February 2025
This Norton which was resurrected in February 2023 has begun to exhibit some tuning problems that seem to point to its Mikuni carb being the main suspect. It had performed pretty well at the time after a good clean up, but is now running pretty rich in the mid-range, while being happy at both ends of the range.
My first suspicion was the choke arrangement, as like the Mk2 Amal they have a seal on the bottom of the piston which performs the choke function, and if it doesn't seal properly when "off" it can add enough excess fuel to the mix that it will cause such symptoms.
Fortunately, it comes out easily.
The seal does look a bit suss, and has eroded away on the top edge in the pic.
However, the piston also has signs of some interaction with the bore of the body, so I cleaned that up and made sure that it operated smoothly before reinstalling it.
We shall now test it for a bit while I see if I can find a new rubber.
Sure enough the bike continues to run a bit erratic at lower speeds and throttle settings, and the plugs still look kinda sooty, so we need a new seal for the choke at least.
As luck would have it, I managed to find the whole plunger in Australia courtesy of a crowd who sell Mikuni kits for various bikes, and besides being affordable the freight was very reasonable which was a surprise.
It arrived in only 1 week and fitted perfectly, so I am hoping the results will be self-evident.
Amend that - yes, it is running much better and pretty much sorted, although I figure we shall need to at least make a mixture adjustment, as there was too much fuel in the circuit last time.
The Norton has settled to its former well behaved self which is gratifying. It is commendable and rather rare that this repair was made easier by the fact that the Mikuni has the "enrichment" bits in a place where they can be removed with ease, so the major part of the job was to identify the culprit.
It visited yesterday and while it was warm I was able to improve the idle and mixture settings, so hopefully it will be more fun less annoyance from here on.
Restored to its former glory - albeit with hardly any signs of improvement externally.
It makes a welcome change to have had such a concise fault to deal with and an obviously successful conclusion without any "can you fix this bit too.." items
We are complete.