400 four knocking noise.

[stevie]

Hello all, can someone please give me some advice about a problem with my 400 four that I am restoring? On tick over there is a knocking noise coming from the bottom end (quite bad). This goes off when the revs are raised. It is the same when hot or cold. I have split the engine and all of the shells look ok and there seems to be no play on the conrods. The crank journal shells also have very little rub marks. Any ideas? Some one suggested that it may just be the carbs that needed balancing. Also can someone tell me how if or how you check the primary chain needs replacing?

Thanks
Steve

[hairygit]

It certainly sounds like carbs needing balancing. It is not to difficult to do yourself if you have or can beg/borrow a set of vacuum gauges, but before attempting to balance carbs you must ensure that the engine is in a good state of tune generally (points gap/ignition timing, valve clearances, engine oil condition/level and air filter. If you don't feel confident enough to give it a go yourself, try to find an independant motorcycle shop to give you a fixed price for doing it, it shouldn't be too extortionate

[Tomb]

If it knocks at tickover and goes off if revs are lifted, try pulling in the clutch lever, the noise should reduce the same-ish. Its clutch rattle and pretty normal on Hondas of this era. It is made much worse when carbs are out of balance so do as HG says. 

[K2-K6]

Are you sure it's running on all 4 at tickover? start it without choke if poss and run for about 30sec, switch off and check if all the exhaust header pipes are getting hot.
If one is cold it cold be a slow speed carb jet blocked and preventing one cylinder from firing properly at low revs which starts as you lift onto more main running jets, can give a quite low thuding that sounds as if it's bottom end.

Sounds as if the bearing shells are all intact, are the gudgeon pins also ok (it's unlikely these are worn but just possible)

As the other posts say, just a general checkover of ignition etc may turn up something.

[stevie]

Hello everyone thanks for the replies. Checked it is running on all 4 and it was. Also tried the clutch and the knocking continued. Am going to start the rebuild in the next couple of days. I am replacing all parts that are worn as I go. I will check and set up carbs and see what happens.

[UKROBK7]

Did you check the bearing shell clearances with Plastigauge (gage) when you had the engine down to check the clearance against the Honda spec?  A couple of my shells on the 750 looked great but they were down below tolerance.

[Lynx]

Stevie,

What was the outcome of this? I have a very similar problem on my newly rebuilt engine.

Cheers
Steve

[Dan]

I would put money on this being carbs out of balance. I had exactly the same problem also. The first time I tried balancing the carbs using vacuum gauges I got nowhere.
I cured this by removing the carbs and setting the carb slides as best I could by eye, then refit carbs and make fine adjustments with the gauges. Result - knocking gone and fears of bearing/primary chain problems allayed.

Search this on You Tube "cb400 4 carb synchronisation"

Dan

[Lobo]

My newly rebuilt engine too has this knocking in its boots; and I'm at a loss at to whether its 'normal' or otherwise. It was professionally rebuilt, inc new can chain, tho' not primary... which I'm told is fine. I've balanced the carbs pretty bloody spot on (I think!)... the only query here is they pull only 16"Hg @ idle versus the typical book figure of 20-24". Comments on this? (My '72 750 same & runs fine)
The knocking is a little alarming @ 900rpm which is rather slow I guess, by 1200rpm the knocking is 'ok', and by 1400rpm the motor's just silky smooth. Due bike not registered unable to give it a true shake-down, but don't imagine this knocking will mysteriously disappear.
Would really appreciate a definitive answer, its had new clutch basket & springs.
Not truly alarming... Its just that the rest of the bike is so good!

[Lynx]

Well I'm pretty sure in my case it is carb related as carb 2 still doesn't seem to be working. Looking as old posts this seems to be a common problem on new rebuilds. If I raise the idle above 2k it souns OK but very little throttle response. Below 2k it sounds like the engine is about to break in half.

Spark plug, cap, lead, coil all tested OK. Compression fine. So I've stripped the bottom end of the carbs again, cleaned jets and passages with Wynns carb cleaner and blown through with compressed air. Re-installed but no change at all . Tonight I'll try replacing the pilot fuel jet, fuel valve and float and cleaning again.

[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.