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Discussion Starter #1
Hey all,

I recently bought a 1981 Honda CM400T, around 65,000km on it. Been working on it for a while now and have it running but now having problems. First noticed that 1 exhaust wasnt getting hot so pulled the plug lead while running and yep that cylinder (left side) wasnt firing...checked it has spark though. So did compression test and the Right cylinder (working one) had compression of 185psi, and the Left cylinder (not firing) had compression of 210psi. That led me to think its not valves or piston. Since i had spark i figured carb might be clogged, so i cleaned carb and put it back in and the Left cylinder was firing for about 5mins but the sounds remained. This morning i started it back up and Left cylinder wasnt firing again, and noise is still there.
Any idea on what the noise is? Or why cylinder not firing?
Have had some people say sound is a loose/broken bearing, but want some clarification before taking the engine fully apart.

Here's the video of the bad noise:


Thanks,

Matt
 

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Can't help with the noise but someone who hears will be along.
Higher than normal compression is a sign of carbon build up, possibly from that cylinder not firing properly over time.
With the plugs out turn the engine over by hand, it should rotate fairly smoothly. If you suddenly hit a really stiff spot that requires a lot of force that would indicate a rod bearing is failing/failed.
 

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Discussion Starter #3
I can turn it over quite easily by hand and has a couple times that get slightly difficult but still hand turnable (believe its just reaching top on the stroke). But when i was riding the bike about 2 months ago then it broke down and died while i was riding it. After it died i tried turning the flywheel but was completely stuck, ended up forcing it and after many many many turns it started moving freely again. Was that a mistake, when it was completely stuck and i forced it did i break a bearing????
Also is there a way to clean the carbon build up so that i can do that then start the bike and see if that fixed it?
 

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Lesson learned I think. Never force anything on the engine. You would be surprised how little force it takes to bend a valve on these engines.
I think that you're going to have to take the head off to see what's going on in there.

I'm of the same thought that your piston is hitting something (possibly a valve)
 

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Discussion Starter #6
Haha thanks, ya still learning...and that wasnt the smartest thing i've ever done! If it is the piston hitting the valve is that something relatively easy i can fix? And if so what does it entail me doing to fix it?
Thanks again, and sorry for the beginner questions
 

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To me, the sound would fit with carbon buildup, as would the high compression. Pull the plugs and compare what you can see in the normal cylinder with what you can see in the other. I have a cheapo e-bay endoscope which works quite well in these situations. Or, just pull the head which is the best way I know to clean the carbon off anyway.
 

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Discussion Starter #8
Thanks all, i'll try looking in there and possibly taking the head off this weekend when i get time. So far looking for carbon build up, or if the pistons are hitting the valves.
If the piston is hitting a valve, how can i tell when examining it, and how would i fix that, does it just mean a valve spring is shot and needs replacing?
 

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Likely there would be marks on the top of the piston. A broken spring would result in a bent valve and a lot more noise than you have. It also would not be accompanied by high compression.
 

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Discussion Starter #10
So i am in the process to taking the enigne apart to get a better idea of where the sound is from. Here are some pictures so far, only taken the top end off so far.

Still waiting to get a valve spring compressor to get a better look at them, but are they supposed to be concave like that? Arent valve heads flat (at least all iv seen on cars are)? Also looks like my cylinders need honing, them and pistons kinda scraped up.... Lastly 1 of the cam bearing is really damaged and the seat it goes into on the head is very very bad figure something got in there, could that be where the sound coming from? And any way i can fix that or do i need a new head and cam shaft? Looks like things are about to get pricey $$$
 

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