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Rebel 450 Knocking

2088 Views 42 Replies 3 Participants Last post by  DD23
Hello all. I have been trying to pin point this knocking that came out of nowhere with my 86 rebel 450. It’s been a dreaded journey for the last week or so trying to trace this and figure out this knocking sound I have. The chain of events that happened that day before it started to knock :

1: I changed the exhaust and was told to re jet the carbs so it wouldn’t run lean. Changed slow jet from stock 35 to 40 and main jet went to a 130 from I think was a 112. I changed the exhaust and rejetted the carbs then rode the bike and tested out the jetting. The oil may have been contaminated with gas also because I smelt So there’s that possible problem

2: came home and pulled the plugs after bike started to act up while riding. Plugs were black and sooty. While checking The bike out….

3 bike fell lightly into the dirt on left side. (Kickstand was in soft dirt) Peg and shifter lever went into the dirt
4: started the bike after it dropped and it started knocking
5. After it started knocking I was told to check out the valve clearance to make sure that wasn’t the problem. I adjusted the valves which didn’t make a difference in knock and returned the exhaust and carbs back to stock. Was told that it may be the top end. It’s very hard to figure out where the sound is coming from because the sound is equally loud at the bottom end and the top end of the motor. I also synched the carbs thinking the knock could be from unsyncd carbs. Knock was still there
I’m chasing down top end but trying to think out of the box maybe it’s not top end but bottom end from the the shifter hitting the dirt. I’m shooting in the dark but just trying to think logically. Nothing was bent shifter wise though and the shaft that the shifter bolts onto seems to be okay. It does move in and out slightly though but I think that is normal .
Other things I’m thinking about if the spark plugs were black and sooty it could be loaded with carbon and having a pre detonation problem but I really don’t know. It’s so hard to pin point where the sound is coming from. I’m going to get a stethoscope from harbor freight today and see if that helps alittle more. The shifter didn’t bend or nothing just had dirt on it from hitting the ground. Things that were also brought up after I had the valve cover off was the cam adjuster larger wedge doesn’t spring downward. It’s stuck up so maybe the cam chain is not adjusting? But then when I start the bike with plugs out or even in with plugs in and the wires not attached there is no knocking at all. If there was something internal of the top end it would be still knocking I would think. The bike only knocks when it is running at idle speed. If you rev it the knocking goes away. So now I’m thinking pre detonation? maybe the new cdi I bought is not functioning properly or the ignition coil. It does get spark though. I have just been a hamster wheel going nuts about this noise. Was looking for some insight or maybe if anyone has had this knocking before …
Thank you!! Here is a video of the knocking

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Have you adjusted the balance chain lately?
As above, and was the flywheel cover on when the bike fell over ?

You rode it with thin petroly oil !?

How is the oil light behaving...not many cmx450 made it to UK but engine is sohc 6 valve shell bearing type.

Remove plugs and check for a "bright" ring on piston crown, or evidence of piston crown striking cylinder head...

Any difference in knock if clutch pulled in ( disengaged )....?
Thanks for the response! No I haven’t checked the balance chain yet. That would be next. I have not taken the right side cover off yet
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thanks for the response!

The cover was on when it fell over.

The oil wasn’t very thin from what I noticed but it did smell like gas. Even after a oil change the oil still smelt like gas. The oil light is normal. It doesn’t come on when the bike is running. There isn’t a knocking sound when just using the starter and cranking the engine. The knock is only there when the engine is running. I’ll check the piston tops when I get home today. Knock is still there when the clutch is pulled in.
Try not to use it but you may have to which I understand.

May be a gear selection mechanism problem but I doubt it, even cotton wrapped around a tyre can sound like a disaster happening at 30mph, ask me how I know...

Do your research, you know of

Find out if your balance system uses cush drive weights....any lumps of black rubbery type stuff seen in the oil.

Petrol in oil is not good, but it does "clean" the insides a bit.

Check piston crowns but as Mike says, balance system check.

Do your research.

Feedback and pictures to help others...
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I adjusted the balance chain through the inspection hole on the side of the crank case. It self adjusted and moved up but the knock is still there. The piston tops look normal. I don’t see any score marks on the cylinder walls either. Was just wondering if it’s normal to see the exhaust valve when it opens through the spark plug holes?
Did it land on the exhaust pipe? Sounds kinda like a leaky exhaust pipe joint.
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The balance chain adjusting quadrant and shaft is spring loaded to turn the quadrant anticlockwise to adjust. So the quadrant slot would move in a downwards direct, not up.

Can you clarify and how much slot is left ?

You should just about be able to see the balance shafts DOT, which should be above the horizontal...

Could be an exhaust leak, makes sense, phone audio has a good mic, does exhaust leak ?
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Hey. No it didn’t hit the exhaust. I also replaced the copper crush Gaskets.
But I just noticed something today It hit the left peg and the gear shifter had dirt on it from the ground. I got to thinking let me check around the shifter rod. Since the knocking happened after it felll. But I just noticed something today.

I was looking and hearing around the bottom end. I noticed that the shift rod that goes into the trans flexes when pushing it down in first gear. Not sure if it’s supposed to or not. Or does the side case eliminate that flexing when the cover is on? The sound seems to be more pronounced there and on the shift rod. But also the same sound at the top by the valve cover. So not sure if that flexing shift rod is normal or not. Here is a YouTube video. Thanks!

