QuestionI have a '78 Honda CB 750k. The bike is making a "metal churning" sound in/around the motor. The shop I have it in thinks it's the timing chain. How can I tell for sure if it is the timing chain or not? If it does need to be replaced, how big a job is that? What is involved with changing it out?
AnswerBrian, it could well be the timing chain, but there is an adjustment on the tensioner, located on the back of the cylinders. It is a 10 minute job, at the most.
Remove the dyno cover on the right side and find a good sized wrench/socket so you can turn the motor over slowly by hand.
Remove the valve cover caps on that #4 cylinder, so you can watch the valves open and close. Once you see a valve open or close, pay attention to whether there is a delay between the back and forth rotation of the crankshaft and the motion of the open valve. Turning the crankshaft back and forth should result in no delay/lag between the motion of the valve. If you feel a delay or free play in the crankshaft rotation, loosen the locknut on the locking bolt for the tensioner and watch for the end of the tensioner to jump inwards. Turn the crankshaft back and forth about 20 degrees so you can watch the end of the tensioner rod.
http://www.bikebandit.com/houseofmotorcycles/honda-motorcycle-cb750k-1978/o/m945... CAMCHAIN TENSIONER illustration block
If the bike has been running with a loose camchain for a long time, there could be damage to the chain and tensioner rollers and the inside of the cylinder block from camchain whip motion. Mileage has a lot to do with the condition of the parts, as well.
The stock camchain is endless... so it loops around the crankshaft sprocket in the middle of the crank. To remove the chain, you have to remove the crankshaft from the cases, after you have removed the whole top end of the engine. BIG JOB after you have wrestled the 176 lb motor out of the frame.
Some people have gotten the correct length camchain and bought a matching master link for that make/type of chain. Grinding the sideplate pins off of the camchain will divide it and you can string one end to end with the other one, feeding it through the engine, around the crankshaft sprocket and back up to the camsprocket. That only requires pulling the top valve cover assembly from the motor, but it still has to come out of the frame for service.
If you have tools, patience, some money and are willing to learn something new, it can be done. A shop manual would be a good investment if you have to go deep into the motor.
Bill Silver
