Tuesday, May 19, 2020

Under-sparked Dominator

Another instalment of lockdown mechanics... This time the Norton Dominator has been receiving some loving. Just for a little background, up until about three years ago I was using the Dominator very regularly and regularly taking it on longer journeys and ocassionally abroad. The bike got laid up for a little while whilst I concentrated on my Velocette - the thing about the Dominator is that it is just so damned good and easy too that it means you tend to neglect everything else. The Velo scratch itched I put the Dommie back on the road last summer, but since then it has been a progressively more and more recalcitrant starter. A real shame as it is normally a first kick bike.

Over the last few months I had done various quick and easy tune up jobs to try and get to the bottom of the issue. Each time I did something like get the tappets spot on, clean the points or make sure the fuel was fresh there was a noticable improvement but it didn't stop the slide and the bike progressively became worse and worse. To be honest I was hoping that it would eventually conk out altogther and give me a good old school hard fault.

With the extra workshop time that lockdown granted me it seemed like a good opportunity to sort out the Dommie's starting fussiness once and for all.

Going back to basics this problem had to be ignition or fuel related. The bike would sometimes start second or third kick, sometimes not at all. If it started easily you could run it for a minute and then stop the motor and then it wouldn't go at all.

I knew some of the wiring in the ignition circuit was a bit tatty but rather than just get stuck in to that this was a job that needed a methodical approach. First up change the plugs. No improvement. Clean the carb and thoroughly check it through. - the fault did give some symptoms of fuel starvation and after kicking for a while and taking the plugs out they did look to be quite dry.

After a while the carb was as clean as a whistle but the problem was the same. The finger of suspiscion was beginning to point clearly at ignition.

Next step was to clear up that tatty wiring and start testing electrical components. I double checked and made sure that the points were clean and correctly gapped. The ignition circuit was re-wired with new cables and crimps and whilst at it I put in new HT lead.

Lucas distributor with fresh wiring.

From my experience HT leads do ocassionally fail and when they do it can be very hard to diagnose. You are as well off to replace it altogether when working on the ignition as to try and fault find it. I used braided cable, it looks a bit different and not everyone will like it but whilst not factory spec it is period correct and not offensive to my eyes. So, the moment of truth. Will it work? The tank goes back on and the bike starts first kick. I stop the engine. It starts second kick. I stop it again and then it doesn't start at all. What a tease. At that point I packed it in for the evening and went back in to the house to drown my sorrows.

Snap connector fitted on the fuel line.

For the next step I decided I was getting way too fed up taking the tank off and disconnecting the petrol pipe. The slimline Featherbed Norton is a very well thought out and designed by riders and mechanics sort of bike - the seat comes off in seconds with just one dzuz fastener and for the tank you just take off the bracing bar, undo an elastic band and then slide it off. However the petrol tap does have a habit of catching on the frame, opening itself up and pissing out petrol all over the floor (I don't think my bike has the original tap). So to make life easier I fitted a quick release coupling in the fuel line. These are great things - just press a button and they break apart closing off the supply as they part. The only thing you have got to watch out for with these couplings is to make sure that they are fitted in a straight length of pipe - from experience they are only available in plastic and do not take kindly to side force.

Fresh braided HT lead.

All that was really left was the capacitor and the coil. I disconneted and checked the coil - a regular 12v bike coil should have resistance of around 3.0 to 3.5ohm on the primary resistance (between the +ve and -ve poles). Also worth testing is that resistance to ground from any of the poles is infinity and that the secondary resistance (from either pole to the ht lead socket) is suitably large (typically 5,000 to 20,000ohm). The primary resistance I measured was 5.5ohm. Fairly high, but seeing as I did not have a coil on hand but I did have a capacitor I had a go at changing that to see what it would do anyway. The answer to that was a big fat nothing.

So, on to the coil. Seeing as my local bike shop was closed I had to go online. It arrived in four days and I put it on the bike. Just for a reminder here the coil should be wired so that the terminal wired to the points is the same as the earth on the bike - ie negative earth and negative terminal on the coil to the points. Tank back on, fuel pipe clipped back together and lo and behold it started first kick. I stopped the motor and then kicked again. The bike burst in to life. One more time to be sure, yes sorted. The next day I needed some eggs and veg so on went the panniers and the Norton became my shopping bike as we went to the local farm shop. All good.

Now we are at the end I'll confess that the coil was the major suspect from the start after previous tune up attempts failed.  Funky, intermittent starting and running have all the hallmarks of a coil issue. I still worked through making sure everything else was in good shape all the same. Throughout the bike looked like it was sparking ok when you took the plugs out to check. A bit weak perhaps but not too bad at all. The spark obviously diminished or died when there was a load applied. When the coil was changed the intensity of the spark increased greatly. At first I didn't really want to believe it was the coil as the one fitted really hadn't lasted to long at all (ok maybe 10 years, but not a huge mileage)..

Now all that is left is to get out and enjoy a Dommie in fine fettle.

And the works in all their glory.


  1. the story of out life? one day or another sometings fails...

    1. Hah ha yes. We all age and fall apart but at least our old bikes we can fix!

  2. Fit a magneto, no more trouble.

    1. I've had plenty of magneto woes through the years too!

  3. i always suspect the coil because it is an easy replacement