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Discussion Starter · #1 ·
Hey, so recently got my first bike: 1981 honda cm400t. So i had it running before then while i was riding it just around the block "learning" to ride, it just completely died and shut off on me, and skided to a stop and wouldnt start. Ended up taking off the left crankcase and the flywheel was stuck, ended up freeing it by just using a socket and just kept turning it till it was loose (accidently went backwards a couple times, later found out thats bad). And now it runs, as in video below but has this knocking noise inside that sometimes isnt there at idle but gets very loud when reving engine.
My thoughts so far have been loose cam chain, ball bearing, or some type of timing issue with the stator/flywheel.
Those are just guesses, anyone know what this sounds like is wrong?
P.s. I tried tightening the cam chain with the adjuster nut which is supposed to automatically tighten when the bike is idling, but it didnt get rid of the noise. But i have read that on these older bikes with more km (mine has 68,000km) that the spring can go and need to be tightened manually, is this something i should try.
Long post, hope someone can help, im new to bikes and really have no clue what is wrong/how to fix
Thanks!
 

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Discussion Starter · #2 ·

couldn't attach video so had to upload to youtube, here it is. Please help don't know what is wrong....
Thanks
 

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OK, remove the plugs and turn the engine over in as complete a turn as possible using a ratchet/socket, breaker bar/socket or wrench. You're trying to get a complete turning cycle w/o stopping, do this a few times. If the engine suddenly binds/sticks or won't turn without a huge amount of force I'd say that the left rod bearing has failed.
No video posted so you might try getting that up again.
 

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though I'm not all that familiar with the 400 series, from his description of what happened (skidded to a stop) and what it sounds like right now, I'm inclined to agree, Jim - it very well could be a rod bearing gone bad, especially considering the miles on it. I wonder if the oil level and condition was checked prior to riding, since it wasn't mentioned in the first post...
 

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Certainly sounds like a rod bearing to me. Can you replay with the bike at proper idle speed?
 

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Discussion Starter · #7 ·
Unfortunatly i wanted to get a jump on taking it apart so already have the engine out, cant get another vid of it running. But im going to definitely look into the bearing rod, thanks.
Anyone know if the gasket kits of ebay for like $50-70 CAD are good enough quality?
 

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Oh, that sound hurts! I had a lawnmower that made that noise. I bought a new one....

Did you drain the oil or drop the pan yet and see what's in the oil?
Definitely take a look before you buy any parts.
If it's a rod bearing it may have trashed the crank as well.
If the crank is badly scored it may not be worth rebuilding the engine.
 

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Discussion Starter · #9 ·
Yeah, very unpleasant sound. But already fallen in love with the bike so kind of want to make it work, and learn during the process which is going to be long!
And ya drained the oil which had a fair amount of metal shavings but no big chunks. The metal shavings were hardened steel though so leading me towards bearing....
 

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Remove the plugs and watch piston movement for any "dead spot", loss of relative piston movement, also look for shiny patch of piston crown around the edges, one shiny ??
 

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Noisy, metal bits in the oil and the engine out of the frame just go ahead and start taking it down. This is one of the easy motors to work on. If you havent done this get a FSM to start with.
I like to put parts in plastic zip lock bags and label where they came from to help on the rebuild. Take off the top end and work your way down till you find the problem .

There are a lot of 400 folks on this site so plenty of help.

Bill
 

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Discussion Starter · #14 ·

So i recently took the engine out and was skeptical about taking it all apart before i fully knew what was wrong, but while it was off i noticed the cam chain at the top was slightly loose so i manually tightened it (the automatic adjustment as per the FSM didnt work) and also had the right crankcase off and saw the balancer chain was loose too (the stopper plate had bottomed out on the adjuster bolt) so i moved the spline over 2 notches and that tightened it up. So with those fixed i wanted to see if that fixed the noise, thought it might have been the chain smacking against the front, and now have put the engine back in and started it up. Above is the video of it running, its sounds great, basically purrrrs at idle but with throttle the sound is still there......have heard its might be a rod bearing, or thought it was a timing issue. But the last video wasnt great, can anyone clarify those diagnoses or offer new ones with this better video?

Thanks again,
Matt
 

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Discussion Starter · #17 · (Edited)
Still trying to fix the noise...since im not the greatest mechanic and new with motorcycles trying to avoid taking apart the head for now. But recently was just trying some stuff and then noticed that one exhaust wasnt getting hot, yep was firing on 1 cylinder...but was getting spark and air intake was fine, so figured it was carbs. Took out carbs and yes they were clogged up, so cleaned them and just put them back in. Now firing on both cylinders but that noise (from video posted above) is still there....really dont care if i have to take it apart but i just want to know what i need to be looking for and where rather than go on wild goose chase. Is it time i took the top end apart? Or anyone got any ideas of where this sound is definitely coming from....man i didnt know bikes were going to be this frustrating

Thanks,

Matt

P.S. also when i noticed 1 cylinder wasnt firing i did compression test and the Right (working cylinder) had 185psi and the Left (not working at the time) had 210psi. If it was low i figure might be a valve or piston but since its high im clueless as to whats causing this sound....
 
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