Carburettor help

The 88ss should have a pair of monobloc carbs but concentric are cheaper. If you already have one then maybe that's the way ti go but if it really is worn out then it's up to you. Adding another is not all that simple. You need spacers, heat insulators, balance pipes, and three studs and one socket head screw (because its impossible to get a spanner on a fourth nut between the carbs). Plus cable splitter and new throttle cables. Lots of fun!

In any case if it's new to you  I'd recommend Bacon's Norton Twins Restoration. Lots of info you won't find in a workshop manual. Enjoy it, read up as much as possible and don't take things apart if they don't need it!

A lot depends on how you intend to ride the bike, and whether strict originality is important to you, or if you want to keep your late relative's personal set-up.  And whether the amount of wear in the Concentric is affecting the good running of the bike.  If it isn't a real problem, I would advise leaving it alone and get used to riding the bike and setting it up well.  Mods can come later.

The Concentric carburettor was extremely prone to wear.  Models now available are cast in aluminium instead of Mazak, and you can buy anodised slides.  They are much more durable.

Strictly speaking your bike should have a pair of 1 1/16" Monoblocs, the right-hand one with its float chamber chopped off.  They are exactly the same as were fitted to the 650 ss.  It would cost you a lot of money to buy the full kit of parts for that set-up.  It will also be a lot more difficult to keep the carbs in balance.  Unless you are after maximum performance, you are probably better off to stick with a single carb.

You can get lucky at Jumbles.  I recently stumbled on a pair of Monoblocs which would be perfect for an 'export' Atlas, twin carbed with a 689 mirror-image right carburettor.  The 689 had a new slide in it, and was in superb condition.  They cost me £50 the pair, the vendor almost paid me to take them away.  No, I'm not letting them go, they are already restored and fitted to my project bike.

Paul

Is it an SS? If it's 1961 it might well be. In which case it originally had two. On mine the engine number is stamped SS but I suspect they weren't always.  The SS came with a hotter cam and twin carbs. Lots of info on this site under 'models'. The standard 88 had one. They were 1 1/16" monobloc.

(Sorry... Paul posted at the same time with more detail. I'd agree that I think you should get used to it first. I do prefer twin carbs just for the extra pull on hills with 500cc. And fit the cable adjuster lock nuts nice and tight. And I fitted chrome plated slides once I was happy with the settings.)

Paul, I would love to see a picture of how these carbs fit. I have a good many export Atlases, all with the wrong carbs.  There is no photo of these in Bacon's book nor in the Walker books. The annoying thing is I probably have the right carb bodies in my pile of Monoblocs but I can't identify them without help.

The 61 SS would have come with the splayed manifold , and has more room for the carbs. The early SS was very rare and many std bikes were mocked up  to look like the real thing. I find the monoblock to be easier to work with and a better carb all round. IF I was building an 88SS now I would use a remote float to bottom feed the 2 carbs. 

Good pictures of twin monoblocs on pages 61 and 63 of Bacon's Norton Twins Restoration.  Since the SS models appeared in spring 1961, and only the earliest 100 or so had the earlier head, a 1961 engine might have either. (I wonder how many 88SS were built that year?)  A photo would help but not prove which model it is after all this time, unless it is stamped SS. If it is not, I don't think the absence of SS  disproves it either. Mine has twin 28mm concentric (alleged to be a bit too big in theory but it came with a single 26mm which was gutless).

Looks very nice. It has the 'downdraft' head. The colour looks too good to be original but if it was always like that then it's not a normal SS colour.  SS also should have a shallower front mud guard, and a magneto, and it's hard to tell from the picture but it looks like it has coil ignition.  Not that it matters nowadays. What size carb does it have? 926 or 928?

The best way to see an image of a 689 Monobloc is to look on EvilBay for an ad by someone wanting to sell a matched pair of 389 / 689 from a bike.  There is a steady stream of them on US EvilBay removed from BSA A65s - they are almost always 1 5/32".

The carburettor body fitted to 'export' Atlas was the 389 / 236 and its pair the 689 / 236.  Reading down the data, you want a body with stamping / 236, or / 237, or / 241 or / 242.  The only difference was the main jet size.  But seriously, you just need a Monobloc with the right bore size, all the jet parts can be changed readily.

You can find all the data on this page:

http://archives.jampot.dk/technical/Carburation/Amal_Monobloc__numbers_type_and_use.pdf

If you have a Monobloc, put a mirror on its right side, and you will get a reversed image, of a carburettor with the float chamber on the right side.  That's what a 689 looks like.  The carburettor slides are not interchangeable, but all the other secondary parts are the same as 389 parts.

