Running a little lean...

Tick-over mixture is altered by the tick-over mixture screw. Initial throttle response is altered by the slide cutaway. Mid range performance-acceleration is the needle, while top end is the main jet. This is on Amal carbs of course.

.. Alan was saying that it depends where in the rev range it's lean. You need to follow the full carb tuning procedure although personally I fit a main jet at the upper limit to start with  and make sure I get all the rest right before touching it as I'm very rarely on wide open throttle.

Amal do a good little guide to tuning their carbs, heres a link to it on the official Amal website.

amalcarb.co.uk/downloadfiles/amal/amal_tuning_guide.pdf

Its also possible than nothing needs changing  ,but that the small passages  or jets are restricted by crusty deposits and the fuel flow is not what it should be.  Its also true that the engine was not set up to run on what passes for fuel these days ,so settings may need altering from standard.

Mike you look positively 'noble' in your latter years, I remember the young  'erbert in tatty white leathers! And as for posing in front of a Cathedral! What ever next?

Thanks for the compliment Al! Sadly the beard has succumbed to the virus as I shaved it off when the mask stuff came about.Yes well the years have indeed gone by since I first met you living in that bedsit in Hanwell! Still have those leathers and my fringed Lightning jacket, but the beer belly means they are ornamental rather than functional:(

Not just "a" Cathedral, its the one in Cologne, bit of a sentimental story but NOC not the place to tell it.

Hope you are keeping well in deepest darkest Norfolk.

Agree with Robert, modern fuel burns a lot cleaner these days and it is very hard to tell if you are running lean just by looking at the plugs. Set the bike up so it is running nice and then get it on the dyno, with sensors placed in the exhaust they will tell you exactly what is happening throughout the rev range and how quick the carb fuelling response is when changing gear. This will then give you a base line to work from. A dyno run is not that expensive, and it really gives a lot of information on your set up so you can make adjustments from a known position. 

I agree, a rolling dyno is £100 well spent! But I’d also add that if the bike is running on its original carb(s) then it would probably appreciate a new one, trying to tune a worn out carb is never easy! I’ve bought new carbs for all my bikes Jap and British and have never regretted spending the money. 

With regard to whether it’s running lean, yes raising the needle (lowering the clip) will make it richer.  I’d suggest that most of our riding is on the needle/slide unless you are on the track, and as a general rule if you close the choke a little and it goes better you know it’s lean.

Of course the other rule is that 80% of carburation problems are electrical! So it would be sensible to check your timing at the same time.

dan

I always remember being told years ago that on the Concentric carburettor that "wear is rapid". The pilot circuit is drilled directly into the casting and is extremely small. It wouldn't do any harm to have the carb body ultrasonically cleaned and see if an improvement has been made. But as others have already pointed out "where in the rev range is it lean"? The link provided by Michael will explain matters more clearly. How old the carb is and could it even be drawing air through the cylinder head/carb gasket. If the carb is showing signs of wear a replacement could be a consideration. Amal now do a 'premier' carb with a removable pilot jet like the monobloc, that's what I'd fit on my machine if I were replacing the carburettor. Assuming the needle has 3 notches on your carburettor in my experience the middle setting  usually gives the best running.

Graham

Wear is rapid, I doubt that. I suspect that they were way out of tolerance when made and doomed from the start. When compared to the modern replacements this seems to prove the old concentrics made in the 70's were no good when new.

Was rapid on the original concentrics, especially for those liking to use a bit of throttle. At least mine did.

Anal, in their infinite wisdom chose to run similar materials for both slide and body, and as a result they were not good at resisting wear. The modern units, with the benefit of hindsight have changed this such that the slides are available either anodised or made of different materials that now are better matches and as such don't wear so badly. 

Also, most of todays users don't ride so hard or do so many miles as they did 'back in the day', so the wear is spread out over a longer period of time so is less obvious.

... petrol good enough for my ES2!

https://www.youtube.com/watch?v=y6IMRXgZE8k

The concentric actually works quite well if it is set up correctly. George F stated his carbs wore quickly, you only have to look down the bore of a Concentric that hasn't really done that many miles and scoring is often present. They were manufactured from Mazak {a zinc based alloy} these carbs are not tough-don't drop one on the floor like I did.Thinking further about the problem I think it is unlikely to be a main jet issue so work backwards from there. Does it spit back when rolling the revs off if so unlikely fo be a pilot jet problem? Perhaps consider needle jet and throttle cutaway next and work backwards to the pilot jet. If the engine response is not crisp when the throttle is lifted from tick over to around 2300 rpm it could be the pilot circuit/jet. One last thing and then I will shut up, is the fuel getting to the Carburettor could the petrol tap strainers be full of S*1T

I had a few problems with the premier carbs on an 850 check you have the right size jets first.  Mine were fitted with 17 (2 rings) the bike started and ran but weak, when I checked and asked the questions I was told that it runs better on a 19 jet (3 rings) and it did, I also raised the needle to the middle notch which improved things even more.

I had carb float issues starving and flooding one carb but that's another story.

Good luck.

Amal themselves now recommend changing to size 19 slow running Pilot Jet instead of the 17 originally fitted on their Premier carb for the 850.

All Amal Norton 850 carb packs are now supplied with the size 19 jet. The improvement of using this jet is not the starting or tickover as the size 17 jet did that. Most notice that the handover from pilot circuit as the slide raises is far better and more responsive with the size 19 jets. On the dyno the fuel response when changing gear is very good, showing very little lag to demand as the throttle closes and then opens.

The final setting up of the carbs must be done with the engine at working temperature, not partially warm.  

The 17 jet has a hole of 17 thou, exactly the same as the old 25 pilot bush. The rerouting of the pilot gallery for the Premier has had an effect on how well the fuel flows hence the change to a 19 jet ie the old 30 pilot bush. I have confirmed this by upgrading an old 932 body to take the premier pilot jet and finding the same issue. Once you go to a 19 pilot jet then the drill size to clean it out goes to 18 thou from the 16 thou used on the 25 pilot bush and the 17 premier pilot jet. The 1 thou less ensures you do not oversize the jet by cleaning it, the crud breaks up so the jet is left clear even with the 1 thou smaller drill.