I have a Norton model 7 . 1954. The stamping on the carb. flange is 376/42. I measured the bore at 1 & 1/8". Every article I read shows the 376 carbs. only go to 1 & 1/16". I do know how to read a caliper, it is 1& 1/8". What gives ? I would like to re-build this or replace it because it floods and is now very difficult to start !
Your bike (and possibly carb too) are 67 years old. Unless you have known the bike from new, who is to say that a previous owner hasn't bored out the carb in order to try and get a few more mph? Whatever that shouldn't be the source of the flooding unless there is a breakthrough between carb bore and float chamber.
As to the flooding, then the float, float needle and its seat are all suspect here. Has it just started happening?
My first choice is the float needle. Treat yourself to a new viton tipped one. Nylon doesn't like ethanol. While you have the carb apart, if you shake the float, can you hear any petrol inside it? If so, then a new float is needed as well.
You could of course, depending upon how much you wish to spend, simply buy a brand new replacement that is made from far better materials than the original that will be set up with everything correctly set for your bike.
I bought the bike on E-bay from Belgium earlier this year. It started 2nd kick when I got it. It was advertised as 1st. or 2nd. kick to start ! Maybe it's the U.S. gasoline ? I would buy a new carb. I received a reply to this question from Surry Cycles. As I have read about the Amal 376 , the largest bore is 1 &1/16" . I do measure 1 & 1/8", no question, but I do not think it has been machined out. The intake 1 to 2 manifold is also 1 & 1/8" @ the carb. flange. They say it must be a 389 Amal ?
So : do I look for 376 or 389 repair/ replacement ? I will take the carb. apart this winter season and get to the bottom of things. Other issues are a leaking primary chain case and leaking fork seals ! Oh ! The joys of getting back in to British motorcycles ! I have owned many over the years.
Thanks again for your reply,
Its an important question you have here. For me, if the carb you have is in good condition as to slide wear etc and the bike was running well with it then I would fit a new float and needle valve and keep it. If however, the bike didn't run very well, then replacing the carb with the correct one might be the better option. Good news is that the bolt on bits for a monoblock are mostly the same for both models.
As a thought, the previous owner may have fitted a new float already. If it is one of the latest ones (marketed as 'Stayup' floats) then I experienced very rich running on my 650 with one fitted (I have a pair of 389's on it). The cure for me was an additional 2nd gasket between the float cover and the carb body. A quick, cheap and easy thing to try.
Where did they measure the size? At the head/manifold face, or, was it at the centre of the venturi?
E.g. a 1 1/16" carb is 1 1/16" at the flange, or is it that a 1 1/16" gauge will seal its bore at the main jet location.
Different manufacturers had different ways of describing this.
I'm sure I read Amal measure at the flange. The venturi is a little narrower to accelerate the air to help suck the fuel, but it's also not easy to measure. My carbs all measure the book figure at the flange.
Having got by with knackered amals for 55 years I bought a new monobloc for my 99SS.
What a difference it makes to starting snd tickover.