1928 Model D - Carburettor

[tony.barnett]

Hello

I have a few carb related questions. I've been building a 1928 Model D since just before Noah started on his Ark - now I'm getting to carburettors. I don't have an original AMAC so will have to do with an AMAL, but I have several to choose from.

Firstly - do you know what choke size carburettor I need? I have always assumed 1" and have a couple of these carbs, but the inlet port is bigger than that. I also have most of a 1 1/8" carb but its incomplete. Will the 1" carb be OK?

Second - I have several 1" choke 276 type carbs and a bare brass body - is there any advantage in using a brass carb body (other than it looking good)?

Cheers

Tony

[Dave.Barkshire]

Might be worth sending Joe the Carb an email just in case he has a correct AMAC in his secret carb box.

joe.kickstart@ntlworld.com

[cmfalco]

do you know what choke size carburettor I need? I have always assumed 1" and have a couple of these carbs, but the inlet port is bigger than that. I also have most of a 1 1/8" carb but its incomplete. Will the 1" carb be OK?

Oversimplifying, a too-small carburetor will allow a bike to be started easily, and will work fine at lower speeds, but will restrict the top-speed performance. A too-large carburetor will make the bike harder to start, can be somewhat unresponsive in mid-range due to the reduced vacuum signal, but will have good top-speed performance.

Anyway, based on the fact the 1-3/32" that came on my Model C when I bought the bike works great at all speeds, and is a one-kick starter whether cold or hot, my advice would be that a 1" would work but it wouldn't be optimum. However, the additional 1/32" step up to 1⅛" shouldn't make a significant difference in performance from my 1-3/32"

Having said that about the 1⅛", an important caveat that applies to all motorcycles is to avoid using a carburetor that results in a step up at the head. The pilot mixture flows along the base of the carburetor and, if it encounters such a step, the vapor can condense. This in turn can cause difficulties in starting, particularly when cold. If your head has an inlet of at least 1.125" then there shouldn't be any problem at all using a 1⅛" Amal. For reference, the inlet port of my Model C is 1.158" so yours is very likely the same, and I wouldn't be surprised if the original Amac was 1⅛".

NB. in looking up information to answer your question I discovered a missing hyphen in my 1928 Model C shop manual. If owners of that book will turn to page 257, make a note that my Amal 276 is 1-3/32" not 13/32". The inlet port diameter is on page 255.

https://www.amazon.com/Ariel-Chronicles ... B0BN61Z6H1

[tony.barnett]

Thanks for the answers. Mulling it over its probably best I find the couple of missing parts for my 1 1/8" carb - its only missing the top and ring. The port is 1 1/8" so it should match ok.

[nevhunter]

A brass body doesn't go weak AND BRITTLE and distort like the diemetal ones do and you can sweat a new brass piece in(solder) where it wears. Don't go the whole way down Just where the slide runs, Finish to size after sweating it in place. It's not really worth spending much on diemetal . it's just as likely to crack after you've spent the $$$s. Nev

[cmfalco]

you can sweat a new brass piece in(solder) where it wears.

For what it's worth, the post linked to below and the several that follow show how I sleeved the bore of a worn Monobloc to result in the proper 0.004" clearance:

https://www.britbike.com/forums/ubbthre ... Post583198