If the timing chain breaks on my '85 CB650, will I crash?

I think I'm hearing some chain slap from the timing chain on my CB650 Nighthawk and I was wondering if the timing chain breaks while riding, if it will lock up the rear tire and make me crash? I have been told it's an interference engine so it won't (not really sure why it won't if it IS an interference engine...), but I just want to be extra sure before I ride it any more. It's shaft driven if that makes any difference.

If you lose the motor you can clutch it in and freewheel to the curb. If you seize the transmission, it's going to slide to a stop (one way or another). If it is an interference engine, it means that the valves can hit pistons if the cam quits spinning. Whether or not that creates a hard stop on the motor, you can still release the rear wheel with the clutch.

Usually when the timing chain or belt breaks the engine doesn't seize. On an interference engine you will have bent valves. One way to tell if the chain/belt broke is the engine will crank very fast because of either bent valves or stuck open valves prevents the engine from making compression so she turns over easily and therefore fast.

Stating the obvious I know, but why not fit a new timing chain ? It will be a lot cheaper than a broken motor ?

Or adjust or repair the timing chain adjuster?

Griff said:

Stating the obvious I know, but why not fit a new timing chain ? It will be a lot cheaper than a broken motor ?

Click to expand...

From what I understand the entire engine has to be split apart to remove the timing chain. I don't have a place to work on it where I currently live or the know-how to perform such a task. A local mechanic quoted me $650 plus parts. It's an old, ratty bike, not worth the expense in my eyes when something else soon will probably go wrong. I was going to hang onto it to teach a couple of my friends how to ride, I just wanted to make sure that if the engine blows they will not get hurt.

To ddeulin's question, have you attempted to adjust the tensioner? If it is working correctly, this only takes a minute or so to do. It is a hyvo type chain and it would take a lot to break it. More likely the tensioner will fail and it will skip a tooth causing all manner of damage and grief - but probably STILL not breaking the chain. Even if the tensioner is not working correctly, if the guides are still OK, then you can adjust it manually. Your "local mechanic" should be able to sort this out for you. How many miles on the bike?

Check nighthawk-forums.com. the 83-85 Nighthawks are prone to cam chain noise. Usually the tensioner spring. OEM springs are not available. If you need a chain, it can be replaced w/o splitting the engine, but you will need to rivet a chain link.

I'll look into how to adjust the tensioner. The bike has about 56k miles. My "local" guy is more than an hours drive, it's not very easy to get there and back. Everyone up where I live either only work on Harleys or won't work on an old bike.

If it is the same as my CB500-4 there is a nut on the rear of the head that you loosen with the engine running. A spring resets the tension and you tighten the nut. On mine, the spring seemed like it wasn't up to the task and I had to remove the valve cover and adjust it manually with the engine off.