FM Bike Works

I hadn’t really planned on doing work on the CB today, but I had a few questions plaguing my brain.  I spent a good portion of last night wondering why half the engine stays cold.  I decided it was time to check the ignition and the timing.

First I started with the ignition.  The CB900 has two ignition coils that send spark to the 4 spark plugs.

Two ignition coils on the CB900.

Each of the ignition coils has two lines that come off of it.  Properly wired coil 1 (left hand side) is connected to cylinder 1 and 4, and coil 2 is connected to 2 and 3.  The bike was all connected properly, so that’s a good start.  Next comes checking the resistance between the two outputs from each coil.  This was all well within spec for the two coils.  Using a spark tester on the spark plugs, I ran down the light and tried starting up the engine; all seem to fire without a problem.

So, then I thought about the timing.  Maybe something is off that’s not quite firing on the second side.  I pulled off the timing cover, adjusted the cam shaft to top dead center and took a look.

View of the timing layout for the CB900. Cam shaft at TDC.

I had to do a static timing as the bike isn’t in much shape for using a timing light.  In this way, setting the bike to TDC, the pointer on the cam shaft should be at a marker on the left-hand pulser generator.  The alignment is a little bit off, but quite close.

With a lack of knowing what else to look at, it was time to get into the engine.  Taking the cam cover off, I was able to take a look at the valve clearance for all the cylinders.

Left-hand side; cylinders 1 and 2.

The process is a bit dirty and a little time consuming, but not too bad.  Using feeler gauges, you rotated the cam shaft to different positions allowing you to take proper readings for the different valves (distance between a non compressed cam to the valve).  The clearance was all over the board:

According to the manual, the intake and exhaust clearances should be (cold) 0.06-0.13 mm, so things are definitely a bit off.  I might double check again in a little bit just to see that my numbers are right.  In the meantime, it looks like it’s time to find a shim kit and a valve lifter tool.