Topic: CB750 Tapping Noise from Top End after Rebuild

[KickstandDan]

Yeah I understand it would be hard to diagnose a noise without hearing it but it sounds like a small wrench tapping a metal surface. It speeds up when reving the engine. It’s a higher pitch noise but it really seems like it is towards the top of the engine not near the cylinders but I might try that homemade stethoscope idea. I feel like the valve hitting the piston wouldn’t make sense because it’s not constant and none of the other cylinders are making any noise. But I don’t have a lot of experience with engines.

[scottly]

The hose stethoscope is more for exhaust and vacuum leaks than hard mechanical noises, where a mechanic's 'scope or large screwdriver often works better. You might as well pull the cam cover off and look for something wrong.

[KickstandDan]

The mod is drilling 8 small holes into the part of the cam carrier that holds the rocker shafts so that oil can seep in. I had done this but I fear the walking drill bit may have gone too close to the edge causing a weak point. If I can't figure out this problem today I will start removing some parts so that I can get access to the valve cover and check things out. Luckily I have installed a splice kit so I technically could disassemble everything down to the bottom end if I needed too.

This image is before I ran the engine. This images shows the 8 countersunk holes I added (per side). Also note the incorrect cam carrier bolts. Let me know if this could be the problem too. They are the correct size and torque properly but they are a different head

[KickstandDan]

Alright, the next thing I am going to try is grabbing a stethoscope from Harbor Freight as a lot of you suggested and try and pinpoint the issue. I will also try and loosen the valve lash just to see if the problem is the valve hitting the piston. I will keep you informed on where the noise is directly coming from.

Thanks for your help!

[Stev-o]

Since you have a frame kit, I would remove the head cover to look things over to see if anything obvious sticks out. Rotate the motor manually to try to replicate the noise.

I've always been very cautious running a motor after an expensive rebuild...

[MRieck]

Why did you use the Phillip head screws for the towers? I have replaced the originals with 12.9 Allen's that are longer for complete thread engagement. I don't think those screws would cause e a knock but.....😄
 Personally I don't see the need for all the oil holes....it's not necessary. Also never had problems with the 5mm screws holding the shafts...a solution looking for a problem IMO.
 As somebody else said turn it over slowly...maybe a valve is bent a bit and hanging in the guide. And change those screws out for the correct hardware.

[KickstandDan]

Interesting update. I started the motorcycle and heard the noise again and I looked down and I could see a little bit of smoke coming out of cylinder number one. As I was watching that it dawned on me that I had never put on any exhaust sealant. I also removed the exhaust clamps and each of the clamps had some soot on the backside from where I assume exhaust was leaking. So I think the next thing I should do is apply exhaust sealant and put the headers back on.

I would like to add that for about five seconds in the last run in my garage the left side stopped making noise and the right side started to make noise. But then soon after it went back to the left side, making noise. To me this sounds like I’m getting exhaust leak out the headers and that’s what’s making the tapping noise. I will try this tomorrow when my headers aren’t hot and go from there.

[M 750K6]

I don't use sealant, but I do replace the crush rings if I've removed a pipe.

[scottly]

Use a piece of hose to probe around the exhaust ports. I found a pinhole leak in a set of headers once that I could hear but not locate. Also, I never had any issues with the stock K1 cam towers.

[KickstandDan]

The copper crush rings have been replaced and torqued twice and so I feel like the issue is still exhaust leaking because of the soot on the backs of the clamps. I am going to remove the headers, retorque the flange bolts and apply some sealant.

I know there is a leak because I could see smoke coming out of the engine at cylinder 1 header. Right where the noise sounds like it’s coming from.

If you have a reason against exhaust sealant I would love to hear it though.

[M 750K6]

When I took my 750 apart, one of the exhausts had 2 crush washers and sooty stains around the header area. Only found it by luck.

[newday777]

Did you dig out the old exhaust donuts before putting the new in place?

[KickstandDan]

I removed the headers and the exhaust flanges, and I noticed that all of my copper donuts were compressed properly and there was no blow by that I could tell on the head to the flange side. I am guessing the leak is coming from the flange to the header connection. I am going to remove the old donuts and reinstall new donuts.

I have an interesting idea, though. If the problem is exhaust leak, could I run the engine without headers to see if the noise persists. I know it’s not great to run an engine without headers, but I’m just curious about this idea.

[bryanj]

You can run it without but wether you can hear anything is a moot point

[KickstandDan]

Well, if I can still hear the tapping noise then I know the issue is not related to the exhaust because the exhaust is not in. This would help me figure out the root of the problem.

[Oly]

Dan... not looking to deviate from the issue you are looking to solve with your top end noise issue, but could please let me know if you drilled the additional 8 holes described in Mark's cb750 book by hand or did you perform this on a drill press?  If by hand was it a challenge to hold position?

[scottly]

Well, if I can still hear the tapping noise then I know the issue is not related to the exhaust because the exhaust is not in. This would help me figure out the root of the problem.

If you can still hear the tapping with open exhaust ports it must be pretty damn loud!

[KickstandDan]

I drilled the 8 holes as described from Marks book. It was by hand not with a drill press so the bit moved a little more than I would have liked. But I have moved past thinking that is the problem until I test no headers tomorrow.

I think the issue is the exhaust leak so doing a test without headers means I should not hear any tapping. No tapping means the issue is exhaust leak not that the tapping is so loud I would hear it above the engine without headers. To me this is simple deductive reasoning.

[KickstandDan]

Just wanted to say you guys were right. Running the bike without headers was a terrible idea. My ears are still ringing and I couldn’t hear a thing over the noise. Definitely a lesson learned from my 23-year-old brain. Tomorrow I’ll reinstall the exhaust with some sealer and check for leaks or if the tapping noise is still there. Appreciate the guidance.

[newday777]

I removed the headers and the exhaust flanges, and I noticed that all of my copper donuts were compressed properly and there was no blow by that I could tell on the head to the flange side. I am guessing the leak is coming from the flange to the header connection. I am going to remove the old donuts and reinstall new donuts.

I have an interesting idea, though. If the problem is exhaust leak, could I run the engine without headers to see if the noise persists. I know it’s not great to run an engine without headers, but I’m just curious about this idea.

But did you ensure that there are no old crushed exhaust donuts in the ports before you put in the new ones?
Use a good flashlight.

[KickstandDan]

I 100% made sure to remove the old copper crush washers. Someone else was having issues with 2 gaskets stacking up.

[newday777]

I 100% made sure to remove the old copper crush washers. Someone else was having issues with 2 gaskets stacking up.

Wanted to be sure as it has happened before by many.

[KickstandDan]

I appreciate it!

[Remcod]

Are you sure you replaced and tightened the conrod bearing bolts properly? So its not the pistons hitting the cilinder?

Verstuurd vanaf mijn SM-A536B met Tapatalk

[MRieck]

Just wanted to say you guys were right. Running the bike without headers was a terrible idea. My ears are still ringing and I couldn’t hear a thing over the noise. Definitely a lesson learned from my 23-year-old brain. Tomorrow I’ll reinstall the exhaust with some sealer and check for leaks or if the tapping noise is still there. Appreciate the guidance.

I kinda figured that would be your reaction.