MKIIA 850 carb needle: bean can vs peashooter

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L.A.B. said:
There wasn't a '74 version manual, so your manual does not actually cover the '74 models, also the 'A' models were initially made for the European market as the US market got the Mk2 although some Mk2A models were sold in the US in late '74 so the Mk3 definitely wasn't the first model to have the black caps and plastic airbox even in the US, Edit: '316170' supposedly being the first US Mk2A.
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Which makes the history of my 1974 US sourced Mk2A (311032) with a build plate dated December 1973 either an oddity or imported from the UK at a later date.
Perhaps I need to get a history check if it is available.
 
Time Warp said:
L.A.B. said:
There wasn't a '74 version manual, so your manual does not actually cover the '74 models, also the 'A' models were initially made for the European market as the US market got the Mk2 although some Mk2A models were sold in the US in late '74 so the Mk3 definitely wasn't the first model to have the black caps and plastic airbox even in the US, Edit: '316170' supposedly being the first US Mk2A.
Click to expand...

Which makes the history of my 1974 US sourced Mk2A (311032) with a build plate dated December 1973 either an oddity or imported from the UK at a later date.
Perhaps I need to get a history check if it is available.
Click to expand...

Well, you could try the VMCC or UK NOC.
[Broken Link Removed]

AN doesn't have records for that period.
https://andover-norton.co.uk/en/factory-records/
 
Makes me wonder, no idea what was originally on the bike, stamped Feb 74.
Came with datebox rear plate, no other airbox parts, connected headers but no
silencers. Tossed by the previous owner.
So I guess it was a Mk2.
 
Check the battery tray, MK2 battery long side front to back on the bike, MK2A battery long side across the bike.
 
A rather more interesting update. I fitted the 4-ring needles today, and there's definitely more go! Not a huge amount, but enough for me to feel it was worth it. I've now checked everything in the carbs, and, apart from the new 4-ring needles, they both have stepped spray tubes, .106 needle jets and new 260 main jets. Great. I haven't put in new .106 needle jets because it didn't occur to me to order them in time for the most recent delivery. I may add them to a future order if I remain power-curious. But the bike now pulls nicely at 80mph, which is quite fast enough for me (besides being illegal on public roads, ahem) given the standard riding position and standard European handlebars. At some point I might risk trying for the ton, see what happens.

One other thing I did today, which might've muddied the waters a bit, is to check the fuel levels using a spigoted plug in the bottom of the float bowl and a length of clear hose (https://andover-norton.co.uk/en/shop-details/20569). I'd checked both float heights recently while cleaning the carbs after the 12-year lay-up, and they were spot on – but the clear hose told a completely different story! Fuel levels in both carbs were way too low, by about 4mm/0.157"! So I raised them. Whatever difference the 4-ring needles made, I guess raising the fuel levels by about that much ought to have made a difference too. One curious thing, the fuel level in the right carb is now at about the minimum of what I understand is the acceptable range – i.e. 0.24" below the top of the float bowl – but the float valve seat is as far down as it'll go. Maybe I need stay-up floats, if I get those I wonder if I'll have to set the fuel levels again from scratch!

Thanks for all the thoughts and support people, as far as my bike's performance goes I'm a happy camper right now! :D
 
NortonMKIIA850 said:
I'd checked both float heights recently while cleaning the carbs after the 12-year lay-up, and they were spot on – but the clear hose told a completely different story! Fuel levels in both carbs were way too low, by about 4mm/0.157"!

One curious thing, the fuel level in the right carb is now at about the minimum of what I understand is the acceptable range – i.e. 0.24" below the top of the float bowl – but the float valve seat is as far down as it'll go.
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It's not that curious if the float needles the old Viton-tipped brass type.

http://amalcarb.co.uk/optimising-mark-1 ... uel-levels
Needle Valve

If your float chamber is fitted with a brass needle valve, you may find the valve sealing under its own weight, before the float has risen far enough to press it shut. Symptoms of this problem can be that the carburetter takes a long time to tickle, hesitates on pickup and does not idle reliably. A Viton tipped aluminium needle valve is now available that overcomes this problem. It is now fitted as standard equipment to all new Mark 1 Concentric carburetters.
Click to expand...


NortonMKIIA850 said:
Maybe I need stay-up floats, if I get those I wonder if I'll have to set the fuel levels again from scratch!
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The StayUp floats aren't likely to make any appreciable difference to the fuel levels but the alloy float needles probably will, and they are an improvement in my opinion.
 
Right, of course! Thanks L.A.B., I had become aware of the aluminium float needles at some point, and then forgotten them – I have the brass ones, so an order to Amal is due! I'll get new .106 needle jets while I'm at it. Only, unless I'm being dense, which is perfectly possible, I don't see the aluminium items for sale separately at Amal's online shop, only as part of a stay-up float kit – am I missing something? Maybe I ought to take the plunge with the stay-up floats anyway. Thanks again for all the help!
 
When I am tuning a bike, I always set two things first - float levels and ignition timing, then jet to them as fixed settings. As I have said previously - with petrol, half a thou of an inch wear in the needle jet is the difference between flying and sluggish. If the ignition timing changes, the symptoms can be the same as a change to the jetting.
 
