Hitchcocks Forum

My '89 350 will fire but not pick up and run properly. I am working through possible causes and i can say that the carb (new Mikarb, correctly jetted) is OK, compression is good, plug is good, HT lead seems OK, so causes are narrowed down to the Boyer ignition and the coil. Do coils break down like that? Can you test a coil?

Simplest test is to simply kick the bike over with the plug held against the motor but where you can see it. If its chucking a big reliable spark that probably isn't the issue. Have you altered something prior to this issue?

To answer your question: yes, coils can fail like that. I take it you mean it will start and tick over, but won't rev when you blip the throttle?



The internal windings in the coil can partially short out such that you get a much weaker spark. The voltage you need to make a spark is much less when the throttle is closed (not much pressure in the cylinder) than when you open the throttle. Thus it may be possible it is sparking when the throttle is almost closed, but cannot generate the necessary voltage when the throttle is opened. The problem is, it isn't very common for coils to fail like that these days.



Having said that, another distinct possibility is some kind of fuel starvation, which causes almost exactly the same symptoms.

You could try running the engine in complete darkness in a shed or garage to see if escaping electricity is visible somewhere it should not be when you open the throttle. (Sounds silly I know but I have used the method as a diagnostic tool several times. )

Have you tested the plug cap for resistance?

Just because the carb is new doesn't mean it's OK.

It will sometimes fire and sometimes run briefly with the throttle closed but will not pick up when its opened. It has run OK recently on the current carb, which has a richer slide (and needle setting) because of a free=flowing exhaust and air filter. Symptoms have gradually got worse, and originated with the previously fitted Amal Mk1 concentric. So I don't think its the carb or fuelling.
I could try a new plug cap, but why would it cause problems? How do you test it for reistance? I have a multimeter, what should it read?
The coil is not old, it was replaced about six years ago and it is one of the supposedly better ones from our hosts.
Might also be worth getting Boyer to test the ignition box I suppose.

have you checked any connectors on the ignition system are both corrosion free and tight?

Yes, all connections thoroughly checked, cleaned and tightened.

I have had a coil on a Yamaha SR500 which would work at speed but would not let the engine idle properly, to the extent the it would die when stopped at stop signs etc. Another one did the same thing on a CB350 Honda. Problems solved with new coils.