CB400F only running on 2 cylinders.

[Riceman]

Hi and Happy New Year Folks.

Im looking for a bit of advice and guidance. Bearing in mind that as far as mechanics go I would class myself as a novice, so here I go
I tried to start my bike (400F) the other day with no joy, the battery was a bit low so  put it on charge. I tried again this morning and still no joy, i removed all the plugs and cleaned them as they were a bit sooty and tried again. The bike started but was only running very roughly on 2 cylinders. I deduced this by touching the pipes and the 2 centre pipes were cold to the touch unlike the outer ones which were red hot. I checked the coils and one was warm and the other cold, so I am thinking that this may be the issue. Should I buy a new ignition coil or do I need to check elswhere? is there a simple diagnostic to confirm that the coil is indeed us?
Also just as a  side note, I noticed after running the bike that quite a lot of fuel was leaking from the carbs. Other than that everything is hunky dory 

[Nurse Julie]

I'm guessing the cold coil is the one that fires 2 & 3, or does not fire in this case 😀

[K2-K6]

Good diagnosis so far.  The coils are paired so they run the outside two cylinders from one coil and the inside two from the other.

It would suggest first a look at 2/3 numbered points that are switching that coil.  If not working it will effectively leave the coil isolated. The wire that comes from the coil to the points is the earth connection,  this remains earthed by the points until they open.  As soon as they open the coil fires a spark at the two plugs. 

So you need to verify if you've got switching first before you can see if anything else has failed. Hold off on paying out money until you can trace the true fault.

[Riceman]

Thanks for the advice, I will have a look and see if there is anything obvious with the points, I imagine that they are easily  replaced and much cheaper as well.
My Haynes manual says that it could possibly be one of the capacitors that may be causing the problem   I now wish I had been more attentive in physics in my youth 

[hairygit]

First of all, check the connectors from the points wires where they enter the main loom, the same point at which the rear brake light wires join. It often happens that the heel of a boot can dislodge the wires, so check the ends are clean and bright, and fit tightly into the socket parts, which should also be clean inside. Also a lot of spray from the rear wheel can hit that area, so once all is clean and tight, make sure the joints are inside the sleeve of the main loom connection.

[Riceman]

Update:
I have lightly cleaned the points and they are arcing when switching on the ignition so I assume that means they are ok. I traced the wires from the rear break light and found the earth wire was loose so have connected securely all other wires appear to be ok. I will try again once I have recharged the battery as it is very low on charge already, which makes me wonder if the battery could be the culprit

[ka-ja]

Your weak battery may be the cause, but only if there is a problem with the 2/3 coil or its circuit, have you checked if the 2/3 plugs are sparking when out of the head

[Rob62]

If the motor starts and still only fires on 2 cyls, remove the points cover and run the motor briefly to see if there is excessive sparking from No2/3 points.... this would indicate a faulty condenser and would certainly cause a missfire... do it in the shade so you can get a good look at the sparking.

[Riceman]

Update:

I tried starting the bike this am with a freshly charged battery and it would not fire up 
The unusual thing is that the coil which was cold yesterday is now warm to the touch and the other one which was warm yesterday is now cold 
Could this mean that my coils are both ok?
Confused or what 

[ka-ja]

Depends on which coil has a complete electrical circuit on the primary side with the engine not running as to which coil will warm up ie. which set of points are closed with the motor not running.
If you have the test gear, check the continuity and resistance of the coil/s low/high terminals and compare to specs, check you have a spark at plugs with plugs out of the motor

[K2-K6]

They are generally reliable and get caught on something very simple and fundamental.  Quite likely the coils are both ok.

Slightly difficult doing it remotely with checking all the basics but it's really not far from running.

If you've been trying to start it without success it could be a little flooded.  Try making sure the choke is off and crank it over with the throttle open to see if it'll respond. This just puts more air into the engine to reduce any excess petrol in there.

If points spark, it suggests the circuits are intact. All the condensers do is absorb the current as the points open and try to stop it jumping the spark gap as it just begins to open. They usually function,  even if impaired,  but with some compromise.  So although they may need changing you should get at least some firing.

[mike the bike]

I rode my 400four home 70miles on 2 pots.  Sounded like a tractor.  The capacitor had died.  I held a new capacitor in place and the revs picked up instantly.  You could swap the 1,4 capacitor with the 2,3 one to eliminate it.

[Lobo]

Riceman, the coil primaries are energised (+ve feed) from the kill switch, ie via a black wire with white stripe - which splits into two (ie feeding each coil) within the Electrical ‘boot’ under the tank - from memory.

The current from the primary coils then flows down its respective yellow / blue wire to the contact breakers, through these... and to earth (ground).

As each breaker opens, the primary winding is de-energised, and it’s associated magnetic field collapses - inducing a high voltage in the secondary winding - which leaps to ground across the spark plug gap. Yea, boring... ZZZzzz.

Whilst I’ve never tried it, my thoughts would be to (you’ll need a basic multimeter):

(1) put a spare spark-plug into caps 1 & 2 (or 3 & 4), and ensure the body of the ‘plug is grounded. (Eg wrap a copper wire around the thread and ground the other end to the bike)
(2) disconnect the yellow and blue leads from the coils, (ie the low voltage primary circuit), turn the ignition ON, kill switch ON, and manually ground each blue / yellow to the frame in turn.
(3) Then pull the yellow / blue away - and at this instant see if you have a spark across the associated plug.

(Note: you are purely ‘handling’ the low voltage 12v primary here, and should not expect any shocks / tho’ possibly might see minuscule sparks as you break the earth (this is why you have a condenser in the circuit - it to minimise the points faces pitting through constant sparking))

If you get a healthy spark across the plug ...your fault lies elsewhere, and is not the respective coil.

If there is no spark, be sure to check the associated black / white +ve supply is indeed +12v.

Next up I’d check that when the ‘breakers ARE OPEN, the “moving” half of the breaker is 12v (with ign & kill switches ON) (See first pic) (suggest you disconnect the condenser here to minimise duff readings).

..... and that the other side of the ‘breaker has a good earth. (2nd pic)


Throughout all of this, do not energise any primary coil longer than necessary...  ie having a complete circuit across it - whether it be you holding a yellow / blue wire to earth.... or the ‘breaker closed (same thing), as you run the risk of burning out the coil.

As I’ve said, not done this trouble shoot myself, and happy to hear any thoughts.

Lobo

[Orcade-Ian]

I think Lobo’s post should become a sticky for anyone struggling with our simple but sometimes confusing ‘Kettering’ system - very informative and helpful.
The OP doesn’t say how long he tried to run the bike or how long it had been standing before the problem.  Another important thing to check is that there is no ‘doubling up’ of resistors, i.e. wrongly fitted resistor plugs with resistor caps which can cause similar symptoms, especially as he said the fault seemed to swap over another time.

Ian

[Riceman]

Thanks for your input everyone, I am still no further forward really and now have acquired a multimeter to carry out the checks advised by Lobo.
I have ordered a new set of plugs because non of them are sparking now which may be that they were at the end of their life.

Just to clarify the bike was running fine when I last rode it has been stood  in a heated shed since the end of Oct, the tank was full with a fuel stabiliser added.

Once again thanks for all your input

Andy