Problem with electrical interference on 1970 Interpol

This is my first post here. I recently restored a 1970 Norton Commando Interpol. Everything on the motorcycle was replaced or rebuilt, including all electrical components & mechanical parts. All components work perfectly when the engine is off, but when it's running, the blue lights & siren work erratically & the headlight pulsates. I have already ruled out the charging system as a problem by disconnecting the diode, infact the system runs better at a higher rpm when it's putting out more voltage. The battery is new. I checked the voltage & amps, but it is steady & not erratic, I don't have a scope. I think it's caused by a high pulse voltage draw from the mk3 ignition box that I upgraded to. I'm not sure if the capacitor is supposed to make up for the voltage drop, or if It's safe to add an additional capacitor to the system. I'm not sure how to test it to see if it's operating correctly though. So far, I added a noise filter to the lights & they seem to work alittle better, but not like they should. Has anyone else had this kind of problem adding accessory's to their Commando, or a similar problem with an Interpol?

you have stated you changed ignition box check: coils... and if electronic ignition i don't think a capacitor is required so something up with that.
Doxford

The coils are new, I took special care not to clamp them too tight.

and I disconnected the capacitor while testing the voltage & it didn't change anything. Another Norton collector advised me to leave the capacitor connected since I have alternating lights, siren, a 2 way radio & etc.

Welcome JSPayne9207,

This is an intersting problem, as we don't have many Interpol owners here (I think?), especially Interpols that still have their special equipment on them!

JSPayne9207 said:

I think it's caused by a high pulse voltage draw from the mk3 ignition box that I upgraded to. I'm not sure if the capacitor is supposed to make up for the voltage drop, or if It's safe to add an additional capacitor to the system.

Click to expand...

Presumably, the capacitor you are referring to is the Lucas 2MC 'blue can' item?

The 2MC is there to aid starting if the battery becomes discharged, or will (if you are lucky?) even allow 'battery-less' running, but it shouldn't affect the operation of any electrical components whether it is connected or not, and I doubt that fitting two would change anything, but I've never known anyone fit two 2MCs before?

If you have the old points set, why not refit them and see if it makes any difference?

I have heard that many Interpols (before the 850 MkIII) were fitted with special high output alternators?

Are suppressed plug caps (or resistor plugs) fitted?

JSPayne9207 said:

This is my first post here. I recently restored a 1970 Norton Commando Interpol. Everything on the motorcycle was replaced or rebuilt, including all electrical components & mechanical parts. All components work perfectly when the engine is off, but when it's running, the blue lights & siren work erratically & the headlight pulsates. I have already ruled out the charging system as a problem by disconnecting the diode, infact the system runs better at a higher rpm when it's putting out more voltage. The battery is new. I checked the voltage & amps, but it is steady & not erratic, I don't have a scope. I think it's caused by a high pulse voltage draw from the mk3 ignition box that I upgraded to. I'm not sure if the capacitor is supposed to make up for the voltage drop, or if It's safe to add an additional capacitor to the system. I'm not sure how to test it to see if it's operating correctly though. So far, I added a noise filter to the lights & they seem to work alittle better, but not like they should. Has anyone else had this kind of problem adding accessory's to their Commando, or a similar problem with an Interpol?

Click to expand...

Disconnect the wires from the charging system, either at the rectifier or at the alternator and run the engine, if you get the same pulses, then the ignition may be the source of the problem, if not, then check the whole charging system.

Jean

The alternator that was in it was a stock commando unit, all of the schematics that I have gone over have shown a standard charging system on the 1970. The early Interpols had 2 small battery's, the larger single sealed battery that I have now has an equal about of amps & capacity. I already unplugged the alternator, diode & rectifier and it didn't make a difference. I'm not sure if the Lucas 2MC capacitor is good because i don't know how to test it, or if it will even make a difference with interference in the electrical system as some capacitors will. I know that if I disconnect the battery, I can sound the horn for a split second so it's holding some power. The old point system that was on the bike is beyond repair because it sat on a shelf in the weather for 35 years. The plugs are champion N7YC, the wires are NOS Norton. I'm pretty sure that it's the ignition, I'm not sure if a capacitor or noise filter added to that circuit will help, or if there's something actually wrong with the module. The module I'm using now is an original mk3, the one I had originally (about a month ago) was a new boyer ignition which fried within a minute because of a bad coil.

Are you sure the coils you now have are meant to be used with this electronic ignition? Many electronic ignitions will "fry" if the coils have too low primary resistance. Also, make sure you follow the manufacturer's instructions for the wiring of the ignition module; the coils must be in series if you are using two of them, the total resistance should be about 5 ohms (check the data sheet), you must remove the old capacitors and the ballast if there was one.

The old blue can can be removed, I never put one when I wire up bikes. Is the battery OK? The battery itself will smooth out any impulses coming from the alternator / rectifier, the blue can is there in case the battery dies. I did run my 69S without a battery, but then the blue can was new, I could kick that sucker so fast it just HAD to start or else, at my age and with an electronic ignition no way I could do that anymore.

Did you re-do the wiring? You could have a bad connection in there somewhere (new or old). Do you have a ground going from the frame to the engine?

Jean

Are the lights and siren relayed from a good seperate power supply :?:

Yes, the coils are 6v & will work with the ignition. I double checked their resistance before I plugged them into the ignition. All factory ignition wiring has been bypassed & I wired it up according to the boyer diagram. The harness is new, it is a 68-70 which is what the bike had originally (it has an ammeter & no blinkers). It didn't have the integrated interpol circuit like the 71 & up harness has (i can't get one that does), so I wired up the circuits myself using the proper colored wires & specs. Everything is wired according to the diagram, the horn controller is wired to the ignition switch & the blue light flasher is connected to the 2mc capacitor.

Problem fixed! I installed a new "blue can" capacitor & everything smoothed out. I guess it does make a difference with alot of accessory's. I could eliminate it if I converted to a 3 phase alternator like the later model interpol has, it's much smoother & charges way better at idle (almost the same amperage output at idle as at high RPM) but it wouldn't be original.
I will post pictures of the bike once I get another photobucket account up.