MkIII 850 commando uncommanded revving

If its still using points ignition then a stuck AAU will do same, to find an air leak spray WD40 progressively over the joints, when you get to the leak the engine revs will change.

Kevin Traill at Alverstoke Restorations in Gosport, Hampshire

His details are: KJTRAILL@qinetiq.com Telephone: 02392 580708 (after 16:30).

Details are old so may not work but his work is excellent.

Remove the slide from the carb. Take a CLOSE look at it. I bet there is a chunk missing from the bottom corner. Let me know how it goes . 

I used to rub the slides and body with some powdered lock graphite . Now I have an air filter I use the thinnest smear of moly grease. Any more will affect the allowable air leaking and richen things up.

Before you take the carb off, just to be sure its not an air leak at a joint do the spray test, include the carb float to carb joint and all the manifold joints. A worn slide alone would have to be really worn to get to 4000 rpm, as standard they have 4 thou clearance.

The couple of times that happened to me it was because the slide did not close.  4000 seems a lot for anything else. Is the cable sticking...maybe too sharp an angle as it comes out of the top of the carb? Test run without air filter so you can see it or touch it. Does it go down smoothly even if you touch its side lightly?

Try taking a look at the manifole joints, Start the engine then spray the manifold with WD40. If the revs change, then there is an air leak. Alternativelt, smother the manifold joints with vasaline and see if that has the same effect. 

I had a similar issue, which turned out to be down to perished O-rings on the throttle-stop screws. See the thread at https://www.nortonownersclub.org/node/12112.