Author Topic: CB750K7 rebuild, bad sound from engine (Read 4458 times)

stou · « **on:** October 06, 2018, 12:50:44 pm »

I just completed the rebuild of my CB750K7. I rebuild the engine with a 836 kit. The cylinder was bored and the head had been rebuild (valves guides replacement) at a very good shop. I replace the valves and seals, the timing chain and every engine seals. I checked the primary chain and all bearing and they where in spec. Valves adjusted and carb sync, new plugs. I have an electronic ignition. The bike start very well and I did a very small road test and it run fine. I have an oil pressure gauge and the presure is around 70 psi. I took compression and every cylinder is around 150. So it look good on this side.

But the problem is that there's a very bad sound coming from the engine. At first I tough it was the clutch. So I removed the cover and the lift plate, everything look good. Now I wonder if it can come from the kick start assembly. It's like some metal parts are rubing together. I tought maybe the cam chain but I adjusted it another time and can't why it can be that. I drop the oil and I don't see metal flakes in it.

For now unless someone have an idea of what it can be, the only option I can see is to remove and open the engine one more time.

Here's a video where we can ear the metal rubing sound:
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ekpent · « **Reply #1 on:** October 07, 2018, 05:01:08 am »

That is a scary sound you have there,wish I could give you an answer. You would think that if it was coming from the kick start area that you would maybe be able to feel a vibration by holding the kick start shaft with your hand or see some vibration in it. I assume it doesn't change when in gear. I have to mull that over some more. Was there any play in the clutch basket on the shaft like you forgot to install a circle clip or washer/spacer to hold it in place ? Hard to tell in the vid exactly where the sound is coming from.
Very nice looking bike,hope it something simple..

stou · « **Reply #2 on:** October 07, 2018, 06:48:52 am »

The more I listen to it, the more it sound like a chain sound. I will have a look at the primary via the oil pan and the cam chain via the cam chain tensionner hole.

Thanks for the comment ekpent!

stou · « **Reply #3 on:** October 07, 2018, 07:01:52 am »

I did a small test this morning. I removed the spark plugs and make the engine turn with the starter. As you can ear, there's no bad sound when I do this. Very strange!

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stou · « **Reply #4 on:** October 07, 2018, 12:20:02 pm »

I just remove the oil pan to check the primary chain to be sure everything is ok. I have a 67mm gap, so it is good. But look what I found in the oil pan

$:-\$

I wonder where this piece come from? It look like a casing corner.

754 · « **Reply #5 on:** October 07, 2018, 12:52:11 pm »

Could be dry cam journals pop a few caps, see if its oily or dry in there.
4 outer ones, if you see dry pop the 2 beside them.

calj737 · « **Reply #6 on:** October 07, 2018, 12:53:38 pm »

I’d be yanking that motor out, pulling it down, and repair what’s wrong. It will only get worse.

stou · « **Reply #7 on:** October 07, 2018, 03:49:59 pm »

I found where this piece come from. When I bought the bike, the guy give me many old parts and an old engine. I already toke the top end apart last year because the cylinder of this motor was in better shape than the one of the bike. The one on the bike had many fins broken. So this afternoon I took the base of this engine apart to see the inside of it. The piece I found in the oil pan has gold paint on it, like the inside of the engine crankcase. I saw in the old motor that a small tab on the casing was broken just under the cam chain. It looked very much like the piece I found. So I looked on my bike through the oil pan hole, and I saw the same part broken. I probably forced it with the cam chain when I install the cylinder. I was alone and I had to play with the crank to insert the pistons. The cam chain probably get caught on this tab and when I turned the crank, it broke the tab of the crankcase.

I don't know if the loose piece can cause all that bad sound, but I will put the oil pump and the oil pan back and try it to see if everything is ok.

Here's some picture of the old engine with the tab broken and the last one is a picture taken via the oil pan hole.

ekpent · « **Reply #8 on:** October 07, 2018, 04:01:16 pm »

Be great news if it was that broken piece pinging around somewhere in there but what would have made it suddenly go down in the pan ? Fingers crossed for you.

calj737 · « **Reply #9 on:** October 07, 2018, 05:34:17 pm »

I doubt that broken piece was the cause of the noise. I suspect (since you mentioned it) that the cam chain and tensioner were dislodged and thus broke that piece.

It would be highly advisable to pull down enough of the engine to confirm proper installation and location of the cam chain/tensioner to prevent further damage.

754 · « **Reply #10 on:** October 07, 2018, 06:40:24 pm »

Just listened......maybe the cam bolts are backing out ?
Sounds pretty bad, I cannot see it fixing itself.

stou · « **Reply #11 on:** October 07, 2018, 07:28:45 pm »

I will remove the valves cover tomorrow to look if everything look ok with the cam, chain and bolts. Today I looked trough the hole of the pan and everything was looking ok from under. So a look from the top and maybe from the chain tensionner hole will be a good thing.

scottly · « **Reply #12 on:** October 07, 2018, 08:56:41 pm »

I've seen pics of the same broken piece here on the forums before, but I don't recall the cause. Since it it broken on the primary chain side, I would check to make sure the primary chains are properly centered on the raised ridges on the tensioner roller before looking at the top end. There appear to be some black bits in the pan as well?

