Topic: CB750 Tapping Noise from Top End after Rebuild

[KickstandDan]

Hey everyone,

I’m finishing up a full rebuild on my 9/71 CB750 K1 and could use some advice. The engine is running and sounds strong overall, but after a few startups and heat cycles, I’ve noticed a tapping or clicking sound coming from the top end. The noise is coming form the top left only. It seems to follow RPM and sounds like it’s coming from the rocker arms or somewhere in the head.

Here’s a quick overview of the build:

• Stock displacement with 10.5:1 compression pistons
• Head and crank work done by Ken at Cycle X (balanced crank, lightened rocker arms, new valves, springs, titanium retainers and jam nuts)
• CX-1 cam
• Stock airbox with a UNI foam filter
• Delkevic 4-into-1 header into a Cone Engineering 18” Quiet Core muffler
• Stock carbs (still dialing in jetting)

I primed the oil pump before first start-up and there were no unusual sounds at first, but now that it’s run a few times, this tapping has become noticeable. The bike is mostly assembled, but I’m using the Cycle X frame splice kit, so pulling the valve cover is easy if needed.
 
I also did the cam tower mod added 8 oil holes shown in page VI A-22 in the HondaMan, My CB750 Book. Maybe I drilled too close to the edge and part of the cam tower broke under load so it is making that clicking noise.

Thanks in advance,
Dan

[BenelliSEI]

I’d start by checking valve clearances and mak sure an adjuster hasn’t come loose?

[KickstandDan]

3 times I have checked the valve lash to make sure it is within spec. Once before running and twice after running and all 3 times it is within spec. Now I am using the CX-1 cam from WEB-CAM and the valve lash is 0.004" intake and 0.005" exhaust per the spec sheet that was with the cam itself. This is bigger then stock but not enough to make a noticeable tapping noise.

[scottly]

Are you getting enough oil to the cam on the left side? Compare to the right side.

[KickstandDan]

When checking the valve lash I have seen oil on the retainers so I assumed there is enough oil. When rebuilding I made sure to put the oil control orifices back in. What would be a good way of checking to make sure the left side is getting proper oil?

[scottly]

Pull the tappet covers off on both sides and start the engine.

[KickstandDan]

So I should be looking for both sides to have the same amount of oil "splashing" on the retainers and rocker arms? There is not very much room to see everything.

I will add that the carbs are synced too so that isn't the issue.

[KickstandDan]

I guess follow up, lets say I look at the rocker arms and retainers and they are getting oil, what would be the next thing to look into with this tapping issue?

[scottly]

I suggested checking the oil flow is because if it's wrong, it can destroy the brand new cam, the rockers, and cam tower in a very short amount of time.

[KickstandDan]

I suggested checking the oil flow is because if it's wrong, it can destroy the brand new cam, the rockers, and cam tower in a very short amount of time.

I do think that oil flow could be the issue but I am just trying to figure out the best way to check. If I need to pop the valve cover off I can do that but it is just hard to see inside the tappet holes. What should I be looking for if I open the valve cover? An even amount of oil covering all the components?

[scottly]

Have you checked for oil splashing with the tappet covers off yet?

[KickstandDan]

No I have not. I will try that first and get back to you.

Let me know if you or anyone else can think of another reason for this tapping noise.

[scottly]

exhaust leak at the head, valve hitting piston, sticking valve from tight guide, improper lash adjustment technique, just to name a few...
Is the noise louder with the tappet covers off? 

[bryanj]

I spent days chasing a tappet noise on a 550 that was cured by nipping up the exhaust pipes

[BenelliSEI]

I spent days chasing a tappet noise on a 550 that was cured by nipping up the exhaust pipes

Me too! Good reminder and one to check…..

[MauiK3]

Exhaust leaks sure can sound very mechanical

[KickstandDan]

I have new exhaust copper gaskets in the exhaust flange but I can check to see if maybe after some heat cycles the flange loosened up. I still need to figure out if I am getting the same amount of oil on the right and left sides and to check the volume of the tapping with the tappet covers off.

[bryanj]

New gaskets frequently bed down and leak a bit

[KickstandDan]

I checked to see if the exhaust flange bolts were loose and to my chagrin, cylinder 1 flange had semi loose bolts. So I tightened them and then tightened the other 3. Tomorrow I will start up the engine again and see if the problem has been solved. I am hopping it isn’t an oil issue.

[KickstandDan]

 I started the bike again. The tapping noise was softer at first but got louder as the engine warmed up. The noise is not constant. There was a period where I thought the problem was solved but after a minute or two of runtime the noise came back. I checked for oil flow by removing the tappet covers one by one oil shot out of the exhaust cover on cyl #1, so I shut it down quickly. The intake side showed steady oil flow, and all four cylinders seemed to be getting equal oil.

Since oil flow looks good and the carbs are nearly synced, I’m wondering if the issue could be the cam tower. I drilled the extra oiling holes but the bit walked a bit, and I’m starting to suspect a small crack may be causing the noise. I’ll run the bike a bit more and if the sound stays, I’ll pull the valve cover to inspect if needed. Curious what you guys think.

[scottly]

I'm not familiar with the mod to the cam towers. Did you go back and recheck the exhaust bolts? Also, you can use a length of rubber tubing like a stethoscope to help pinpoint noises sometimes be holding one end next to your ear and probing around with the other end.

[KickstandDan]

Hondaman talks about the mod here:

http://forums.sohc4.net/index.php?topic=59423.0

I did tighten the flange bolts and the noise persisted.

[jonda500]

A large screw driver also makes a good mechanics stethoscope - handle pressed into your ear and the tip held against different parts of the engine. Never tried using a tube, I have to try that one!
John

[KickstandDan]

So just to confirm the exhaust flange bolts are tight and there is enough oil flowing in the head. My valve lash is set properly but what would too tight or too loose of a lash do? Just curious.

[scottly]

It's hard to diagnose a sound we've never heard, but since it's only on one side it warrants looking into. As far as lash, let's say you had a valve hitting a piston. If you increased the lash on that valve the noise would lessen, and if you decreased the lash, the noise would get worse, but I wouldn't recommend that.