Info Manual

Motorcycle Camshaft Decompression Pin

We all know some motorcycle camshaft especially those SOHC engine have a so called Decompression cam or other called it Decompression Pin. But what it is really for what? Take a look at the picture of a shogun 125cc motorcycle equipped with this pin. The hook like pin located at the cam gear of the camshaft is the decompression pin. With that position (hook is resting towards the center, it is actually in the ON position. It already trigger the exhaust valve to open so that when we turned off the engine, the combustion chamber releases unwanted compressed mixture at the exhaust. This scenario makes it easier to start the engine by allowing fresh mixture to enter the chamber when we turned on the engine again. There is more, the unwanted kick back of the kick starter to which can cause injury when turning the engine by kick when the combustion chamber have leftover combustible mixture if this pin is not present.

Others find it difficult to understand why it was there, and they tend to remove this pin saving weight on their motorcycle in order to produce a little more top end, but what they do not actually know. As of my findings in my 10 year old shogun 125cc was. when my decompression pin stop operating normally due to the spacer break down as shown,
i have a real hard time starting it in the morning and kick always kick back to me, and so i decided to replace it with such that can cope the tromendous heat build up inside by handcrafting the spacer from a fiberglass PCB as shown.
I was so lucky that the broken plastic part of the valve train did not manage to create havoc inside my engine because it was trapped somewhere that i manage to pull out when i checked my clutch lining (an article is being written after this)..

I also found out that without the pin attached to the camshaft..and start the engine, abnormal vibration can be felt at the head area, and so i believe it has a function that when the camshaft is turning and the hook begins to move outwards near the edge of the cam gear due to INERTIA, it acts as a balancer of the camshaft for the hook part is against the LOBE of the cam.

So to those who are eyeing of removing this pin..take a closer look at the photo on how this part actually works. It was there for a reason., and removing this will not add you some extra HP or gain you some at the top end.