Topic: Diagnose my engine noise please. 72 CB750

[Malcolm]

Need help diagnosing the clanky engine noise on my 1972 Honda CB750.

Video:

Any ideas?

[Brian G]

One thing's for sure, Malcolm - it isn't going to be good news.

I might very well be wrong but it makes me think of a cam chain flailing around with little or no tension. When the revs pick up a bit and there's a more constant load on the chain, it seems to quieten a bit but I don't think you'll be able to fix this one with the engine in the frame.

Not what you want to hear - especially at this time of year - but from the looks of her, she's worth the effort!

Regards,

[Malcolm]

Thanks for your input Brian,

Yesterday I loosened the cam chain tensioner while the bike was running and it sounded like a chain being dragged across a washboard. Shut the bike off and adjusted it the usual way and it stopped making that noise, but I took this video today to address the usual sound my engine emits.

Perhaps it's not the cam chain?

[pjlogue]

Timing chain noise.  When you adjusted it did you have the timing marks for 1-4 15 deg past TDC? 

-P.

[ekpent]

Timing chain noise.  When you adjusted it did you have the timing marks for 1-4 15 deg past TDC? 

-P.

with the tappets on #1 loose or tdc.

[Keith]

It is important to follow the instructions precisely...wondering why you tried with engine running?

[seanbarney41]

One thing's for sure, Malcolm - it isn't going to be good news.

I might very well be wrong but it makes me think of a cam chain flailing around with little or no tension. When the revs pick up a bit and there's a more constant load on the chain, it seems to quieten a bit but I don't think you'll be able to fix this one with the engine in the frame.

Not what you want to hear - especially at this time of year - but from the looks of her, she's worth the effort!

Regards,

this is absolutely not the case.  Chain might be a little loose but every cb750 I have ever heard run will make that exact sound when the 3k tune up has not been done or has been done incorrectly.   Sounds like you have the idle speed set at about 800 rpm, which is about 300 too low... and the carbs are out of sync.

[Malcolm]

Timing chain noise.  When you adjusted it did you have the timing marks for 1-4 15 deg past TDC? 

-P.

Yes I turned the engine to 15 degrees past TDC, checked that the valve on 1st cylinder was loose, and loosened the tensioner and tightened it up again.

[Malcolm]

One thing's for sure, Malcolm - it isn't going to be good news.

I might very well be wrong but it makes me think of a cam chain flailing around with little or no tension. When the revs pick up a bit and there's a more constant load on the chain, it seems to quieten a bit but I don't think you'll be able to fix this one with the engine in the frame.

Not what you want to hear - especially at this time of year - but from the looks of her, she's worth the effort!

Regards,

this is absolutely not the case.  Chain might be a little loose but every cb750 I have ever heard run will make that exact sound when the 3k tune up has not been done or has been done incorrectly.   Sounds like you have the idle speed set at about 800 rpm, which is about 300 too low... and the carbs are out of sync.

Just got the bike back from the shop, carbs are freshly synced. Unless they didn't do it properly.

[seanbarney41]

One thing's for sure, Malcolm - it isn't going to be good news.

I might very well be wrong but it makes me think of a cam chain flailing around with little or no tension. When the revs pick up a bit and there's a more constant load on the chain, it seems to quieten a bit but I don't think you'll be able to fix this one with the engine in the frame.

Not what you want to hear - especially at this time of year - but from the looks of her, she's worth the effort!

Regards,

this is absolutely not the case.  Chain might be a little loose but every cb750 I have ever heard run will make that exact sound when the 3k tune up has not been done or has been done incorrectly.   Sounds like you have the idle speed set at about 800 rpm, which is about 300 too low... and the carbs are out of sync.

Just got the bike back from the shop, carbs are freshly synced. Unless they didn't do it properly.

ride the bike til fully warm(at least 10 min averaging 45 mph), set idle speed to 1100-1300 rpm and then enjoy.  If it hasn't quieted down but it runs fine, I would not worry about it.  The previous poster that I disagreed with was correct that the only way to have a quiet idling cb750 is to have absolutely tip top chains and associated hardware.  It will most likely run fine for 10's of thousands of miles whether it is quiet or not.  Is the lack of noise worth it to you?

[evinrude7]

my 750 k6 made a similar noise though not as bad until i adjusted my valve tappets. 

[jaytee-nz]

The noise is very similar to my bike's noise - mostly due to the good old clutch rattle issue and a slightly loose / worn primary chain.
If you pull your clutch lever in, does the noise decrease (but not entirely)? If so, it's the dreaded clutch rattle which is a case of "they all do that".

[Malcolm]

The noise is very similar to my bike's noise - mostly due to the good old clutch rattle issue and a slightly loose / worn primary chain.
If you pull your clutch lever in, does the noise decrease (but not entirely)? If so, it's the dreaded clutch rattle which is a case of "they all do that".

Pulling in the clutch doesn't affect the sound.

[PeWe]

It is for me the typcal sound of a CB750.
If I remember correctly.....  Hardest sound is clutch rattle with primary chains. You will most likely hear the sound from that area. If you press the clutch lever, the sound will change.

Cam chain has a lighter sound.
Maybe a perfect ignition setting will make the engine to run smoother on low rpm and less rattle.
New primary chains with tensioner, + cam chain with tensioner when everything is apart will reduce the rattle.

You can measure the primary chains status when oil pan is removed. Distance between case and tensioner. See Honda shop manual. New chains max 65mm slack, replace >70mm.

Increase Idle,  no problem with 1200 rpm which will ensure oil pressure at the cam

A balanced APE crank will make a smoother idle. New cush rubbers in primary hub when the other stuff is done too. $$$$

[ekpent]

Timing chain noise.  When you adjusted it did you have the timing marks for 1-4 15 deg past TDC? 

-P.

Yes I turned the engine to 15 degrees past TDC, checked that the valve on 1st cylinder was loose, and loosened the tensioner and tightened it up again.

One other thing on the cam chain tensioner. It is not unheard of for that shaft in the tensioner to have a light seize,especially if the bike has sat for a long time, and for the shaft not to move when the bolt is loosened. On new to me bikes I usually take that off with the 3 bolts,easier when the carbs are off, and clean and lube to make sure all is free. Also if overtightened in the past a dimple can be created on the shaft and when the bolt is tightened it will slip back into the same position on the dimple.Filing and dressing out the ramp can remove the dimple. At least tap the shaft end with a screwdriver next time you do the cam chain.