, the suspending section 421 is arc-shaped, and an inner concave surface of the suspending section 421 abuts against the head of the corresponding fastener 3. As shown in fig. 8, one end of the swing arm 42 is connected to the side of the hinge portion 41 opposite to the sprocket 2, and the bottom surface of the swing arm 42 facing the sprocket 2 is higher than the limiting protrusion 21, so as to prevent the swing arm 42 from interfering with the limiting protrusion 21 during the outward throwing process of the throwing block 4, and ensure that the hinge portion 41 of the throwing block 4 abuts against the limiting protrusion 21 after the throwing block 4 is thrown.

As shown in fig. 6, the throwing direction of the throwing block 4 is counterclockwise when viewed from the front side surface 22 of the sprocket 2, so that the throwing block 4 can conveniently swing clockwise in the resetting process of the throwing block 4, and the tightening direction of the fastener 3 is also clockwise, therefore, the action force for driving the fastener 3 to loosen reversely is not brought to the fastener 3 at the moment when the throwing block 4 is contacted with the fastener 3, the fastener 3 is ensured to be always in a tightening state, and the mounting stability of the sprocket 2 is further ensured.

The working principle of the present pressure relief structure is briefly described below:

as shown in fig. 6 to 8, when the engine is started, the camshaft 1 starts to rotate, when the camshaft 1 reaches a certain rotating speed, the throwing block 4 can be thrown out due to the action of centrifugal force, the throwing block 4 drives the pressure reducing valve rod 6 to rotate after being thrown out, so that the pressure reducing pin 8 can be retracted into the pin hole 14, the working end of the pressure reducing pin 8 is lower than the cam base circle of the exhaust cam 13, the pressure reducing structure does not play a role in reducing pressure, the limiting protrusion 21 at the edge of the sprocket 2 abuts against the hinged end of the throwing block 4, and therefore the limitation of the maximum throwing stroke of the throwing block 4 is achieved.

As shown in fig. 2 and 3, when the engine is shut down, the throwing block 4 is reset by the reset elastic piece 5 and abuts against the fastening piece 3, and the working end of the pressure reducing pin 8, which is matched with the exhaust valve, is higher than the cam base circle of the exhaust cam 13 to play a pressure reducing role, so that the pressure reducing effectiveness is ensured when the engine is started next time.

Example two

The structure and principle of this embodiment are basically the same as those of the first embodiment, and different points are as follows: the elastic part 5 that resets is the extension spring, and the one end of extension spring is connected in 4 free wobbling one ends of getting rid of the piece, and the other end of extension spring is connected on sprocket 2.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications, additions and substitutions for the specific embodiments described herein may be made by those skilled in the art without departing from the spirit of the invention or exceeding the scope of the invention as defined in the accompanying claims.

Although 1, camshaft is used more herein; 11. a connecting seat; 111. a flange face; 12. a central bore; 13. an exhaust cam; 14. a pin hole; 2. a sprocket; 21. a limiting bulge; 22. a front side; 23. mounting holes; 3. a fastener; 4. throwing blocks; 41. a hinge portion; 411. a notch groove; 42. a swing arm part; 421. a suspended section; 5. a restoring elastic member; 6. a pressure reducing valve stem; 61. a linkage disk; 7. a pin shaft; 8. a decompression pin; 9. deadbolting, etc., but does not exclude the possibility of using other terms. These terms are used merely to more conveniently describe and explain the nature of the present invention; they are to be construed in a manner that is inconsistent with the spirit of the invention.

Claims (10)

1. A motorcycle engine starting pressure reducing structure comprises a camshaft (1), a chain wheel (2) and a pressure reducing valve component, the chain wheel (2) is provided with a fastener (3) and the chain wheel (2) is fixedly connected on the camshaft (1) through the fastener (3), the pressure reducing valve assembly comprises a throwing block (4) arranged on the side surface of the chain wheel (2), one end of the throwing block (4) is hinged with the chain wheel (2), the throwing block (4) can be thrown out along the radial direction of the chain wheel (2) under the action of centrifugal force, the throwing block (4) is connected with a reset elastic piece (5), characterized in that the throwing block (4) is reset under the action of the reset elastic piece (5) and then is abutted against the fastening piece (3), the edge of the chain wheel (2) is provided with a limiting bulge (21), and the hinged end of the throwing block (4) abuts against the limiting bulge (21) after the throwing block (4) throws away, so that the throw stroke of the throwing block (4) is limited.