I am not fitting my pair to a parallel twin, but a V twin. 

BTW my 389 / 27 was made for a Francis Barnet 250cc 2-stroke (the spray tube is cut away at the engine side), and I changed the choke tube for a 4-stroke one from a damaged 1 1/8" carb body.  It also had stripped float chamber cover-plate threads.  I drilled deeper and re-tapped the threads.  It just needs longer 2BA screws to hold the cover-plate on.  I got some nice brass ones, but they are countersunk, not countersunk cheeseheads.  The long screws offered by Burlen for extended float chambers are cheesehead but not countersunk....  I prefer the steampunk look of the brass 

Paul

I doubt that your bike started life as an SS.  However no reason to not create one!.  It does not appear to have a downdraught head.  However the 61 SS  started life with the type of  head you have.  The two  11/16 376 monoblocks were not handed or chopped .It also had coil ignition with the headlight and PSR8 switch  that you have , The front guard you have was more usually seen on the 88DL  (which your bike may have been. Look for signs of ground off panel lugs ), The later Downdraught head SS came with a magneto. Your frame and engine numbers could throw more light on what you have. Could the cases be stamped 88c?. If so that would make it a DL or Std bike.I like it as it is, Tasty tomato!!.

Hi Alex - As far as I remember all the 88SS's had magneto ignition and whilst it is possible that someone retro-fitted a coil set up it would make no sense so to do.  The lighting and ignition switch on your headlamp is an obvious sign that it had coil ignition from the build.  On the drive-side crankcase half there should be a stamping of 122 SS near the brass badge or above the breather pipe exit.  There were 88 SS's made in 1961 but other models as well.  Like Robert I am unconvinced but yours is a splendid bike already.  If you wish to make it to SS spec you're likely to spend quite a lot of cash.   Ride it and enjoy is my tip.  Good luck, Howard 

Bruce Mainsmith  did a road test early in 62 of a 61 88SS   from the factory. I have large photos. The bike clearly has coil ignition  and twin monos  which were not paired or chopped and were fitted on a splayed manifold.  I liked the look of it so much I built a replica back in the mid 1960's after my 88DL was  wrecked by an unaccompanied learner driver  outside the Bromley Court Hotel. Unfortunately the police attitude to motorcyclists was very poor at that time and they believed his lies  and false witnesses.  Fit a headcam!!. Just checked my papers,the bike was collected and tested by the Green-Un mag one month after it was registered. A press demo bike I suspect.

Hi Bob - Now you've reminded me,  I do recall seeing that roadtest.  Did it not also have a siamese exhaust system a la 99SS ?  I had a 1963 one with a Lucas Competition magneto and a 1965 version with a standard K2F magneto.  Both bikes were fliers - best of the Dominators to my mind !!  Obviously Plumstead were spending less on Norton specs !  I expect that in 1961 at Bracebridge Street they were sharing parts with the 99SS that never had a magneto (unless specified as an  option).  By the time the 650SS was running on the production line the 88SS would share parts with that model.   So Alex's steed could be an 88SS.  The numbers will obviously be key to this mystery........Ride safe. Howard

Yes it came with the small bore Siamese system   which was tested by Norton and found to improve acceleration when used with the 60 head  ,Manxman cam , tapered alloy pushrods and harder valve springs. The bike was said to have a stronger crank which had a different core.It was also said to have bigger valves ,but I think the sizes were std for the head and just bigger than the earlier pre 60 head. (Don't know for sure and the parts lists have plenty of errors) .Won plenty of production races ,although sometimes a Dommiracer motor got slipped in!!.  

Hi again Bob - Yes, I too have heard those rumours.  Most probably spread by jealous Velocette and  Triumph entrants.  I still have an 88SS and as you say the crankshaft is quite different to a standard 88.  The big-end journals are a good deal thicker than standard and the flywheel lighter.  An obvious cure to stop them breaking at the high rev range of which they were capable.  Unlike the Domiracer the big-end o/d remained at 1.5 inches however.  I did have a 1960 big valve 88SS head on one of my 500 racers in the late sixties and I can assure you that when I substituted a semi-downdraught type it made no difference at all to its performance.  It did look a lot neater of course and the missing float bowl was a nuisance until I fitted an external float chamber.  There were two versions of the big-fin pre- downdraught head.  The first one had inlet and exhaust valves the same diameter and the later one, for the SS versions, had the same inlet size as the 650.  Cheers, Howard