November 2024
1974 Norton Commando 850
Black beauty has returned after a long leave of absence, which is a good sign, as it has been mostly running fine for the intervening five years.
A recent bout of bad behaviour had led the owner to initially suspect a carb tuning problem, but due to his observations of spark plug and piston crown appearance he was concerned that perhaps the head was in need of attention.
He turned up late this afternoon and we did an immediate compression test, which revealed two rather healthy cylinders with 150+ psi present and accounted for.
As I have an ancient colourtune spark plug we gave that a try too, and while it picked up a slight mixture discrepency on the right side, it refused to work properly on the left, giving indication that the left side spark was a bit low on energy. Next step would be to lift the tank and get involved with coils and wiring, so the bike stayed here and I gave him a lift home.
If it is true that life imitates art, I guess I prefer them undressed...
The coils and wiring got fully checked over and I could not fault them at all. The carbs felt a little reluctant to drop back to the idle stops cleanly so initially I altered the routing of the throttle cable and it seemed they were a little snappier to return.
I also read my writeup on these pages from 5 years ago to see what I had done then, and I was reminded that I had at that stage replaced the needles and had set them in the richest position as the bike had been exhibiting lean symptoms such as spitting back when blipping the throttle while changing down. As the colourtune had looked a bit rich now I lifted the slides out and dropped the needles to centre position. Other than that it was just the usual slide synch and pilot adjustment and it all went back together.
A mate dropped in on his Commando so I elected to fire this one up and ride home with him just to see how it felt now. It went very well but there was still a faint hesitation when changing down, with a bit of a sneeze if you didn't get it right.
I rode home again and proceeded to tune the carbs now it was a hot idle. It did not want to run on one cylinder so I worked with both, starting on the right which was the healthier idler and soon had that where it needed to be. I then started on the left but when I backed off the idle screw nothing happened. I screwed the idle back in and it was more than a turn before it began to raise the idle speed. The slide had been hanging on the cable rather than sitting on the idle stop. Suddenly we were cooking, and now it made more sense. I got both idles as near to level as I could then raised the cable abutment to give equal freeplay at rest.
I rang the owner and said it was ready to come home and that it was feeling a bit outnumbered at my place..
The ride to his place was brilliant and the bike felt happier in every respect, with all trace of the hiccups gone and a much more even exhaust note when accelerating from low speed. Very pleasing.
He is as happy with it as I am, and it is surprising how a small error can have such an annoying effect. I am suspicious that the idle screw may be in the habit of backing itself off, but now we shall keep an eye on it.
Lovely bike to have the experience of.