My cam chain broke!

[lusciouslemur]

I had to wait a day until I posted this because of how upset I was, but on my 1980 CB750C, I had just installed the engine after it was apart, only to have my cam chain break a day later.

I was starting the bike to see how she would idle, and after I closed the choke, I heard a hard CLICK and the engine cut off - opened the valve cover and the cam chain had broken (not the one linking the intake and exhaust camshafts, the one from the crank - camshaft).

I guess in retrospect that chain was immensely tight. I used a piece of steel as a lever to get the chain on the sprocket to begin with. Anyway, my question is, is this type of engine one that is vertically split or horizontally? I imagine i'll have to split the crankcase to replace this chain. Also, any special tools needed to do this? I hear of a pulling tool but never peered into the bottom end of an engine. ALSO, what's the likelihood of a bent or damaged valve? Thanks in advance.

[TOOLS1]

I am not sure if that is an interference engine or not, but if you had to use a prybar to install a cam chain, there is something wrong inside that engine that needs to be addressed.
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[LukeM]

When the long cam chain on my '79 bike broke, it did bend a few valves.  I guess it depends on the revs when it broke.  It's hopeful that at low idle, the pistons just kinda pushed the valves back into the seats, but that's just wishful thinking.

Luke M

[lusciouslemur]

Your 79 is a DOHC right? Is there a guide online for splitting the crankcase? Did you use any special tools?

[TOOLS1]

Yes there is a guide online. It is the Honda Factory Service Manual which can be downloaded for free from the manuals thread at the top of the home page. Basically just turn the engine over, remove the side covers and all the components behind them until you get to the case. Then remove the bottom of the case. Just take a lot of pictures as you go, and keep everything organized. You should be fine.
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[lusciouslemur]

Update, have successfully split the crank case and replace the chain, however it is still impossibly hard to put on. With the tensioner tab pulled out I still can only get half the chain on the sprocket. I must be doing something wrong  but I don't know what. Any thoughts? Bad tensioner?

[TOOLS1]

Remove the tensioner from the engine and try putting the chain on.
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[lusciouslemur]

Ok, tried with the tensioner out of the bike and it's still a no go. The chain moves smoothly when I crank so it's on the crankshaft teeth okay.

[Rodgil]

Pull up on the tensioner and clamp it, to keep the tension off the chain. Then put the chain on the cam sprocket BEFORE you bolt the sprocket onto the camshaft.

[lusciouslemur]

I've tried that, but unless it takes superhuman strength to do, there isn't enough slack in the chain.

[Re-run]

Count the links in your old chain and compare it to the new chain. The new chain may not have enough links or the pitch is wrong.

[lusciouslemur]

Another update, thanks everyone, with some elbow grease I went with the suggested method and bolted the sprocket on after the chain was place correctly. My timing was set with no problems and all cylinders are getting 100+ psi compression-wise.

I don't know if I should start a different thread for this, but after assembling the bike, it will turn over but not start. I remember checking 1 or 2 of the spark plugs on the head for spark, and I had drained the gas out of the carbs before removing them for the chain replacement. Is 5-6 weeks really enough time for the carbs to get gummed up again? When I try spraying some starter fluid through the carbs, the bike will puff a few times to indicate starting, but then die out.

[TOOLS1]

That should not have been enough time to gum up the carbs. Do you have the wires to the coils swapped? The yellow wire goes to the coil for cylinders #2-3.
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[lusciouslemur]

I haven't really messed with the coils, the yellow is connected to the 2/3 cylinders.

[lusciouslemur]

So I decided to check all 4 spark plugs and two weren't firing, the connectors to the boots were somewhat frayed and after fixing that, the bike did start! It won't stay running unless I give it either throttle or choke, so perhaps I do need to clean the idle jets. Not sure.

[makanimike]

lusciouslemur wrote

with some elbow grease I went with the suggested method and bolted the sprocket on after the chain was place correctly.

I'm struggling with exactly the same issue you did. I suppose it is obvious when you have done it once, but I am really having a hard time. Could you describe the process and solution more in detail, please? What was the aha moment?
 
Or maybe I'm doing something wrong with the tensioner. When we pull it up before locking the nut again, it's just moving 2mm or something like that right?

[Rodgil]

You should be able to pull the tensioner up a long way. An inch perhaps? I clamped mine with vice grips so that I could pull on it hard.  Did you undo both locknuts on the tensioner? And the chain has to go over the sprocket BEFORE bolting it onto the camshaft.

[makanimike]

ok. If we are talking in the ballpark area of an inch then I'm fairly certain we have pinpointed my root cause.  
I undid only the lower nut. I stuck to what the Clymer manual writes.  
I'll try it again while loosening both nuts and pulling it up on the tensioner harder. Just to be sure: I'm pulling up the entire tensioner, or am I just pulling up that part of the tensioner that is behind the spring?  
I have been pulling on the red part. Should I be pulling on the blue part?
 

[TOOLS1]

Neither. You should be pulling where the pin is that connects the fiber/plastic rubbing block to the spring loaded tensioner.
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[makanimike]

oh man! So easy when you figure out what an illogical fool you have been (as always :) ! Thanks!  
 
Doing it this way I could pull it up 2-3cm instead of 2mm. Pulling the chain over the sprocket, and subsequently the sprocket over the exhaust cam was very easy now.  
 
What had thrown me off is that the upper tensioner bolt had a couple of mm leeway. I thought I was supposed to pull that part up.