Topic: 77 cb750K Cafe Finally done, but still weird engine noise (DISCOVERED ISSUES)

[Xander^]

Hi everyone,
first i'd like to thank everyone on SOHC forum that helped me along the way of my 3 year cafe racer rebuild project with my bike! wouldnt have gotten through it all without some solid information and some shipped parts as well!

hopefully this last kink can get worked out, but in the video is how the bike is sounding after having a top end engine rebuild. It sounded worse before i got the timing done right. Since then i got the timing spot on and have put in thicker oil but that knocking sound still seems very weird and mysterious.

i have done a few test rides, have put 5 kms on the bike and nothing has blown up on me lol.. so i feel like this sound is serious, but not too serious since its still working at all.

could this be something caused but not rebuilding the bottom end also? when i put my ear closer to it, it sounds like its coming from the pistons moving up/down.. the gent that rebuilt the top end advised that the noise is a bit louder than usual but its nothing to worry about as these are "old bikes" etc

see attachment for video but in case that does not work here is the youtube version:

[Kevin D]

Exhaust leak?

[Airborne 82nd]

Hi everyone,
first i'd like to thank everyone on SOHC forum that helped me along the way of my 3 year cafe racer rebuild project with my bike! wouldnt have gotten through it all without some solid information and some shipped parts as well!

hopefully this last kink can get worked out, but in the video is how the bike is sounding after having a top end engine rebuild. It sounded worse before i got the timing done right. Since then i got the timing spot on and have put in thicker oil but that knocking sound still seems very weird and mysterious.

i have done a few test rides, have put 5 kms on the bike and nothing has blown up on me lol.. so i feel like this sound is serious, but not too serious since its still working at all.

could this be something caused but not rebuilding the bottom end also? when i put my ear closer to it, it sounds like its coming from the pistons moving up/down.. the gent that rebuilt the top end advised that the noise is a bit louder than usual but its nothing to worry about as these are "old bikes" etc

see attachment for video but in case that does not work here is the youtube version:

First try to pin down where the sound is coming from. This is cheap but it works.
https://www.harborfreight.com/Mechanics-Stethoscope-63691.html

[Xander^]

Exhaust leak?

I don’t believe so, i’ve put in new crush gaskets recently, however have removed and put back the exhaust a few times since then. Should i be replacing the crush copper gaskets every time?

[Kevin D]

When you use new ones, you will likely need to retighten. DAMHIK
If they stay in place when removing/installing exhaust they might be ok to reuse.
Make sure the old ones are gone so they are not double stacked.

I Found this quote at a parts supplier site:"Why You Need This: That tick, tick, tick sound you hear could very well be an exhaust leak. If you've ever pulled the pipes off your bike there is a good chance there is some leakage around these copper crush o-rings."

[Xander^]

When you use new ones, you will likely need to retighten. DAMHIK
If they stay in place when removing/installing exhaust they might be ok to reuse.
Make sure the old ones are gone so they are not double stacked.

I Found this quote at a parts supplier site:"Why You Need This: That tick, tick, tick sound you hear could very well be an exhaust leak. If you've ever pulled the pipes off your bike there is a good chance there is some leakage around these copper crush o-rings."

i hope this is all that it is.. again, the engine may have gone bad on me if it was interanl.. going to swap gaskets this week. (they were staying in place and i did remove old ones though). well see

[Remcod]

Check the carbs for false air. Are the synchronisation screws in? Did you sync?

Does the noise disappear with a bit of throttle?

My f1 made similar noice untill i found out that the sync screws were wissing 3 out of 4. Still not perfect but still need to sync

[BenelliSEI]

I agree it sounds like an exhaust leak. Start there and also recheck valve clearances and then ignition timing. Too much advance at idle can make some weird noises too. Have you done a compression check lately?

[Xander^]

I agree it sounds like an exhaust leak. Start there and also recheck valve clearances and then ignition timing. Too much advance at idle can make some weird noises too. Have you done a compression check lately?

Yeah going to start at exhaust leaks.
Ignition timing was done and was set perfect, since then noise got better.
Noise goes louder/stays the same when reving the throttle.
Compression was good last time i checked, been since early 2019 since i checked but feels good still.
Valve clearances is another thing to check i guess

[Dolomite]

You may have solved this but I had a eerily similar noise. Sounded like a slapping noise, thought maybe the cam chain was just too loose. Turned out it was a bolt from the #4 con rod connector coming loose. The noise stopped when the bottom of the con rod fell out and seized the crank shattering the piston sleeve and bending the con rod.

[Xander^]

You may have solved this but I had a eerily similar noise. Sounded like a slapping noise, thought maybe the cam chain was just too loose. Turned out it was a bolt from the #4 con rod connector coming loose. The noise stopped when the bottom of the con rod fell out and seized the crank shattering the piston sleeve and bending the con rod.

oh god.. i hope now. i still havent solved this though..

[Xander^]

today i had my cb750 engine taken apart for the second time, this time done by a local professional shop, and the discoveries were ridiculously alarming.. the issue i was having (heard in the orignal post video) was due to 2 major issues that went wrong in the initial top end rebuild..

1, oil gallery to the top end of the engine was clogged on one side, which resulted in a ruined camshaft and holder.. as you can see in the pics, 2 o-rings snug around the rod blocked any oil trying to go up into the cam and the other side had no o-ring at all.

2, valves were completely off

3&4, other careless things that results in damaged dowels.. see pics

long story short, i wanted to post this as an example to others to prevent anyone else from having this happen to them... i was somewhat worried and intimidated to rebuild the engine myself (also lacked tools for valves/machining etc) but going forward with the help of this forum, i will be attempting everything myself or using this amazing local shop have only recently found out about. i trusted a guy with "experience" on CBs who worked out of a home garage part time and the results show. glad i caught the issues when i did and didnt ride this season..

[Kevin D]

Dang, Xander^, that is very far from an exhaust leak.

[Xander^]

Dang, Xander^, that is very far from an exhaust leak.

yeah.. no kidding. Hopefully this helps others in the future by ultimately preventing such a thing

[PeWe]

Another reason of no oil to head good to know.
It look like a blind mechanic has worked hard with a sledge hammer damaging those dowels.
I have ruined some when taking them out when sitting hard and they must be removed. Pinch them where sticking out, hammer them out with a pin from other side.
That' s why I have several new of different lenghts as spares, they are not expensive.

It is very easy to damage the oil pump dowels too if not going on/off absolutely aligned parallel with the block.

[Kevin D]

Does the engine "builder" know about his errors?

[Dolomite]

I should have a spare set of cam towers if you need them. Let me know.

[MRieck]

Another reason of no oil to head good to know.
It look like a blind mechanic has worked hard with a sledge hammer damaging those dowels.
I have ruined some when taking them out when sitting hard and they must be removed. Pinch them where sticking out, hammer them out with a pin from other side.
That' s why I have several new of different lenghts as spares, they are not expensive.

It is very easy to damage the oil pump dowels too if not going on/off absolutely aligned parallel with the block.

Yeah......I don't know how you do that.

[Xander^]

I should have a spare set of cam towers if you need them. Let me know.

Thanks for the offer, but I’ve picked up a camshaft and a tower from a local guy that was on the forums. Only needed the one tower