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[Bandit12]

Hiya

I've a 1959 big head bullet I'm restoring and the monobloc carb that came with it is seized up and knackered. Both the slide and the body have deep grooves in them and the slide keeps getting stuck so I've decided to replace it with a new carb. Question is what?
Another monobloc or should I go for a concentric? I plan on riding this bike so am not fanatical about perfect originality but would like something that looks appropriate if possible so i'm not planning on a flat slide mikuni anytime soon-chuckle. The previous owner was an engineer and has ported and flowed the head and i would like to take advantage of this if possible.

Any tips and advice are gratefully appreciated

Bandit

[PO51UHD]

That depends...

Monoblocs in my experience look the part but can leak.

Concentrics don't leak, look ok and (again in my experience) are easier to set up correctly.

Depending on how seriously it's been 'ported', consider an Indian Mikcarb or similar Mikuni.

...or a dell'Orto...
Personally I'd go with a Conc. but that's my choice based on familiarity and ease...

Stephen

[Adrian]

Your carb choice will to some extent be limited by considerations of space if you're retaining the original toolbox/air filter housing, both the Mk1 Concentric and the Monobloc will at least fit straight on, anything else you might struggle to fit it in, even if it's a much better carburettor! For a Mk1 Concentric I'd want a chromed brass slide for it, or I'd fit one of the Premier versions which are an improvement on the old zinc alloy bodied things. 32mm is as big as they go.



As for (eg) a Mikuni VM32 or a Dell'Orto PHF32 you MIGHT get one to fit but it will need a one-piece flange rubber to keep things as compact as possible. You will also need to see that flange rubbers with 2" centres are available, or you might have to plug the carb stud holes and re-drill and tap for 60mm or 2.5", if the flange on the inlet port has enough metal on it. On the Hitchcocks' reproduction Big Heads it was plenty big enough, I can't speak for a Redditch original.



You could of course forget the tinware amidships, fit a Fury-style battery mount and there is a lot more room to play with carb exotica, but I assume you're trying to keep things original.



A.

[Presto]

A no-brainer in my opinion: Amal Mk 1 Concentric Premier – with new style pilot jet and anodised slide. Much criticised by theorists but a very practical and successful carburettor. Easy to tune and simple to maintain.

[Beezabryan]

PO51UHD says "Monoblocs in my experience look the part but can leak."
Are you confusing the Amal 365 with a 276?
Monoblocs do not leak but a 276 can

[Dennis C]

You will get almost as many opinions as posters?, Bear in mind that the monobloc was introduced as an improvement to the previous Amal when cost was less important, the concentric was introduced due to hard up bike makers demands for a cheaper carb when times got harder, is the concentric better?, Personally having used both over many years I think not.

[Mark M]

I think I'd go for the Monobloc. It is a better carb than the Concentric. Reasoning is this: the Concentric was created by Amal to meet the demands of the industry for a CHEAPER carb. Amal did this by using inferior materials and a simplified design. The new Amal Premiers are being made to address both these problems and they work very well (I've just fitted them to my Mk 2 Interceptor) but what they do is bring the Conc up to the spec it should have been in the first place. The Monobloc will look better behind that wonderful big head (the Conc is quite small,) and is very tuneable, probably better than the Conc as you can get at both jets easily. The Mono is also physically stronger than the Conc and less likely to warp, there's not much Amal can do about that on the Premier unfortunately. You say the head has been ported, that could mean a lot of things. If the inlet tract has been opened up do make sure the new carb you buy has the right diameter to match. If the porting is mainly cleaning and polishing (most likely) then you probably won't need to do much unless the compression ratio has been increased and/or performance cams have been fitted. The Bighead will only take mild tuning before strengthening work needs to be done so be careful! Remember that in contemporary reports the Home Market BH didn't go any better than the previous 500 Bullet. (The US Woodsman/Fury is a different and fragile beast!) Finally, our Hosts run their Scrambler competition machine with both a Bighead and a standard head (different head for different tracks they tell me,) and they've got a lot of experience of jetting so whichever Amal you choose they can advise on alterations from standard jetting which is where I'd start. Amal were very good back in the day and their standard jetting for road bikes is usually bang on.

REgards, Mark

[Presto]

Mark M mentions our hosts 612 Big Head used – very successfully – in completion, always with a Mk 1 Concentric fitted!

[Mark M]

I see I will have to further explain my thinking. Our Hosts probably use the Conc on the Scrambler because it does one thing better than the the Monobloc which is that as the float is Concentric (get it?) with the main jet the fuel supply does not suffer from starvation when accelerating hard and leaning at extreme angles. The Mono with it's float on the side can suffer this way but it's not going to be a problem on a roadster Bullet, Bighead or not. Other reasons could be (just in case someone is also wondering!) the Conc is lighter, smaller, available in larger bore sizes than the Mono, it has a "Smoothbore" racing version and also just possibly, there is (there certainly was some time back,) a magnesium bodied version for extra lightness. None of which should make any difference to the Original Poster's question.

REgards, Mark

[Presto]

I think the reason for using the Concentric in competition is that no one (well virtually no one) would use a Monobloc in preference - the Concentric was used for years in umpteen highly successful racing, enduro, moto x and trials applications – and in too many road bikes to begin to list. It is in practice a perfectly adequate item.