Topic: CB750k7 clank when cranking after rebuild

[Catfish]

Hi there all,

I'm in Australia, feeling for you good folks in other countries less fortunate with the wretched virus. My project is a 750 k2 with a k7 engine. It was pretty much barn find standard when I bought it and I have slowly been working through everything, starting with rebuilding the top end - rebore with 2nd oversize piston kit, head professionally redone with new valves etc. New cam chain with master link and tensioner parts. I didn't touch the bottom end or starter motor. Before the tear-down I had managed to start it and ride it around the block (after replacing the coils and patching up the wiring loom).

So I am now at the point of starting my pride and joy after the rebuild, cranking with the starter motor with kill switch activated to build up oil pressure (yes I poured some oil in the top). While the starter is cranking everything sounds normal but when I take my finger off the button there's an almighty CLANK in the top of the motor as the motor comes to a stop. Obvioulsy I am thinking I will pull the motor again and have a look under the tappet cover, but I also wondered if others on this forum might have some clues. Are there some simple checks I could do before I pull the motor?

Thanks in advance.

[PeWe]

Cam chain tensioner housing with its rod, spring and rubber is mounted?

[Catfish]

Yep, sure is

[pjlogue]

Is the cam chain properly adjusted?  To do so for engines through K-6, and I believe the K-7, you need the #1 piston 15 deg past TDC and then loosen the cam chain adjuster nut so any slack in chain is taken up.  Excess slack in chain can cause a backlash in the cam when engine stops. 

You stated the head was professionally rebuilt.  Was the head milled?  If so, how much was milled off? 

-P.

[ekpent]

  Funny it happens only whenyou release the starter button and its stopping. Sure it isnlt coming from the side starter clutch area ?

[Bodi]

I looked at an engine that started an ungodly clanking at low RPM and as you describe. The cam sprocket bolts had come very loose. I expect you tightened and loctited yours but could that be the issue?
You may be able to see the sprocket through a tappet adjusting hole? If so, try turning the crank backwards and forwards while watching it: if it turns a bit without the cam turning, the bolts are not tight. If the crank rocks cw/ccw much at all without the sprocket turning, the chain is too slack.
Turning the crank the "wrong way" will be quite difficult with the starter motor in place - don't try that via the advancer nut.

[Catfish]

Is the cam chain properly adjusted?  To do so for engines through K-6, and I believe the K-7, you need the #1 piston 15 deg past TDC and then loosen the cam chain adjuster nut so any slack in chain is taken up.  Excess slack in chain can cause a backlash in the cam when engine stops. 

You stated the head was professionally rebuilt.  Was the head milled?  If so, how much was milled off? 

-P.

To me, that's the sort of sound it makes, like some sort of backlash. I spent a lot of time making sure the timing was right and the tensioner was installed correctly, but I don't recall the procedure you mention so I will give it a go. In terms of milling the head, I don't know how much they took off.

[pjlogue]

If you tear down the engine again (have to remove it from frame) I strongly suggest you replace the cam chain with one that does not have a master link or minimum, stake the master link/clip so it CAN'T come undone.  Honda made the cam chain continuous for a reason.

Years ago when I had just gottem my 76' 750K, the owners manual gave instructions for adjusting the cam chain.  It did not mention about making sure the #1 cylinder was 15 deg. past TDC.  Engine made a god awful noise while running.  I can't remember if it mad the click/snap after trying the electric start as this was 44 years ago.  Using the OEM owners manual (not shop manual) gave the cam chain too much slack.  At 15 deg past TDC all the tension is on the front of the chain and slack is at the back where the tension adjuster is.

I am not saying this IS the cause in your case but it would be a first guess.  The reason I asked about the head milling is if they took just a little too much off it could interfere with valve/piston clearance.

-P.