Motorcycle Repair: Honda cm450e 1982, Vibration During accelaration.

Question
As I am winding up to change gears, about 3/4 of the way through the rpms I pick up a vibration that goes away if I give it more gas, or let off for a second. It doesn't last long and its only during accelaration. I have some ideas but I am not sure.

Answer
Chad, one thing that is important to keep these bikes running smoothly is to adjust the balancer shaft to tension the chain.

http://www.cmsnl.com/honda-cm450e-1983-usa_model7290/partslist/E++16.html#result  

If you look at the left side of the illustration, you can see the adjuster arm with the slotted hole. This all resides underneath the clutch cover on the right side of the engine.

You can access the adjuster by going through the hole on the right front corner of the cover...

http://www.cmsnl.com/honda-cm450e-1983-usa_model7290/partslist/E++06.html#result

Open the hole plug and look down inside to see where the adjuster arm is setting presently. Loosening the lock nut allows the adjuster arm plate to move through an arc in the slotted hole. I don't think that the arm is spring loaded, so you will have to move the arm with a long screwdriver to see if it wants to take up some slack in the balancer chain by moving the adjuster arm further in the slot. If you get to the end of the slot and the chain is still loose, it requires removal of the clutch cover to access the tensioner parts. You have to remove the snap ring and take the arm off the shaft splines and then move it one spline so that the adjuster arm is back in the center of the adjustment slot when the chain is tightened.

When the chain is loose the balancer shaft weights get out of phase with the engine' crankshaft movements. This CAN cause vibration in some cases.

Bear in mind that the engine has a 360 degree crankshaft so both pistons are going up and down together,  alternating firing with each rotation. Without the balancer the engine would shake like crazy. That said, all motorcycle engines make vibrations at different engine speeds, especially when it comes up on the fat end of the powerband. The power and torque curves when plotted out are not straight lines. They have a big curvature to them and when the power really starts to build, you will feel that increase in vibration. Engines are balanced for certain rpms and when they are not in that range they will shake more than at other times.

Bill Silver