I bought a 1975 MK III last fall, and it came with a Mikuni single carb. I prefer the original dual carburetors, so I bought a pair of Amal 932s, They are in very condition, and I have cleaned them and the jets thoroughly. The slides look good. I installed the carbs, and the bike runs well, but it stumbles off idle, and runs rich on top. I have the original mufflers, but am currently running the fake Dunstalls that were installed at the time of purchase, and a K&N air filter. Pulled the carburetors off and found it has 260 mains, way off according to the parts book and manual. When I look at the parts book on the pdf it has two columns. One is marked parts number, the next is "maker's or superseded parts #." Not sure what this means. Also, though I can find the jet size on the main, I don't know how to determine the sizes of the needle jets, idle jets and slide cutaways. I am not sure that the combination of aftermarket mufflers and K&N will percolate just right on stock jetting, pretty sure it won't. I would probably to have available piecess/parts to switch jetting when I switch mufflers. If anyone has any suggestions on jetting probabilities for the current muffler/air cleaner set-up, I would appreciate that info. Also, I am running the carbs without choke. The parts book shows a choke spray tube. Is this something I need?
MK III carburetor questions
- Thread starter ElTigre1
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Are the Amal 932 carbs you bought specifically for a Norton ?
This is related to that mention of a "choke spray tube".
That is the brass stub that you see sticking up into the airstream on the floor of the Amal inlet tract - that the needle drops down into.
For later Nortons, the Norton version is partly cut away - the famous "stepped spray tube".
The brass bits are numbered, if you examine them closely.
The length and number of notches on the needles say what they are (hopefully, there are counterfeits about).
The choke slides are the 2 little cable operated airflow blockers that descend down into the airstream,
operated by the choke control on the handlebars.
The engine will tend to stumble a little while cold without chokes, but once warmed up should be able to be tuned to give a smooth operation all the time.
Without those stepped spray tubes, Nortons had a problem with that off idle stumble and some combinations of mufflers and airboxes.
You could try lifting the needles up or down a notch and see if it makes any difference.
(After setting the idle speed with the engine WARM, and if that makes no difference).
Watch that dropping the needles down makes the overall mixture a tad weaker - this could cause engine overheating, which needs to be watched for.
hopethishelps.
This is related to that mention of a "choke spray tube".
That is the brass stub that you see sticking up into the airstream on the floor of the Amal inlet tract - that the needle drops down into.
For later Nortons, the Norton version is partly cut away - the famous "stepped spray tube".
The brass bits are numbered, if you examine them closely.
The length and number of notches on the needles say what they are (hopefully, there are counterfeits about).
The choke slides are the 2 little cable operated airflow blockers that descend down into the airstream,
operated by the choke control on the handlebars.
The engine will tend to stumble a little while cold without chokes, but once warmed up should be able to be tuned to give a smooth operation all the time.
Without those stepped spray tubes, Nortons had a problem with that off idle stumble and some combinations of mufflers and airboxes.
You could try lifting the needles up or down a notch and see if it makes any difference.
(After setting the idle speed with the engine WARM, and if that makes no difference).
Watch that dropping the needles down makes the overall mixture a tad weaker - this could cause engine overheating, which needs to be watched for.
hopethishelps.
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ElTigre1 said:
If a "Makers" part number, then it will be the original manufacturer's (in this particular case, Amal's) own component part number, for instance, the Norton part number for item 10 "Throttle needle" is "06-8102" and the maker's (Amal's) number for that part is "928-104".
"Superseded" means that either the part has changed from one used previously, or in certain cases a new 6-digit part number has replaced the old Norton part number although the part itself may not have changed.
ElTigre1 said:
Check if the carbs have a stepped or a flat-topped (image) spray tube?
http://amalcarb.co.uk/rebuilding-mark-1 ... arburetter
If stepped, (original 850 spec.) the matching needle should be the 4-ID ring 06-8102 (928/104) item.
If flat, then it needs to be the Amal 622/124 2-ID needle (see Amal info in the link).
