SOHC.co.uk Forums > CB350/400

CB400F only running on 2 cylinders.

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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  :D

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.

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