Tuning problems: ignition or carb, Norton commando 750 1970

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I have just got'n my bike back after a restoration job on the engine. :lol:
I have fitted an electrical ignition and an mikuni carb.
But I can't get it to run properly :cry:
That is, it is okay as long as I ride the bike, but when I stop and the engine is just ideling it just "die's" (as if It is out of gas)
The last time I tried the bike, it got progresevlie worse the warmer it got.
Any idee's ?
Regards Norway, Oslo
Will it idle with the enrichener on when the motor is hot? If it will, you might have a slight leak in the intake manifold.

Try starting the bike and speeding the idle up enough to let it run, then spray some WD-40 or similar product around the intake manifold gaskets--keep it away from the air cleaner. If there's a leak, the vacuum will suck the stuff into the motor and the engine will slow down or stop.
Hi Vidar,

I had nearly the same problem a few weeks ago. The engine was dying on
every red traffic light - BUT only when the engine was warm / hot. No problems after starting the cold engine. After 2 days of going crazy we've
found the fault. It was a Ignition Coil (6V) who changed the resistance and had some dropouts when it comes hot ...

Greetings from Germany

Re: Tuning problems: ignition or carb, Norton commando 750 1

[Thanks Guys, I'm not online all the time, but I'll try out your suggestions.
Oliver: How can I measure the resistance on the Coil's.
Measuring Coil Resistance

You can measure the resistance of the coils with any standard ohm meter or electronic multi-meter that measures ohms. You can get these from any electronics store or radio shack for $20 - $45 for a good one.

Measure the resistance (ohms) by putting one lead from the meter onto the positive connector of a coil, and put the other lead onto the negative connector.

For 6v coils you should get about 1.6 - 1.9 ohms resistance.

Measure both coils to ensure that they are both within range.

If you have 0 ohms or much less than 1.6, then the coil is damaged and must be replaced. You can blow out an electronic ignition if one 6v coil fails. Note that total resistance is the sum of the resistance of each coil.

Electronic ignitions like a Boyer-Brandsen require between 2-4 ohms total resistance.

Do *NOT* run 12v coils with such an electronic ignition. 12v coils have resistance of about 3.7 ohms each, and provide combined resistance of about 7.4 or more ohms. Electronic ignitions cannot drive them properly, and you get weak spark and crappy performance throughout the rev range.

'70 stalling

Another possibility is that the charging output/battery are low. This problem is worse when hot because the all the wiring is more resistive and sensitive when hot. I'd try running the bike with the lights off, which will provide more electric power to the ignition. My bike stalls at red lights when the headlight is on, and I know my battery is crap. It comes to a stable idle when I keep the headlight off. I don't remember how to check the output of the charging system, or what value to expect-- any one got this info handy??
Thanks for the replies on my problem

I have now bought new coils, plug wire and plugs. I have also gone over the carb. The result is that the bike is runing like a dream, that is, when it is hot. It is like hell :twisted: to get it to start.
Any idees !

Hi I tested my 6volt coils and found each had 2.4ohms is this good or bad

Have you checked your battery :?: if you come to a stop and the engine dies it could well be a low battery.

mike mcmanus said:

Have you checked your battery :?: if you come to a stop and the engine dies it could well be a low battery.

As this topic was originally started well over three years ago I would hope Vidar has been able to sort out this problem by now?
Vidar, you mentioned that you have fitted a Mikuni. I'm assuming this was just done along with the engine rebuild. If so you will probably have to play with the jetting to get the carb right. You may have too small of a pilot jet. This makes the bike hard to start and messes up the idle.

I was tuning the Mikunis on my Rapide awhile back. For experiment, I changed the pilot jets to the next size down and the bike went from a one kick starter that idled perfectly to a ten kick starter that would not idle.
Incidentally, I also changed the main jets to 210s from 185s and got the bike to go like it should on full throttle, before it would bog on full throttle acceleration in top gear and required the throttle to be opened up slowly. No it just goes, but the old pilot jets had to go back in to get the idle and starting back.

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