L.A.B. said:
This is the most important thing regarding spray tubes... Matching the straight cut one with a 2 ring needle. Or a notched one with a four ring needle. DO NOT MIX. Slides are numbered on the underneath, look for a small simply stamped 3 or 3 1/2. (Note that the rings mentioned are relatively finely scribed above the three much heavier clip location rings)
By the way you in your first post you questioned needing to order spray tubes for your carbs. As per LAB's photo you will see you already have that part. I really doubt that an engine would run at all without one.
My MK3 is running fine with K&N filter, notched spraytubes, 4 ring needle in middle clip groove, 3 1/2 slide, and 240 main jets. Exhaust are open p's.
o0norton0o
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ElTigre1 said:
That sounds like 2 different issues. You have a poorly adjusted idle circuit mixture, and your main's are too big...
ElTigre1 said:
I have a K&N air filter and dunstalls decibel mufflers on my 750 and my jetting is stock. I've tried richer jetting but the stock jetting parts were better.
ElTigre1 said:
Start with the stock settings and the stock parts for your model, and go from there. I have a little mad scientist in me so I tried a little richer jetting with a little more advance, thinking that I would get a little more HP and a slightly cooler running bike. Instead I got a bike that wouldn't accellerate past 80mph. When I installed the stock main jets, I zoomed right past 80mph... which convinced me that the K&N air filter and dunstall decibels have no significant effect on the stock jetting (at least probably not without changing the cam also)
Thanks. all. My problem was that I couldn't determine which jets/slides I have, in order to compare them to original. I find that I do have stock jets, except for the mains. The carbs do have a choke spray tube, but I am not sure of the function and didn't know whether the choke slide was needed or desirable. Sounds like it is. Part of the problem for me and these carbs can't be solved with advice - my 75-year-old hands are not as strong or steady as they used to be. Putting that damned spring and closing the tops, and finding the allen heads that hold the manifolds and carburetors on is painful and clumsy. I am excited about the Norton, it is beautiful, but still needs to be fettled before hitting the road for anyplace other than the nearby. Haven't had a bike with Amals since my '73 Trident.
Thanks, again, I will report back, and will have many more questions. Now I just need younger hands, and maybe a memory.
Thanks, again, I will report back, and will have many more questions. Now I just need younger hands, and maybe a memory.
EITigre1
I think part of the confusion may be over the term "choke". In the context of the "choke spray tube", the book is referring to the location of the spray tube. It is located in the part of the carb that is "choked" down in size compared to the inlet bore and outlet bore...And "choked" down further still by the throttle slide. This creates the low pressure needed over the spray tube to draw fuel from the bowls. The spray tube's function is to get this fuel up in the air stream to spray rather than dribble from the bowls.
The "choke" when referring to the choke slide is referring to starting enrichment, as in "choking" a cold engine. That slide is not needed in normal running and many riders (myself included) have eliminated it finding that tickling alone is enough added richness to start a cold engine.
I think part of the confusion may be over the term "choke". In the context of the "choke spray tube", the book is referring to the location of the spray tube. It is located in the part of the carb that is "choked" down in size compared to the inlet bore and outlet bore...And "choked" down further still by the throttle slide. This creates the low pressure needed over the spray tube to draw fuel from the bowls. The spray tube's function is to get this fuel up in the air stream to spray rather than dribble from the bowls.
The "choke" when referring to the choke slide is referring to starting enrichment, as in "choking" a cold engine. That slide is not needed in normal running and many riders (myself included) have eliminated it finding that tickling alone is enough added richness to start a cold engine.
Florida, even the northern part, would be warm enough to eliminate those pesky choke slides for sure. The carb tops- to- body are much easier to assemble without them, not having to fight with an additional stiff spring. A cut down allen wrench is the ticket for those two inner manifold bolts, but you'll still only ever be able to get a flat at a time before coming up against one of the carbs, very fiddly and of course the tank has to come off beforehand.
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