Sorry I made a mistake with the balancer Before I loosened the nut the plate was up against the stud. When I loosened the nut the plate moved down. The stud is more towards the middle now. I didn’t take a picture of it but I will take a picture tomorrow when I get back to the bike
Here is another knocking video. The exhaust isn’t leaking. I wish it would of been that simple 😂
But that’s not my luck
Here is a video from before. I tried to hold the stethoscope ear piece to the camera mic but that didn’t work out to well. But you can hearing the knocking. In person it sounds loud by the shift rod and crank case but then it’s also loud at the valve cover. So that’s what’s driving me nuts. Can’t pin point it

No vidio opens...

Gear shaft flexing ?

In what way ?

Some, but not much, in and out is normal.

The gear shaft had a spring loaded " central position", to which it returns after each gear selection.

Up and down, or front and back gear shaft movement is not normal. How is the flywheel cover support bush for gear selector shaft.

First vidio showed flywheel cover off and knock at idle. Since cover was on when bike fellow over, the flywheel will not have "picked up" any gravel or similar.

Need to see shaft flex vidio...

The shaft will have some flexibility cos its quite long, but is made of relatively large diameter steel...

Will have another look/listen to the first vidio....
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Here is the other video.

Hi, please post a picture of the flywheel cover which the gear shaft goes thru...

It has a support bush for the gear shaft ?

The flexing is the gear shaft bending a bit which will not happen in service due to the cover support bush.

The shafts "arm" is not vertical when in its central position meaning a loss of mechanical advantage for changing down gear....

As said not many cmx450' s made it to the UK, so "finer details" not known to me.

As long as the gear shaft is not bent, or has no ( other than very slight ) "play" in the c/case, then no problems seen. No oil leaks from the oil seal ?
okay got it. Thanks. That’s what I was wondering. Now I’m back to tracing down the knocking. No oil leaks. Nothing else is bent or compromised by the shifter with the cover on. This noise is driving me mad! Did you get the knocking video to open?
Yes, watched initial vidio again, seen gear shift vdio as well. cmx450c rebel 1986 (g) USA California balancer exploded view shows solid balance wieght, so no cush drive problems to worry about.

Balance adjusting quadrant self adjusted to slot mid point so didn't go over centre, so not completely worn out, dot position will prove this if needed.

So, what is causing this bad sounding noise ?

You have ruled out a leaky exhaust.

I doubt if the cylinder head is loose or a head gasket problem....but check visibly....

A picture of the camshaft would help rule out camshaft problem. I assume both cam sprocket bolts are tight ?

Picture of camshaft please.....
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I have removed the valve cover and clutch cover today and here are some videos of my findings Please any help is sincerely greatly appreciated! 1st video is the valve cover removed and 2nd video is what I found with the clutch cover removed..

Look at your clutch basket thread...

Videos although informative are too jerky to see/check some things....

I keep forgetting this is a cm bring slightly different to a CB.......please forgive me for that, no oil pressure switch seen...well not in the same place as a CB.....

Watched clutch vidio again, can I see the clutch hub, the bit with the spring plate, wobble ?

Hold hub and basket between finger and thumb so they become one, any wobble ?

The hub should be solid as if part of gearbox input shaft.

The basket drive overun cush drives are always a bit slack by now, as long as rivets not moving.

See clutch basket thread...

There is normally some clutch basket in/out movement...see CMSNL exploded views for your exact bike and post a link in this and your other thread...

Two threads on the go at the same time can cause some confusion, so requests answered in your latest thread, the clutch basket one would be great. Try to quote your bike details at the begining of each new thread....

Cmx450c rebel 1986 ( g) USA California ?

Love the tash and two compressors ....
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Hey, I will have a more detailed response later on when I get to the bike. Thank you for the info. The spring plate doesn’t wobble. There isn’t any wobble of the whole clutch assembly. Just that back and fourth play. Where you can hear it making noise. A friend was nice enough to take his cover off his spare engine and he also had that play. Maybe not so much as mine but he indeed have play. Mine seems to make alittle more noise though. I’m just trying to find anything that would make a noise before having someone or myself tear into the engine.

my problem is I can’t Reproduce the sound without the engine running.

the clutch basket nut is definitely tight and still staked. I haven’t tried to tighten it up more yet because I don’t have the socket to get on the nut.

Sorry about the confus with the double thread. I just didn’t want to get the information lost in the other thread bringing up this clutch basket.

Here is a picture of the balancer adjuster. I will get pictures of the cam later on. Thanks again for the help.


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Ok been having a good look on using cmx450c rebel 1986 ( g ) USA views.

Exploded views do not show enough finer details, and if part not in stock, no picture.

Looking at your slotted nut picture, I reckon the slotted nut has been off before, due to the "disturbance" at about 9 o clock. The stakeable nut, looks to have a clean "restake" at about 7 o clock.

Would you agree with that ?

How much in/out play in mm ?

Top cam chain adjuster bolt tight or loose ?

The petrol contaminated engine oil ( thin) would not do the engine any good, but oil light behaving ok.

Yours according to CMSNL is a shell type bearing crank, very similar to CB 400 n.

Cb400n and the 450 version ( 250 the same ) are prone to left cylinder big end failure, as it is the last shell bearing to get any oil.

No marks seen on piston crown.

You may be asked to return to that cylinder once the " onions outer layers" have been eliminated such as gearbox problem ( don't think so ), clutch problem ( a possibility ), cam chain adjuster problem ( check top bolt ) , balance system problem ( not ruled out yet, but CMSNL shows solid wieght rather than problematic cush drive type ) or anything else.

I assume electric start works all ok ?

Hope it's not a buggered big end .

Going down the clutch less engine run route would rule out, it in, Tete would rule out g/box/clutch basket cause, as would engine run with cam cover off.

A quick ten second engine run would be sufficient.....possibly ?

From memory does this bike make that noise when engine is cold ?

At least you have proved the slotted nut is not loose.

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