Topic: Need Help Troubleshooting

[tofan]

Hello All,

Look-time lurker, first time poster. I've learned a lot from you all, but I'm at my wits end here. I have a 1976 CB550K with 25k miles on it.

After day's ride I came home with this sound coming out of my engine: Link to Video. The noise disappears when I reach about 3000rpm. I managed to find a video on youtube with the same engine and noise, and the poster said it was the piston ring. So I get to work and carefully rebuild the top-end, replacing the piston rings, seals, valves, etc. etc. I put everything back together, check and adjust the tappets, and adjust timing. Finally, with some work I manage to turn the engine over, but I still have the noise! Any ideas??

(In addition to that my new cylinder gasket is leaking and having some serious issues keeping the engine running - but I'll get to that later as I don't think it's related).

Today I performed a compression check while the engine is cold and I'm getting 120, 130, 120, 95psi (1-4 respectively). Once I get the carb back in I'll check while the engine is warm.

Thanks in advance...

[b52bombardier1]

Was the compression check done with the throttle held wide open?  The rings are also probably not seated.

  The noise?  Could be rockers, rocker shafts? Hard to tell from the video sound.  Is it top end noise or bottom?

Rick

[BomberMann650]

Thats a lot like how mine sounded before the carbs got synced.

Do you have a mechanics stethescope or long screwdriver?  You can listen in close to different parts of the motor and try to isolate the offending noise

[DH]

I didn't hear the link,(don't want dropbox) but is the noise a ticking or knock? Sometimes you can narrow
it down by the occurance/speed of the noise itself. Faster, matching more to
crankshaft speed, or slower, matching cam/valvetrain speed and IF the noise is
happening in a constant cycle when it occurs.. just throwin that out there.

[flybox1]

Did you rebuild the top end before doing a complete tuneup?
You have not mentioned if youve set timing/advance, adjusted the cam chain, or done a vacuum sync of the carbs.

[Scott S]

 Sync the carbs.
 Does the sound appear to be coming from a particular place?

[tofan]

Was the compression check done with the throttle held wide open?  The rings are also probably not seated.

  The noise?  Could be rockers, rocker shafts? Hard to tell from the video sound.  Is it top end noise or bottom?

Rick

Hi Rick,

The compression test was done with the carbs off, so yes. I believe the noise is from the bottom.

[tofan]

Thats a lot like how mine sounded before the carbs got synced.

Do you have a mechanics stethescope or long screwdriver?  You can listen in close to different parts of the motor and try to isolate the offending noise

Yes, I believe this should be the next step once I clean, bench sync and put back the carb.

[tofan]

I didn't hear the link,(don't want dropbox) but is the noise a ticking or knock? Sometimes you can narrow
it down by the occurance/speed of the noise itself. Faster, matching more to
crankshaft speed, or slower, matching cam/valvetrain speed and IF the noise is
happening in a constant cycle when it occurs.. just throwin that out there.

It's more of a knock. It's frequency increases as I pull on the throttle and eventually disappears around 3k rpm and above.

[tofan]

Did you rebuild the top end before doing a complete tuneup?
You have not mentioned if youve set timing/advance, adjusted the cam chain, or done a vacuum sync of the carbs.

Yes, before the tune-up. I did set the timing/advance and adjust the cam chain afterwards.
I was on my way to perform a vacuum sync, but I ran into other problems so I currently have the carbs off to do a deep clean and bench sync.

[tofan]

Sync the carbs.
 Does the sound appear to be coming from a particular place?

This is my next step. It seems to be coming from the lower front end.

[tofan]

I didn't hear the link,(don't want dropbox) but is the noise a ticking or knock? Sometimes you can narrow
it down by the occurance/speed of the noise itself. Faster, matching more to
crankshaft speed, or slower, matching cam/valvetrain speed and IF the noise is
happening in a constant cycle when it occurs.. just throwin that out there.

DH, I've uploaded it to youtube if it helps: https://youtu.be/K6c3h8DxiAc

[jonda500]

the youtube video is private 
John

[tofan]

Sorry about that, John

https://youtu.be/K6c3h8DxiAc

[jonda500]

Is the noise still there when you put it in gear and start letting the clutch out until the bike is starting to creep forward?
If it is then I would suspect it's the primary chain - it would be louder when the engine's hot after a long ride
John

[tofan]

Is the noise still there when you put it in gear and start letting the clutch out until the bike is starting to creep forward?
If it is then I would suspect it's the primary chain - it would be louder when the engine's hot after a long ride
John

Once I get the carbs back on I will check and report back.

[tofan]

Just a quick update. I took apart my carbs and came to the conclusion I had to get them replaced because the part that holds the main jet was corroded to the point where the main jet seal was exposed and wouldn't stay in.

Also...in the process I learned the carbs on the bike (627B) are not the original nor, technically, the correct model. Luckily, a friend of mine had a really clean set of 087A carbs that I just finished installing today.

The motorcycle will turn on and run, but it needs tuning/syncing. The great, but puzzling, news is that I don't have the noise that started all of this.

[BomberMann650]

Damn that corrosion 

Did you check that the jets and needles were proper spec for your motor/setup?

The 627b carbs were from the cb500.  Iirc, 087A were later years.  The jetting combination, and especially the needle jet profile, varied a lot between carb types.

[flybox1]

Just a quick update. I took apart my carbs and came to the conclusion I had to get them replaced because the part that holds the main jet was corroded to the point where the main jet seal was exposed and wouldn't stay in.

Also...in the process I learned the carbs on the bike (627B) are not the original nor, technically, the correct model. Luckily, a friend of mine had a really clean set of 087A carbs that I just finished installing today.

The motorcycle will turn on and run, but it needs tuning/syncing. The great, but puzzling, news is that I don't have the noise that started all of this.

Hmmm......Correct, clean, working carbs, and now no sound?!?!?
.....im stumped! 

Free lumps: Just think a proper tune-up way back when might have saved you a fresh top end 

[BomberMann650]

Just a quick update. I took apart my carbs and came to the conclusion I had to get them replaced because the part that holds the main jet was corroded to the point where the main jet seal was exposed and wouldn't stay in.

Also...in the process I learned the carbs on the bike (627B) are not the original nor, technically, the correct model. Luckily, a friend of mine had a really clean set of 087A carbs that I just finished installing today.

The motorcycle will turn on and run, but it needs tuning/syncing. The great, but puzzling, news is that I don't have the noise that started all of this.

Hmmm......Correct, clean, working carbs, and now no sound?!?!?
.....im stumped! 

Free lumps: Just think a proper tune-up way back when might have saved you a fresh top end 

At least now with a top end rebuild and new(er) carbs - the bike ought to perform well into the coming years!

[flybox1]

^^^ Definitely! 

[tofan]

Yeah, live and learn.

I had rebuilt the old carbs not too long ago before the problem started, so I had dismissed them as being the culprit.

BomberMann650: The jets are to spec.