1985 Honda CB125RS - Ignition Electrical Problem

[Tedwood]

Hi,

Finally got my bike on the road last Friday and took for it's MOT ( it passed!).

Everything seemed OK electrically, battery charging and direct lighting and everything else.

BUT on my ride to MOT the engine would not rev over around 4000 rpm!

Today I've checked all the connections including all earths and found nothing wrong. However, I disconnected the alternator output wiring, yellow (direct lighting) and white (R/R) leaving only the black/white connected which is the power to the ignition coil (stator exciter coil) and engine runs fine with full revs to the red line!

Completely baffled on this one, I suspect the R/R is dumping too much voltage/current via the resistor at 4000 rpm which may be causing the ignition to break down.

I don't know how to test the R/R so I was going to buy new one to try. I have tried fitting secondhand R/R and resistor from my spares box with no luck but of course they could also be faulty.

Sorry this is a bit of a long post.

Any help would be greatly appreciated. Thanks

[Tedwood]

Decided not to just buy a R/R and try it, as I really don't think that's the problem.

Been unable to do any further testing on the bike as the rain has just not stopped!

Still can't get it into my head as to what the problem could be.

Basically, there are 3 completely separate circuits, 1. Igntion, 2. Headlamp and 3. the Battery Charging Circuit. So I really don't understand how disconnecting the Headlamp and Charging Circuit cures the revving problem.

My attempt at a simplified wiring diagram:

[Teflon-Mike]

IF you have a one-wire system with condensor.. then it would be a points system, and you're feed from the ignition isn't coming from an 'exiter-coil' or pulse trigger, but from the points, which when closed put volts on the coil, when they open, breaking the circuit, the change in voltage on the coil creates a field colapse in the electromagnetic flux, which in turn generates an effof big reverse voltage to opose the change, which, amplified by the transformer windings ratio of the coil, makes the voltage big enough to jump the gap in the plug... condenser, is basically an electrolytic capacitor, and it's job is to act as a 'soak' against reverse voltage trying to jump the points gap rather than the spark-plug gap.

There is no 'exiter coil' or 'pulsar' which generates a very small pulse to trigger the spark, as you would have in a DCi system... upstream of the points, a winding in the magneto makes the volts to send to the points and thence the coil.

Old rule, if you had an ignition fault on a points ignition... before looking any further, or asking silly questions, you replaced the condenser, cleaned the points and set the points gap to the book... and 9 times out of ten 'fault' would go away.

Often the condenser at fault, breaking down and either going open circuit, which causes the points to arc and ruins the timing, or it goes closed circuit and effectively earths the ignition so you never get a spark, BUT horrible bludy devices, they often dont play fair and go 'intermittent' as they break down... often when hot... so happens when riding... then when you have pushed the bike home, and had a coffee, bludy thing starts first kick, and you haven't a clue what's up!

So, IF you have points, that's my first suggestion... replace condenser, clean and gap the points.

Beyond that... connundrum of diconnecting A/C direct winding begs suggestion you have some-how got the windings in the mag crossed over... could this be a possibility?

OR... its not a points ignition, its a CDi system, in which case, you should have two wires to the CDi unit. One from the ignition winding in the mag, the other from the 'induction trigger' that times the spark instead of points.

This latter suggestion, might be more condusive with your suggested sysmptoms, if you have the lighting or charging winding powering up the ignition, not the other wise normally isolated ignition winding....

Solution is to identify accurately which system you have; get a wiring diagram and a multi-meter, and start buzzing stuff out...
____________________
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Current Bikes:'Honda VF1000F' ;'CB750F2N' ;'CB125TD ( 6 3 of em!)'; 'Montesa Cota 248'. Learner FAQ's:= 'U want to Ride a Motorbike! Where Do U start?'

[Tedwood]

Thanks Teflon-Mike,

Yes, it is a Contact Breaker ignition system getting its power from the Exciter Coil in the Mag/Genny. Sorry, I omitted to include the points in my diagram.

The bike has a new ignition coil and condenser (permanently mounted to coil connected via a single connection to the loom & points), new points, spark plug and cap. Also there is no arcing at the points when it is running.

As I tried to say in my original post, all is working fine:

(1) Bike starts first kick and idles OK
(2) AC Direct Head/Side lights all OK
(3) Battery is being charged OK
(4) Battery powered items, Neutral Light, Horn, Brake Light and Indicators all OK.

Problem being that it will not rev over 4000 rpm approx.

I checked all the connections against the wiring diagram and also continuity checked and couldn't find anything wrong. So to try and isolate the problem just to the ignition wiring. I disconnected at the alternator the Direct Lighting (yellow) and Battery Charging (white) coils. With these disconnected the engine runs OK up to red line!

I just don't understand what difference disconnecting these 2 circuits make as, to my mind, all 3 circuits are separate and independent of each other. Although they all do share a common Earth, perhaps that's where the problem may be?

When the rain stops (if ever), I will go through the checks again in case I missed something.

Hope this all makes sense. Cheers, Ted

[Robby]

Try running some more earths back to the battery negative - in particular run an earth lead from the earthing point on the coil back to the battery negative.