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Jubilee - wrong carb

Submitted by dennis_thompson on Sun, 03/03/2019
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Light Twins

When I bought my 1964 Jubilee as a box of parts it came with the wrong size carburettor. I was hoping that I might get away with using it but after a road test this week there is a noticeable fall in power mid-range.

The carb I have is an Amal monobloc 376/35. The correct carb for a Jubilee is I think a 375/43.

The 376 is jetted correctly for a Jubilee and that may also be part of the problem.

It looks like second-hand 375/43 carbs are rare as I have been looking for a while.

The main difference is the larger bore on the 376 at 15/16" or 0.9375 decimal inches versus

25/32" or 0.7812 decimal inches on the 375.

Any suggestions about how to use the 376 or do I have to bite the bullet and find a 375? 

I did find a throttle body for a 375 but to find and fit the missing the parts e.g. slide, jet body, etc would cost more than a new carb from Amal at about £200.

Dennis

 

Submitted by dan_field on Sun, 03/03/2019

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

Submitted by paul_standeven on Sun, 03/03/2019

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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

Paul,

Thank you and good to know that other bikes had the 23/32" bore. I located a list of Amal carbs and their respective bikes from a site called "Pork Lips" (!) so I can cross reference to other possible sources. 

Presumably, I'll only need to change the jets? My old carb has the right jets for the 375/43  and they are new so I am hoping I can swap them over.

Dennis

Submitted by paul_standeven on Mon, 04/03/2019

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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

Submitted by dennis_thompson on Mon, 04/03/2019

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Paul,

Good info and many thanks. I have found a donor carb on a well known auction site and if I get it I will strip it down and refurbish it as you suggest. Cleaning the compensator hole sounds interesting.

Dennis

Submitted by robert_tuck on Wed, 06/03/2019

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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 . 

Submitted by philip_hannam on Thu, 07/03/2019

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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

Submitted by dennis_thompson on Sun, 10/03/2019

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Thanks but I managed to buy a monobloc with the right bore so I will fettle and fit that.

Dennis

Submitted by dan_field on Sun, 10/03/2019

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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! 

Submitted by nicholas_clarke on Mon, 11/03/2019

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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

Submitted by maurice_turlington on Tue, 12/03/2019

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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.

Submitted by dennis_thompson on Tue, 12/03/2019

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Maurice, what number is the Jubilee 375?

There are many 375's at 20mm bore (approx. 25/32") but I need to know the rest of the number e.g. 375/43 to avoid having to buy a new slide as they can be a 3 or 3.5. Assuming it is correct for a Jubilee how much do you want for it?

Dennis

Submitted by dennis_thompson on Thu, 21/03/2019

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Maurice, many thanks for the carbs, they have many useful parts that I need.

Dennis

Submitted by dennis_thompson on Sat, 27/04/2019

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Finally got around to making up the correct carb from parts. I had a 375/43 body but it had no internals. I bought new jets and choke and used second hand parts for the rest.

I was feeling smug about the assembly until I turned on the fuel tap and petrol p******d out from almost every orifice! What I found was that the main jet body needs a fibre washer and the fuel inlet union does not - on my carb at least. The fuel inlet union sits too high if a washer is fitted and therefore the fuel level in the carb is also too high causing fuel to leak into the carb body. Leaving off the inlet union washer allowed the needle valve to close with fuel at a lower level in the float chamber. Paul did warn about this but I forgot to take his advice!

I could not believe  how readily the bike started with the correct carb; first kick after a tickle. I still have some setting up to do as the bike picks up slowly and then gets going even when the engine is warm (needle position?). I have the needle on the third notch. The bike ticks over fine.

Dennis

 

 



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