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 Carburetor size

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Mick Potter

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Number of posts : 125
Age : 61
Localisation : Cheltenham
Registration date : 2007-06-09

PostSubject: Carburetor size   Sat May 12, 2012 10:58 am

Hi All
What size of carburettor is best to use on a Bantam.
I have had various people claiming to be using between 30mm & 38mm carbs on their Bantams.

But what size is best for a Bantam.

Using reference from a well known carburettor supplier I Share this information.

A 26mm carb can produce between 18 to 28 BHP.

A 28mm carb can produce between 20 to 32 BHP.

A 30mm carb can produce between 24 to 38 BHP.

A 32mm carb can produce between 28 to 48 BHP

A 34mm carb can produce between 34 to 50 BHP+

Larger bore size will produce higher BHP at higher RPM if ITíS available, but at the expense of power lower in the rev range.

So the question I ask is, what size of carb are you using & why.

Mick.

Keep it rubber side down.

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Derek

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Number of posts : 1065
Age : 56
Localisation : worcestershire
Registration date : 2007-06-15

PostSubject: Re: Carburetor size   Sat May 12, 2012 11:38 am

Hi Mick

I was on a 34.0mm Amal, the throttle kept sticking so I asked around, now Im on a 30 or is it 32, Mikuni,

"why" - I was adviced to go smaller, and I feel it was the correct advice for me at the time.

am I going Bigger! No plans to at the moment, although I have tried a 36mm lectron on it the throttle stick even worse than the Amal use to seems the throttle return spring may be week of catching inside.

ref most 100cc Kart engines I have seen, produce well over 25/28 HP "in fact a lot are around 30 to 32, max bore on their carbs - Tiliston is aprox 17.0mm to 19.0mm or 21mm (venture).

regards Derek

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mjpowell

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Number of posts : 1074
Localisation : Lincoln England
Registration date : 2006-12-09

PostSubject: Re: Carburetor size   Sat May 12, 2012 9:18 pm

Good question Mick...
I'd say Reed Valve 32mm, Piston Ported 125 38mm, 175 36mm.
Remember- Reeds are pretty forgiving to ride where as pp are
not, if rider ability/experience is lacking start with a smaller carb
(on pp machine).
When playing with different bore carbs you cannot really make
comparisons with you go Amal/Mikuni/Lectron ... etc
Keep to one make of carb, decide what size and then try another
carb make of the same bore size.

Mike
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mscutt

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Number of posts : 96
Registration date : 2011-10-21

PostSubject: Re: Carburetor size   Tue May 15, 2012 1:23 am

Interesting topic Mick!

Using a simulator like MOTA it generally shows that a larger carb gives more peak power and maybe unexpectedly, more mid-range power too.

In reality there are problems with starting and carburation when you get to the larger sizes. On one of our piston ported motors a 38mm carb has been impossible to set up but switching to 36mm cured the problems straight away. We tried a lot of different things and haven't got to the bottom of this but its not really a problem - the 36mm carb works just fine.

In karting and other formulas that restrict the carb size its true they still get 30+ hp but there are probably a lot of other reasons they get that sort of power despite the small carb. Which ever way you look at it smaller carbs can be made to work very well and flow plenty of air/fuel for a quick Bantam.

Mick
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Derek

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Number of posts : 1065
Age : 56
Localisation : worcestershire
Registration date : 2007-06-15

PostSubject: Re: Carburetor size   Tue May 15, 2012 9:41 am

Hi
All this talk about carbs makes me think about my diet, sound like a lot of hot air and velocity to me.

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