2. A motorcycle engine starting decompression structure according to claim 1, wherein the number of the flail blocks (4) and the fasteners (3) is two, the two flail blocks (4) form central symmetry about the center of the chain wheel (2), each flail block (4) is connected with a return elastic member (5), and the two flail blocks (4) are abutted against the two fasteners (3) in a one-to-one correspondence under the action of the return elastic members (5).

3. A motorcycle engine starting decompression structure according to claim 2, wherein a mounting hole (23) is formed in the middle of the sprocket (2), and the fastening member (3) is located between the inner side edge of the corresponding thrower (4) and the edge of the mounting hole (23).

4. The motorcycle engine starting decompression structure according to claim 3, wherein the swinging block (4) comprises a hinge portion (41) and a strip-shaped swinging arm portion (42), the swinging block (4) is hinged with the chain wheel (2) through the hinge portion (41), one end of the swinging arm portion (42) is connected to the side surface of the hinge portion (41) back to the chain wheel (2), the bottom surface of the swinging arm portion (42) facing the chain wheel (2) is higher than the limiting protrusion (21), the swinging block (4) abuts against the fastening member (3) through the inner side edge of the swinging arm portion (42), and the hinge portion (41) of the swinging block (4) abuts against the limiting protrusion (21) after the swinging block (4) is swung out.

5. A motorcycle engine starting decompression structure according to claim 4, wherein the number of the limiting protrusions (21) is two, the limiting protrusions (21) are integrally formed with the sprocket wheel (2), the limiting protrusions (21) are strip-shaped, the length direction of the limiting protrusions is arranged along the circumferential direction of the sprocket wheel (2), and the two limiting protrusions (21) are arranged on the outer sides of the hinge parts (41) of the two throwing blocks (4) in a one-to-one correspondence manner.

6. A motorcycle engine starting decompression arrangement according to claim 4 or 5, characterized in that the side of the sprocket wheel (2) facing away from the camshaft (1) is the front side (22) of the sprocket wheel (2), both throws (4) being arranged on the front side (22) of the sprocket wheel (2), the direction in which the throws (4) throw out being anticlockwise in a view looking at the front side (22) of the sprocket wheel (2).

7. A motorcycle engine starting decompression structure according to claim 6, wherein the fastener (3) is a cylindrical head bolt, the head of the fastener (3) abuts against the front side (22) of the sprocket (2), the swing arm portion (42) has a suspending section (421) extending out of the hinge portion (41) and suspending with respect to the front side (22) of the sprocket (2), the suspending section (421) is arc-shaped, and an inner concave surface of the suspending section (421) abuts against the head of the corresponding fastener (3).

8. A motorcycle engine starting decompression structure according to claim 4, wherein the hinge portion (41) is plate-shaped, the hinge portion (41) and the swing arm portion (42) are both disposed in parallel with a side surface of the sprocket (2), and the hinge portion (41) and the swing arm portion (42) are of an integral structure.

9. A motorcycle engine starting decompression structure according to claim 7, wherein a connecting seat (11) is fixedly sleeved on an end of the camshaft (1), a flange surface (111) attached to a rear side surface of the sprocket (2) is arranged on the connecting seat (11), and the fastening member (3) penetrates through the sprocket (2) and is screwed with the connecting seat (11).

10. A motorcycle engine starting decompression structure according to claim 4, the camshaft (1) is provided with an exhaust cam (13), a central hole (12) is arranged in the camshaft (1) along the axial direction, a pin hole (14) communicated with the central hole (12) is radially arranged on the exhaust cam (13), the pressure reducing valve assembly also comprises a pressure reducing pin (8) arranged in the pin hole (14) and a pressure reducing valve rod (6) arranged in the central hole (12) and connected with the pressure reducing pin (8), one end of the decompression valve rod (6) is provided with a linkage disc (61) extending into the mounting hole (23) of the chain wheel (2), the linkage disc (61) is provided with two shifting pins (9), the hinging parts (41) of the two throwing blocks (4) are provided with notch grooves (411), and the two shifting pins (9) are correspondingly inserted into the notch grooves (411) of the two throwing blocks (4) one by one.

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