Topic: Honda CB400T, top end noise

[Patrick]

I just transplanted a motor from a 1979 Honda CB400T into a 1978 Honda CB400T because my son considered oil to be an optional additive in the 1978 motor and he opted not to add any. The motor is now a great big paperweight. The transplanted motor starts up easily and idles pretty smoothly considering it sat for several years in an unused parts bike, but it has a noise in the left side at the top end when you rev it that I having some trouble identifying. At first I thought something like a nut or a washer had fallen into the cylinder and was rattling against the piston and head when you revved, but I couldn't find anything in there and I now think that something in the cylinder would rattle all the time and not just at higher revs.

I checked compression and I have 125 psi on the right side and 85 psi on the left. I checked the valves and they appear to be operating properly and I regapped them. I now am working under two theories. First, I did not replace the exhaust gaskets and I know the left one leaks, but I don't know if it would sound like a rattle. Second, the rings in the left piston may be stuck, causing some piston slap when you rev it. This would account for both the noise and the low compression. If this were the case, I was thinking that running the bike might free them. What exactly does piston slap sound like?

Any thoughts on this, anyone?

Patrick   

[750goes]

I would check and re-do your valve clearances for the bad compression on one cylinder..also pour some penetrating oil down the bad cylinder and let it sit for a day or two, then start it and run it again...if compression comes up a bit then it could be a stuck ring...but trying to diagnose a "noise" is not real easy.....it could also be a stuck valve...which could explain your noise...hope I'm wrong with that bit..good luck.......

[Slapguts]

I'd check that exhaust first. I went through quite a few gaskets putting the new pipes on, and it made some strnage noises until I got it sealed up right.

[bryanj]

Gasket blow will sound exactly like a noisy tappet, if it runs try using for a while with oil and filter changes at 250, 500, 1,000 miles and hopefully the rings will free up. There are a couple of things about servicing these motors especialy the balance chain tension and cam chain tension, you can email me direct and i will try and prod the brain if you wish

[KB02]

Sorry to drag up an old post, but I'm dealing with the same issue on my 78 CB400T.

The service manual I have states that if the valves are properly adjusted, there should be any "ticking" noises coming from the valve train. If there is, and cam chain adjustment doesn't fix the issue, then it may be caused by the tops of the valves stems being pitted from the rockers, hence making any adjustment null-in-void due to the pitting. In this case they say to replace the valves. Could you just have the tops of the stems machined flat again and then re-adjust?

Also, the ticking sounds in my engine seem to get MUCH quieter when I have the cam chain adjuster loose, but as soon as I tighten it again, the ticking get louder again. There's less than 9000 miles on this bike, so it shouldn't be in need to a timing chain replacement, should it?