Something snapped? CB350

tsivero

tsivero

New Member
Something snapped. I think...

Was cruising along on my '72 CB350 last weekend and all of a sudden I heard/felt what seemed like a cable snapping. Immediately the bike revved down and shut off. Thought it might have been my throttle cable at first. When I finally rolled to a stop, I tried to kick the thing over (no push start) and immediately felt much less resistance than I normally do. The crank continued to spin as well which doesn't normally happen. When I kick it over now, i can hear the internal parts continue to spin freely when I kick it all the way down.

It's worth noting that this snap sound/feeling happened mid-top of the gear range. I wasn't in the middle of any gear change.

Ended up towing it home and was able to figure out a couple more things once I got there.
1) Clutch doesn't seem to be engaging it right. I say this because I can put it any gear and roll it with little resistance without holding the clutch. I can actually feel it going over the gear points, almost feels like equal distant bumps on a road.
2) Little to no compression. Took the spark plugs off both cylinders and I'm not feeling any pressure. With my thumbs off, I do feel some air coming out. Definitely nothing compared to the compression I've felt when setting my points though. I can see parts moving inside so that's good.

I'm thinking the clutch went out but I wanted to post here to get some help with a diagnosis. I'm considering just taking it to the shop since it's definitely something internal. I'd love to do the work myself but need to figure out the cost versus frustration of figuring it out.

Worth noting that I took the bike on a 30+ mile ride the weekend before and it gave me zero trouble.

Appreciate any and all help!
 
I suspect you either broke the cam chain or the upper cam sprocket came away from the cam shaft. Take the plugs out, using the kick start lever rotate the engine. While doing this, place a long thin screw driver in on the top of a piston. as you crank the piston should come up and down. Open a valve adjusting port and see if the rockers are moving as you crank.
 
Thanks a ton for the reply guys. Much appreciated.

@TheCoffeeGuy - I liked where you were heading. Pistons are definitely moving up and down. I went as far to take the cylinder cover off the top of the engine. The sprocket and chain are all in place and in good shape. I turned the kick-start and i can see the chain moving but it's bouncing up and down quite a bit. I pulled off the cam chain tensioner adjustment that sits between the carbs and this could be the culprit. It's hard to tell though. I was able to get some pretty good pics on the iPhone by sticking the camera where the carbs sit.

Any thoughts on this being the culprit?

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The tensioner roller is worn, but I wouldn't think this would cause your issue.
If the chain and sprocket are moving up and down, I suspect the cam sprocket bolts sheared and the sprocket is spinning on the camshaft.
To verify: open a rocker inspection cap. You should be able to see the rocker moving when turning the motor over with the kick starter.
 
I agree. Sounds like the sprocket bolts sheared. If it snapped at TDC it's possible that it might turn over without hitting anything but I'd not be surprised to see heads off valves...

The top end has to come off to see what's wrong.

With the top cover removed as you kick it over does the cam rotate and valves open and close?
 
So if the cam chain tensioner wheel is gone ( they are like a hard rubber) and you say the cam chain is bouncing up and down, then to me, the cam chain has probably jumped a few teeth. This is supported by the position of the chain in your pictures, way off to one side.

Having jumped teeth, even if the cam is going around and the valves are going up and down, they will not be doing it in concert with the pistons at the right time. Hence no compression.

Get the manual out and see where the timing marks on the cam sprocket line up when the timing marks on the crank shaft are set. As i recal the timing mark on the cam sprocket is a small punch mark.
 
If you don't have a Factory Service Manual you can download one here: http://www.common-motor.com/manuals
Good luck sorting it out.
 
Really appreciate everyone's help.

Alzcbz called it. The cam sprocket bolts are gone and the chain/sprocket are turning on top of the camshaft. Once I opened things up a bit more, I could see it spinning over the camshaft while the rockers didn't move at all.

Once again, really appreciate everyone's help as this was the first time I've had to open the engine for something.

My question now is, will I need to take the engine off the bike completely to replace this? In addition, should I be worried that with this detached, my pistons got messed up when it snapped? I will say that when it did snap, the engine rev-ed down calmly and just shut off.
 
You will want to pull the head and check valves and, at least, the tops of the pistons. I would remove the valves from the head, as well, and check runout on the stems.
Disassembly will require you to remove the engine from the frame. There's not much room for engine work on the 350 frame.
 
Thanks alzcbz and everyone who's been helping.

Finally started tearing the top down a bit with the engine on the frame still. Was hoping to at least replace the sprocket and see where that gets me. Unfortunately, I got all the way to removing the rocker pins and rockers and got stuck. The left rear rocker pin won't move and it seems like its the same on the right rear one as well. I'm starting to think that the camshaft actually might have stopped and that's what snapped the sprocket. Any idea why the rocker pins would be stuck? Other one slid out easy as it I've seen in videos.

Next step, is probably to take it to my mechanic since an engine tear down is a bit past my comfort zone and scope of work for my little garage.
 
Typically the pins slide out easily once there is no pressure on them. You need to turn the cam shaft some way to release the pressure on the rocker. The Cam Sprocket bolts snap because they are old or there has been a lot of backfires. this usually happens when an old bike is tried to be restarted.

there is no way the rockers could reasonably stop the cam shaft enough to shear those bolts.
 
Is there pressure on the rockers from the cam lobes? Try turning the pins so you open the gap on the valves. Like already said, you may have to find a way to turn the camshaft.
This is a pretty simple engine. You should be able to do a rebuild if necessary.


Sent from my iPhone using Tapatalk
 
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