Bigger Carburettors.

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lazybandittattoo
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Bigger Carburettors.

Unread post by lazybandittattoo » Tue Nov 29, 2011 10:30 am

A classic Bike Magazine feature from 1992 claimed that a 30mm 0r 32mm carburettor with a 105 needle jet and 220 main jet could be fitted to a standard barrel.

As I understand it the standard barrel in question was from a D14/4 or a B175. These I believe (not actually got one to check) had a 26mm inlet port opening.

My Bantam is a bitza (It's of low value therefore but it goes bl**dy well but there is still room for improvement.) It is a D7 with a D14 bottom end. It is bored out to 187cc with a 64mm domed crown piston and the D7 barrel (with 22mm inlet port opening) has the smaller 1 3/8 inch exhaust.

I'd like to keep the smaller exhaust as it's all nice and shiney as I bought new header and silencer within the last six months.

Last week I decided to be clever. Ok. Not that clever. My 22mm Amal concentric is jetted with 105 needle jet and 140 main jet as per D7. I popped in a 150 main jet as specified in D14 owners manual. I was hoping that the slightly larger jet would improve running but in fact the opposite happened. I couldn't get above fifteen miles an hour - the engine ran over-rich and simply couldn't get the air it needed. I have put the 140 main jet in for now but I have bought (from ebay) a 32mm amal concentric which I intend to try.

Do you fellas think the 32mm carb will be too big for the 22mm inlet? I gave away a 26mm amal a couple of weeks ago to a mate who needed one but I'm sure I can find another somewhere.

Have a good day.

Tyon

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lazybandittattoo
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Re: Bigger Carburettors.

Unread post by lazybandittattoo » Tue Nov 29, 2011 10:31 am

Don't know who Tyon is. Computer witchcraft.

Tony.

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Greenbat
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Re: Bigger Carburettors.

Unread post by Greenbat » Tue Nov 29, 2011 12:21 pm

I suspect a 32mm carb will be too big. Also, unless you open up the ports on the D7 barrel, you'll be restricted by it. I'm not sure of the exact sizes, but I believe transfers were bigger too. A 26mm would be worth trying though.
The big bore pipe might be worth doing anyway, there's not a great deal of difference in size at the port outlet. You need an adaptor ring, which is what BSA did in the first place.

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lazybandittattoo
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Re: Bigger Carburettors.

Unread post by lazybandittattoo » Tue Nov 29, 2011 1:28 pm

Thanks for the opinion Mr Greenbat. I have seen the adaptor rings. Does this mean that the BSA Bantam D14 and B175 had the same size exhaust outlet just a bigger pipe?

I tend to agree - the 32mm will be too big - I'm going to try it anyway as I'm curious as to what it will do exactly (I'm a novice so it's all learning for me.)

I gave away a 26mm amal the other week but I will track down another - if I can get em cheap enough may try a 24mm one too.

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Greenbat
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Re: Bigger Carburettors.

Unread post by Greenbat » Tue Nov 29, 2011 2:44 pm

Yes, pipe spigot is the same. I don't know what difference is in the port size at the cylinder though.
The bigger carb will let it breathe freer, letting the engine rev higher. However, you'll lose bottom end power. The bigger bore reduces the suck when the inlet port opens, meaning the air flows slower and doesn't draw fuel mist off the jets as well. OK when revving high as the speed makes up the suction, but no bottom end power. At least thats the theory, not done it myself so be interesting to hear results. Just be very careful about jetting, you'll probably need bigger ones or t'll seize. Though you may well know that already.

Velton
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Re: Bigger Carburettors.

Unread post by Velton » Tue Nov 29, 2011 3:00 pm

Is there not an intermediate size of Concentric?

An increase for 22 to 32 mm is a big jump, isn't it?

I know that Concentrics were usually a size up from the preceding Monobloc on the same bikes.

FWIW my own D7 went well on Monobloc - or two.

30 odd years ago but I seem to recall that it was a 7/8ths inch originally and went up to 1inch and a1/16th.
D14 barrel, piston, large bore exhaust system, stuffed crankcases, padded crank and Todd head made it go damned well.
Jet sizes? Can't remember!
Just being old doesn't make it a "Classic."

Aaaah! Nostalgia is not what it used to be!

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lazybandittattoo
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Re: Bigger Carburettors.

Unread post by lazybandittattoo » Wed Nov 30, 2011 12:36 am

Thanks. I have a Todd head but it has not been machined to accept the bigger piston and it needs some fin repairs. Also I understand that the todd head increases compression to a point where a timing adjustment is required?

The 32mm carburettor was indeed too big. I didn't get far and it didn't run well. I have put the 22mm concentric back on and re jetted as per D7. 140 main jet. 105 needle jet. needle in position 2.... for now. :twisted:

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lazybandittattoo
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Re: Bigger Carburettors.

Unread post by lazybandittattoo » Thu Dec 01, 2011 7:14 pm

Had a stroke of good fortune. Bought a spare cylinder barrel from an ebay seller. It turns out to have the larger 26mm inlet port and is already bored out to accept a 64mm piston like the one I have fitted. Also in good condition and neatly painted with heatproof cylinder black.

Were the later bantams fitted with a 26mm Amal concentric carb then?

I have some playing around to do. Sourcing larger 1 3/4" bore exhaust and silencer and adaptor ring. I suspect the 22mm Amal Concentric I have fitted to my current barrel will be too small for the bigger inlet. I think I need to try the 32mm carb and possibly a 26mm also if I can get hold of one cheaply.

What would using a smaller bore carb than the inlet port result in?

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Greenbat
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Re: Bigger Carburettors.

Unread post by Greenbat » Fri Dec 02, 2011 3:03 am

Yes, later ones were 26mm. Not sure about smaller carb than the inlet though.

Codger
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Re: Bigger Carburettors.

Unread post by Codger » Fri Dec 02, 2011 3:16 am

32mm ? that is silly for a 175/185?

the enfield bullet 350 used a 24 mm , and a 500 used a 28mm , even a full blown 612 bullet uses a 32 mm

I think (from memory) that I used a (6)26 <26mm>concentric on my d14/4 motor (that was modded) , NOTE: the very last B14 (s) were a last ditch (cock up) by BSA and used a lot of 4 stroke parts in the carb , in a feeble attempt to make the bike run,

from memory (and going back a few yrs) , the concentric was modded , ie: no pilot jet , different lengh needle and jet , not sure on the 2t carb tho , contact surrey cycles or amal for the correct info , as fitting the wrong needle to the wrong jet will *uuck it up

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