Britbike forum Forums British Motorcycle specific Brands Norton bulletin board Proper Carbs for 1975 Commando Mk III

I finally got around to checking out my Norton and the exhaust smoking from a recent ride with a friend. When following behind me he said I had a lot of smoke out both pipes when I accelerated. There is no smoke when idling or normal riding, just when accelerating hard. I just pulled the plugs, which were brand new NGK BPR7ES plugs. The plugs I removed when I purchased the bike fairly recently looked good, but were not the correct heat range, so I replaced them. I actually replaced one of the NGK plugs twice because one new plug failed after a few miles. Although I hadn't ridden the bike or started it since the ride a few weeks ago, the right-side plug was deep black with carbon soot and wet with fuel. The left-hand plug was also deep black with carbon soot, but dry. It is obvious that both carbs are running way too rich. I'm wondering if that was causing the smoke my friend observed when riding behind me. I have never worked on those particular carbs, so whoever worked on them last obviously has them set too rich. I was planning to replace the jets and needles and set them to the #1 position as from the factory and see what happens.

However, I also found something else out. The right-side carb is a 932/33. The AMAL book calls for 932/43. The really strange thing is that the left-side carb has no model markings at all. Even the little raised area where the model number is normally stamped is missing. The carb does have the proper AMAL logo on it, but no model, so who knows? There is no information on the other side of the carb as well. I've never seen that before. Did AMAL make carbs with no identifying information stamped on them? Now, I'm thinking that I should replace the carbs with new AMAL 932/43 carbs so that I know what I have, but that is expensive. Probably worth it since the bike is in excellent unrestored condition and runs well except for the smoking.

The AMAL list is wrong. The Mk3 carbs should be 932/33 (R/H) and 932/34* (L/H)
*(932/24 in the Mk3 parts book which is obviously a misprint)



The raised pad and stamp would normally be on the right-hand side of the carb whether it is a right or left-hand carb (because both L/H and R/H bodies are produced from the same castings).



The correct carb kit for the Mk3 is AMAL PACK 113 (Premier) or ACK 113 (Standard Mk1), not P/ACK 114.
https://andover-norton.co.uk/en/sho...emier-32mm-850-mk2a-mk3-220-jet-pack-113

If your Mk3 has peashooters then the main jets will probably need changing (to 260s) regardless (113 comes with 220s and 114 with 200s).

The numbers on the outside of Amals are only meaningful if previous owners haven't messed with them
- near EVERYTHING is replaceable.
It does give the basic carb size though = 32mm.
Dunno about the un-numbered one.
If it has all the correct parts, it could be identical to a 'proper' one.
Is it a 32mm one ?

What colour is the smoke though ?
Well worn Amals can produce black smoke, if they are so worn that fuel is just hosing through the engine.
The clue to this can be that it won't need choke for a cold start.
Fitting new jets and needles and maybe slides also can considerably reduce fuel use
If the bodies aren't so worn that the slides are a hopeless fit.....
(a problem that can plague zinc bodies with zinc slides)

Smoke can also be oil use - worn rings, or well worn pistons/bores as well.
Sooty plugs can ditto be oil burning symptoms.
Blue smoke though.
I don't quite recall the history here, you rebuilt the engine ?

I realize that L and R side Amals are cast the same, but the left-side AMAL doesn't have a model stamped on either side. Who knows what previous owners / mechanics have done with the internals of the carbs, which is why I'm considering new Amals. I will at a minimum completely rebuild both carbs. My Mk III never needs any choke to start, even in cold weather and as Rohan stated, this in itself would indicate a super-rich mixture. I have never rebuilt the engine, but the engine seems very strong with virtually equal high compression readings for both cylinders. It also uses very little oil. The little oil it uses ends up on the ground when the bike is parked. At least I have another project for the upcoming winter. I don't mind rebuilding the Amals as they are very easy to work on. I don't like pulling the tank though. I ride bikes throughout the winter as long as there is no ice on the roads, but I think the Mk III is down for at least a month or so and maybe longer as I have some upcoming surgery.

Not necessarily as the ticklers alone should provide a rich enough mixture for starting and owners often remove the chokes.

I found my first 850, with just fitted brand new Amals, needed a lot of tickling and throttle juggling
if cold started on a frosty morning without any choke.

Before new carbs, it just needed a short tickle, no choke, no problemo.
Rich as eck, I'd say ....
Exact same setup on old and new carbs.

Smoking under acceleration is usually rings.
Or piston/bore wear if big miles.

A rich mixture alone would not cause smoking under acceleration, but a rich mixture from a needle set too high would wash oil off the cylinder bores making them dry and rapidly wear out the rings giving blue smoke under acceleration. Rule of thumb for a late Commando with oil filter is 2 sets of rings per bore life, so likely you just need new rings a hone and a full carb tune. On a touring bike at 1/4 to 3/4 throttle for most of the time the needle and needle jet will be the most used part of the carb tune.

After the fantastic way my 1969 BSA 441 Victor Special starts and runs after I installed a new AMAL Premier aluminum carb, I decided to go ahead and purchase new Premier carbs from Andover Norton. I hated to spend the money, but the Mk III is such a nice bike, I decided to go for it. I'll post up the results when I get them installed.

Good idea!!!