SOHC.co.uk Forums > CB350/400

400 four knocking noise.

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LesterPiglet:
Post a video and let us hear it, you will find out if it's normal or not.

Bitsa (Ralph Wright - RIP):
Lynx,
Cant remember if the air screw has a hole through it at the bottom like my 750s.If blocked can give this problem with the slow running side.
Cheers
Bitsa

K2-K6:
It does sound as if there may be some missing (cylinder not firing) in some of these cases......imagine if you missed firing on one cylinder for one beat and then it got it the next time, then that can sometimes sound like a dull thud.

A question for all cases would be, are the noises a regular beat or sporadic?

A suggestion is to try running it with a good car battery jumped onto it to make sure there is sufficient voltage supply for the ignition system to supply at very low revs.......connecting and disconnecting while it's running will give you a pointer if it has any effect.

Also run it in the dark to see if you have any visible escaping HT supply tracking to earth anywhere.

Lobo:
In my bike's case the 'knocking' is constant... and due new battery, new loom / wiring / connectors, new HT coils, new HT Leads, new plug caps / plugs ... new everything I'm not 'thinking' electrics / ignition.  Besides, it'll idle fairly happily / smoothly @ 900rpm - tho' I will indeed take a look in the dark!

Due non-availability I've not renewed the cam-chain tensioner springs, and indeed, only adjusted the cam chain by the book.... ie not the screw driver down the hole method which is perhaps the next stop.

Primary shaft bearings new, all other bearings given a clean bill of health...

If the retensioning of the cam chain (via alternative method) doesn't do the trick I'll likely leave it as in all truth not 'bad' - ie I'm possibly expecting too much of 1970s technology.

Finally - just how would a slack primary chain / worn cush's manifest themselves... deep knocking, rattling... ??? (although again, given a clean bill of health)

Lynx:
After the third carb strip and clean, things do seem to be getting better. I could balance the carbs easy enough this time and have a ready of 20 cm mercury. Lat time I just could not get them all the same and the were roughly at 16 cm mercury. I also set the ignition timing with the strobe and set the mixtire using a Colourtune plug (pilot air screw now out 2 1/2 turns).

Now it will idle fine at 1800rpm. No knock and good throttle response. Still if I drop the revs to an indicated 1200 rpm it'll knock badly. But it sounds to me like it douing about 500 revs! I wonder if the tacho is reading fast. I tried 2 other tachos. One read the same as the one on the bike and one read 1000 rpm when the one on the bike read about 1250, but I don't know which is right.

Anyway I think the bike will be quite ridable now but I still need to build the front brake calliper and put the gear change on.

I did make an MP4 of the bike running but it's too big to upload to here.

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