SOHC.co.uk Forums > CB350/400

CB400F only running on 2 cylinders.

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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

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