Like a lot of other things in life, timing is everything, and inside your bike’s engine timing this notion is especially correct.  Making sure the intake and exhaust valves are opening with the correct lift and duration, and that at the same precise moment in the cylinder stroke sequence, that the cams are doing the same ensures that your bike is running properly.  Today in this segment of Shop Talk we are going to talk about Cam Timing—understanding it and how to make sure you don’t have a problem with it.

To begin, a camshaft consists of the shaft, the sprocket or gear, a groove for a keeper or C-clip that keeps the shaft from moving from side to side, and the cam lobes.  The camshafts are kept in place by cam caps that bolt into the cylinder head.  In many modern engines a camshaft may have a position indicator that usually resembles a small, dowel-like rod of metal that interfaces with a sensor that is usually found in the cam cover.  This indicator allows the bike’s Electronic Control Unit to constantly recognize the exact position of the cams.  On many modern exhaust cams you’ll also find a decompression unit.  This is a spring-loaded device that allows the exhaust valve to open slightly when starting the engine, allowing the engine to spin to a start without having to overcome the full compression load of the pistons.

Generally speaking, the taller the cam lobe the more “lift,” the distance traveled by the valve from fully closed to fully open.  The wider the lobes, the more “duration” which describes the amount of time the valve stays open.  In some engines the camshaft lives deep inside the crankcase and is geared to the crankshaft.  These cam lobes communicate with the valves via pushrods that actuate rocker arms in the cylinder heads that open valves that are spring-loaded to snap closed. 

In other engines, the camshafts are located above the cylinder heads.  These are generally referred to as “overhead cams.”  In these engine types the cam lobes directly actuate spring-loaded valves, and are rotated in one of two ways.  Chain-driven camshafts have a sprocket at one end.  Around it travels a chain that is also tensioned around a sprocket driven by the crankshaft.  Gear-driven camshafts have a gear at one end that meshes with another gear driven by a chain from the crankshaft.

The distance between the cam lobe and the valve actuator buckets varies from engine to engine, and is governed by shims which are small pieces of metal machined to an exact thickness.  These shims can wear (valve and valve seat can wear), and to keep the bike running smoothly they will require adjustment or replacement from time to time.  In another Shop Talk session we’ve looked at this kind of valve job.  But as we found out from MMI Chief Instructor Chris Reo, it’s one thing to take all this stuff apart and the tricky part comes when everything goes back together. 

For further information, please consult this episode on video to see just how Chris installs the cams and sets them to the proper specs.  Also, always remember, if you are having any sort of problem of this type, take your bike to your dealership where it can be evaluated and repaired by a certified motorcycle mechanic.

Back to Episode CRN10024