Twin carbs or 2 singles?

Previously Gordon Johnston wrote:

For a start, 389s are too big for a Norton 99. If you wish to stick with twin carbs, a pair of 376 carbs (but tricky to fit) or 626 concentrics would be more appropriate. May I suggest getting hold of a single carb manifold (which your bike would originally have had) and fitting a1 1/16" 376 monobloc. This will make it a nicer bike to set up and ride and reduce the risk of trying to get too much out of a 62 year old engine with predictable consequences.

Many thanks for your advice Gordon all feedback is gratefully accepted. I am in a total quandry about this machine, I previously owned it as a teenager in the early 60s and, as teenagers do, I modified it to cafe racer style. When I sold the bike to an unknown buyer in Liverpool it only had 1 carb and that is the one shown on the left 389/71. I know this for a fact since it has several recognisable features. When I bought it back, earlier this year, it has been modified to twin carbs as shown but I have no idea if it has ever been run with this setup. Since writing my original question I have come across more anomalies. There is no choke slide in the 389/71 carb and the cable duct has been crudely sealed off with mushroom head screw. The 002 carb is complete and obviously much newer than the other one. Can you make any sense out of any of this? Regards. Ron

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Previously David Cooper wrote:

Lots of owners remove the air slide for some reason and, when you do, the hole has to be blocked.

The second number (/71, or /002) denotes the setup (slide, jets etc).

http://amalcarb.co.uk/carburettor-for-a-norton-dominator.html

...says 389/71 was a setup for the Dommie.

So it looks like someone decided to add a second carb (/002) to make a pair with one they already acquired (/71), in which case in would almost certainly end up with a different number.

You obviously know your carburettors David. Looking deeper into the blanked off air slide I found that Norvil actually supply a blanking plug for that purpose but I am no nearer to knowing why it would be removed in the first place.

Perhaps you can help me with another puzzler: I bought a couple of throttle/choke cable adjusters for the carb tops, only to find that the top on my 389/71 carb has threaded holes to accept the adjusters but the top on the 002 has clearance holes making the adjusters impossible to fit. I checked the exploded view of the carbs and it shows a variant of the top with inserts to fit in the clearance holes. Any thoughts on that one?

Many thanks Ron

Previously howard_robinson wrote:

Previously ron_tomkins wrote:

Previously David Cooper wrote:

Lots of owners remove the air slide for some reason and, when you do, the hole has to be blocked.

The second number (/71, or /002) denotes the setup (slide, jets etc).

http://amalcarb.co.uk/carburettor-for-a-norton-dominator.html

...says 389/71 was a setup for the Dommie.

So it looks like someone decided to add a second carb (/002) to make a pair with one they already acquired (/71), in which case in would almost certainly end up with a different number.

You obviously know your carburettors David. Looking deeper into the blanked off air slide I found that Norvil actually supply a blanking plug for that purpose but I am no nearer to knowing why it would be removed in the first place.

Perhaps you can help me with another puzzler: I bought a couple of throttle/choke cable adjusters for the carb tops, only to find that the top on my 389/71 carb has threaded holes to accept the adjusters but the top on the 002 has clearance holes making the adjusters impossible to fit. I checked the exploded view of the carbs and it shows a variant of the top with inserts to fit in the clearance holes. Any thoughts on that one?

Many thanks Ron

Many bikes such as Triumphs had an adjuster in the cable run between the top of the carb and the twist grip so they did not need (or fit) an adjuster in the carb top. Instead a ferrule on the cable went straight into the mixing chamber top. Other bikes, like Norton, did not have the adjuster in the cable run because they preferred the adjuster on the carb (or carbs). Hence the thread for the adjusters (part number 4/035). So you have a Norton type of 1 1/8 inch - probably from a 650 Standard (71) and one I cannot define. It does get a little more complicated because the same logic applies the choke cable adjusters and therefore mixing chamber tops. So some tops have two fine threads for adjusters, some have one threaded and one not and some have neither threaded. The mixing chamber top you'll need is part number 389/064. Obviously to use them as a pair they need to have the same bore size and jets, needles and slides. They would be too big really for a standard tune 99. Many 99SS heads had bigger valves and were gas flowed and polished by the cafe racer fraternity.... to match the Triumph Tiger 110's and 650 Matchless's of the period. Good luck, Howard

Incredible the amount of detail that some of you guys have. Much apprecitated Howard. Thankyou.

Previously robert_tuck wrote:

I do agree that the carbs are on the large side and would only be usefull if the motor has been tuned and capable of high revs. It is possible that it has been ported and flowed and the carbs match the tracts. The motor could run reasonably well although probably missing some of the low down pull the 99 is normally good at. Thats not to say that a high revving 99 would be prudent (although it could have a one piece crank lightweight pistons and Carrilo rods!). We just don't know enough to pass judgement. I have a 250 single with a 29mm TT type carb an awfull port shape and really poor combustion chamber which I ride on the roads and events ,its fast and furious fun but not very practical. I have other bikes for practical. What do you want to use it for?.

Thanks for your input Robert. I know that the head has been played with by a previous owner because the inlet ports have been polished to match the custom made manifold. I haven't explored the the crankcase yet, but I strongly doubt that it will contain the goodies that you have referenced. As regards what I am going to do with it: as a senior member on the wrong side of 75, I will not be taking part in any track days, I just want to get her up and running and presentable. The nearest that she will come to a good thrashing is probably a ring ride at one of the local shows. Thanks again. Ron.

Previously anna jeannette Dixon wrote:

hello if you want to stay with twin carburettors then you must have them in first classic conditionas this makes tuning easy and they say in tune better so a pair of new mono-blocs 376/256+376/257 as was fitted to the 650 Manxman to tune twin carbs first remove right-hand spark plug, and start the engine on one cylinder and get this cylinder to a tick over point then do the same with the other right-hand side when you have achievedthis , then start the engine on both cylinders it will run fast and now get the engine to a tick over point, around 700 to 800 rpm and there you have tuned your carbs, or get a single carb manifold and run on one carburettor witch in our day make more sense, yours anna J

Many thanks for your advice Anna. Your tips on tuning the carbs will be invaluable as I have never dealt with multiple carbs before. Could be fun. Thanks again Ron.