PeWe · « **Reply #13 on:** October 07, 2018, 09:50:40 pm »

Was that piece broken when cam chain was loose, no cam in? Crank rotated, chain locked up against that piece, more force to rotate and click, broken?

stou · « **Reply #14 on:** October 08, 2018, 03:47:43 am »

Quote from: scottly on October 07, 2018, 08:56:41 pm

I've seen pics of the same broken piece here on the forums before, but I don't recall the cause. Since it it broken on the primary chain side, I would check to make sure the primary chains are properly centered on the raised ridges on the tensioner roller before looking at the top end. There appear to be some black bits in the pan as well?

It's under the cam chain, not the primary chain. I checked the primary chain when I removed the oil pan and the oil pump. Everything look good on that part.

I think PeWe is right. Like I wrote above, I think it happened when I assembled the cylinder and the pistons. I pulled the engine up alone and I had to play with the crankshaft and cam chain to get the pistons into the cylinder. The cam chain has blocked a few times. Probably by turning the crank when the chain was blocked, it broke this thin corner of casing that is right under the chain.

stou · « **Reply #15 on:** October 08, 2018, 01:15:09 pm »

I removed the valves cover to see if everything is ok and didn't found anything suspect. Cam gear and cam chain was ok, there was oil everywhere. So I replaced the cover and started it again... same thing.

When it was running I tried the best I can to locate where the noise come from. I'm not sure but it like it is located around the kickstart mecanism.

So now I don't have any other option than pull the engine out an open the base to look what can be wrong!

754 · « **Reply #16 on:** October 08, 2018, 02:51:58 pm »

If you pull the whole clutch out, you can see the whole kicker are and look for something broken or scarred up.
About 1/2 he to pull clutch out, or less.

stou · « **Reply #17 on:** October 08, 2018, 03:34:02 pm »

Yes I did that this afternoon. It look like there's some play in the shaft and the gear, but I didn't saw any wear or damage. That's why I decided to open the engine. The retaining pin was in place but there's a lot of play in the parts. If I push or pull the shaft the gear is rubing on the casing. But to do that you must remove the clutch basket, so this will probably give more free play to the shaft.

stou · « **Reply #18 on:** October 10, 2018, 07:09:44 am »

I just complete the engine removal and crankcase opening. A first look didn't found any trace of metal friction. The only things I notice, is a free play in the kick mechanism and another free play on the crankshaft when I push and pull sideway in the casing. I just wonder how do I know if the cranshaft is at the right place? I can slide the crankshaft of 4mm from left to right.

Here's some video and pictures.

Free play on the crankshaft (4mm):
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Free play on the kickstart assembly:
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stou · « **Reply #19 on:** October 10, 2018, 07:41:03 am »

Maybe I found something. The starting gear on the stator side have a lot of free play. I compare with the one on the old engine which as no free play. Maybe a problem with the seal?

Not a valid vimeo URL
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stou · « **Reply #20 on:** October 10, 2018, 10:16:54 am »

The primary chain is in spec with Honda shop manual, but I find it very slack from side by side. So I order a primary chain kit on CMSNL and a also new starting gear seal.

Not a valid vimeo URL

enwri · « **Reply #21 on:** October 10, 2018, 10:27:32 am »

I can't find a measurement for end float, but 4mm seems excessive. None of the chains will like that, points and advance mech might even tangle up as well.
Would have thought that the alt/starter gear assembly is the only way to control thrust. There's no seal on that side, only the points side.

Edit, by the diagram there is a seal there, what is it sealing?

scottly · « **Reply #22 on:** October 10, 2018, 10:30:29 am »

Stou, I can't play your videos on my steam-powered computer, so I can't hear the sound. Read this thread:
http://forums.sohc4.net/index.php?topic=86429.0
PS I doubt the seal on the starter gear is the issue.

754 · « **Reply #23 on:** October 10, 2018, 10:35:12 am »

I doubt the crank end play is that much with the cases together.
At any rate, i think the cause will be a very worn part somehere, usually followed by a scraped up or scuffed area, shavings that sort of thing.
The cam chain, try lifting it off the sprocket, check for wear, or broken links.. I can't see the noise coming from something you can't see.

enwri · « **Reply #24 on:** October 10, 2018, 10:43:52 am »

How much float on the old engine in the video? is it in the same state of disassembly to compare? That's it for me and my suggestions, its been awhile since I've seen inside mine. I'm curious to find out though.

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Author Topic: CB750K7 rebuild, bad sound from engine (Read 4458 times)