D'oh! Cue Cheese Shop Sketch quote – they're terrifically popular! Thanks again. But I see that the stay-up float kit that includes them is in stock ...! Maybe I'll get those then. But argh, the .106 needle jets are out of stock too! 'Tis the season, I suppose, they'll probably be overflowing in 6 months' time! :roll: I see that AN have the needle jets, but apparently they don't list the aluminium float needle. Around and around I go! The Post Office alone is making a small fortune out of me this summer. :(
 
If you are using petrol, it probably would not be smart to make your own needle jets. But I wonder what the quality control is like with the stock items. Raising and lowering the needles provides a way of adjusting for differences in needle jet internal diameter, but what if you have two needle jets of slightly different ID and you are raising and lowering two needles together ?
 
acotrel said:
If you are using petrol, it probably would not be smart to make your own needle jets.
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Thanks acotrel, but that's waaay beyond my skill set and equipment! About the order of doing things – jetting, timing etc. – I'm still at the stage of getting my ducks in a row for a starting point. But, as I hope you've read, there are already improvements! Cheers.
 
L.A.B. said:
Surrey Cycles?
https://surreycycles.com/carburettors/m ... oncentric/
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D'you know, I've been familiar with that name for years, but never tried them, no idea why ... I'll have a look at their website. But I've already placed orders at Amal and AN, thereby forking out for double the postage ... Still, I'm always thinking of things I meant to get from AN on the previous order, so there are a few other bits on their way from them too, making it slightly more worthwhile. Thanks again L.A.B., you're very kind.
 
Welp – thanks to super speedy deliveries from AN and Amal, both my carbs now have new 4-ring carb needles, new .106 needle jets, new 260 main jets, Stay-Up floats and aluminium float needles, viton-tipped of course! Fuel level in the left carb is kinda midway in the range 0.170" – 0.240" below the top of the float bowl, but in the right carb I could not get the fuel level above what I'm pretty sure is slightly below that range. I tried bending the tabs on the Stay-Up float and moving the needle valve seat, the only change I achieved was when the needle couldn't seat and fuel would've spouted out the top of the sight tube if I hadn't turned it off pdq! All very strange. To me anyway. In that one carb, for whatever reason, the top of the Stay-Up float appears to meet the underside of the carb body when I try to raise the fuel level. The idle/very low rpm mixture seems very weak, so idling is worse than before and there's a bit of popping on the overrun – of course that might be expected given what I just said – but naturally it gets much better once everything is well and truly warmed up. Maybe I need to make an appointment with a carb specialist. Do Surrey Cycles offer that sort of service, or just sell carbs and parts? I can ask them myself tomorrow ... Otherwise it's running very nicely indeed. Cheers.

p.s. Following Bushman's advice ([Broken Link Removed]) I'm waiting for my son to break a guitar string, then I'll see what happens when I try cleaning the pilot jets ... When I first cleaned the carbs after the 12-year layup, cleaner spray seemed to pass fine through all the orifices, we shall see ...
 
Colin,
I noticed something about the Stay Up floats that I have not seem mentioned here before. After adjusting the fuel level to the proper height, I noticed that when the float is in the full dropped position that it still does not lift up on the float needle. That indicates that fuel delivery depends solely on fluid pressure from the tank. I have never seen a carb that worked that way. A lot of needles have a spring that wraps around the float tang that will pull the needle off the seat as the fuel level drops. I tested a plastic float in another bowl and noticed that it will lift the needle as it drops. think the tang needs to be thicker so it both pulls and pushed the needle depending on need.
As an experiment I set the fuel level and then bent one tank up to pull the needle up off the seat as the fuel level in the bowl drops. I have close to 2,000 miles since I made the adjustments and not seem any negative affects. On the plus side I was able to get a higher fuel level and it will tickle a lot faster. I don't know if any of this is helpful but I thought I would share my findings.
Pete
 
NortonMKIIA850 said:
but in the right carb I could not get the fuel level above what I'm pretty sure is slightly below that range. I tried bending the tabs on the Stay-Up float and moving the needle valve seat, the only change I achieved was when the needle couldn't seat and fuel would've spouted out the top of the sight tube if I hadn't turned it off pdq! All very strange. To me anyway. In that one carb, for whatever reason, the top of the Stay-Up float appears to meet the underside of the carb body when I try to raise the fuel level.
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Try checking the fuel level with the float bowl removed from the carb.

Deets55 said:
I noticed something about the Stay Up floats that I have not seem mentioned here before. After adjusting the fuel level to the proper height, I noticed that when the float is in the full dropped position that it still does not lift up on the float needle.
Click to expand...

Alloy or brass needles? The brass needles are heavy enough to shut off the fuel supply on their own with no help from the float (plastic or Stay-Up) which causes a low fuel level.

http://amalcarb.co.uk/optimising-mark-1 ... uel-levels
Needle Valve

If your float chamber is fitted with a brass needle valve, you may find the valve sealing under its own weight, before the float has risen far enough to press it shut. Symptoms of this problem can be that the carburetter takes a long time to tickle, hesitates on pickup and does not idle reliably. A Viton tipped aluminium needle valve is now available that overcomes this problem. It is now fitted as standard equipment to all new Mark 1 Concentric carburetters.
Click to expand...
 
Deets55 said:
Aluminum Viton tippedneedles in .125" bowls.
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OK, well, I've only observed it with brass needles, and it didn't seem to matter if the floats were the white plastic or StayUp.
 
L.A.B. said:
Deets55 said:
Aluminum Viton tippedneedles in .125" bowls.
Click to expand...

OK, well, I've only observed it with brass needles, and it didn't seem to matter if the floats were the white plastic or StayUp.
Click to expand...

Thanks Les,
I'm chasing a problem like Colin but farther up in the RPM range. I think I got narrowed down to fuel flow under hard acceleration, in top gear, after running through the gears. Petcocks will flow 500cc/min each, flow measured through drain plug is 200cc/min (checking one carb at a time). I even cut slots in the float valve to get more flow. Fortunately it only happens in a range I never travel in. I'm not overly concerned with it, but would like to get it straightened out.
Pete
 
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