Jubilee - wrong carb

I had a similar problem, experimented with a concentric, then grasped the nettle and bought a new 375, ..... instant success! 

375/43 Norton 250cc 1958-66; Jubilee Twin Standard

25/32" bore

25 pilot jet

3.5  slide cutaway

130   main jet

0.1065  pilot jet

3  clip position

Any Monobloc body with a 25/32" bore will be good, as long as it is in sound condition.  If you look at the Amal Monobloc carburettor data, you will see that LOTS of Brit bikes used this size, so it will not be that hard to source one.  You just have to make sure that it has the right jets n fings so it will carburate well.

Paul

When I need Monobloc data I go here:

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

It has all the specifications for Monoblocs you could want.

Not many carbs use a .1065 needle jet.  A worn .106 might suffice if you lack one....  but you don't know how accurate that might be.  If a newish slide is loose in the slide bore, don't bother with the body, find a better one or get it sleeved by an expert.  That's expensive...

When I recondition a carburettor, I strip it right down and remove the jet block.  Make sure the mounting face is flat, I use some wet 'n dry on plate glass. Clean thoroughly.   Do not forget to clear the tiny compensator hole which vents petrol from the pilot jet into the main bore, on the engine side of the spray tube.  If it's blocked, you get some odd and elusive 'lean mixture' conditions when coming off idle.  I use a fine wire, a pair of tweezers and a magnifying glass to see what I am doing, but do this carefully so as not to cause wear to the hole.  Oversize means too much petrol flow....  

When you re-assemble, give it a new gasket set and ethanol-resistant float, float needle, fuel filter, and fuel pipe, and for your peace of mind, a new needle and matching jet.  I like to check that the float level is correct.  To do so, you need to attach a clear pipe to either a main jet cover nut or a float chamber cover plate.  The cover plate has a little protrusion just above the AMAL logo to indicate correct fuel level.  If the level is low, there are shim kits.  You might get lucky and find a fibre washer to put under the feed union which gives the correct level - they can be filed thin.  Be aware that early Monoblocs had no fibre washer under the feed union, late ones had it, and the washer is supplied in a gasket set.  Fitting one to an early carb will give the wrong float level

Paul

Watch the prices of parts list, it can be better value to buy a new one, especially if you resleeve it! 

Dan

The "Compensator" hole is also called the By-Pass and helps with the progression from pilot to needle jet. At some point it supplies air and then mixture so if  the wrong size can make things both weak and rich . 

How about fitting a Concentric 626 series carb? You can pick these up at an A/J for half the cost of a Monobloc.

These were fitted to the last of the BSA Bantams. Ok the choke is a fraction larger than a 275 but it might be a solution.

Firstly, the 375 / 43 body for a Jubilee is 25/32", which is 19.84mm.  A 20mm Concentric will be correct, if that's what you want to do. A 26mm Concentric will be far too large - the existing problem is that the 376/35 already on hand is too big, at 15/16" or 23.81mm.  Don't do it.

Be very careful before fitting a carburetor off a two-stroke.  They have a different spray jet - it's cut away in some form at the engine side.  Some are cut at an angle, some cut completely away. This alters the petrol delivery.  It's actually more of an issue fitting a 4-stroke carb to a smoker than the other way round.  But if you do this sort of thing, do not expect standard carb settings to be relevant.  You are on your own.

I seem to remember reading somewhere that a particular 2-stroke engine needed a 140 main jet, but a 170 if it had a 4-stroke type carburetor fitted. 

I bought a nice 389 at an AJ, which turned out to be from a Francis Barnet.  The engine side of the spray tube was cut away at an angle, in similar fashion to a 627 for a Trident.  I needed a matched pair of carburetors for a twin carb bike, so I fitted the body with a jet block from a damaged carb to solve the problem. 

But the suggestion here is to fit a Concentric off a Bantam.  You can't change the jet block in a Concentric, so I think it's stuck being a 2-stroke item

Paul

I think I’m right in saying that no one has managed to get a concentric to run nicely on a jubilee., but am happy to be told otherwise! The monoblock is a pretty good carb, but as you say you need the right spray jet. 

A mikuni would be interesting but you’d need a big box of jets. One of the supplies offered to jet one for me if I lent them my bike, don’t know if the offer is still open! 

For what it's worth Dennis, I think it is more cost effective to bite the bullet and just buy a new Monobloc, by the time I had rebuilt mine I had spent more on parts and postage than buying a new one - to say nothing of the time spent stripping and rebuilding it several times fettling it!   My only solace is it is running really well now and I kid myself the old carb body has a wonderful 'patina' now I've polished it! 

Nick

If anyone is interested I have 3 x 375 carbs I want to get rid of. I think one is for Jubilee at 20mm and the other 2 are 7/8ths for Navigator/Electra. Only one of them is complete with 3 1/2 slide. 140 main, 105 needle and 25 pilot.