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Mercury concentric carb throat size

Submitted by dominic_gomes on Mon, 28/05/2012
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Heavy Twins

Isee that my Mercury concentric carb throat is a 32mm unit. In the miniscule amount of data I have for the bike I see the throat should be a 30mm. The 650ss appears to have two 30mm carbs. Would there be any performance benefit to increasing the single carb size to a 34 or 36mm throat?

My bike runs fine with the 32mm.

DOminic

Submitted by Anonymous (not verified) on Mon, 28/05/2012

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I am sure that those more expert in these matters will comment further, but two thoughts:-

(1) Gas flow. You are looking for laminar flow between carburettor and cylinder head. Anything that upsets this and causes turbulence in the gas, e.g. a jump in size between the two items, is going to have a negative effect on performance. That's what I have been told, anyway.

(2) A rather eminent fettler of Commando cylinder heads in the USA for whom I have great respect says that going to a larger carburettor can sometimes have a negative effect. The linear velocity of the gas is lower because of the wider throat and the gas does not flow into the head as well.His performance heads work, amongst other ways, by restricting the inlet tract and not enlarging it.

Submitted by gordon_johnston on Mon, 28/05/2012

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I have used a 928 single carb on a Mercury head satisfactorarily. I don't think it really matters if the inlet tract is bigger than the carb throat - there will be minor turbulence downstream of the joint face but this can actually result in better mixing of the fuel/air entering the combustion chamber and give you better running. What you don't want is a bigger carb than the inlet tract. Low airflow rate has the opposite effect. (O.K., the exception is the 350cc Gold Star but that'a another topic for another forum). Gordon.

Submitted by Xanna_jeannett… on Mon, 28/05/2012

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Hello You guy Well I am going to have to say this, That the Norton Manxman The first 650s used a Sleeve in between the inlet manifold and spacer and with this sleeve you got 3 more BHP . because late on the 650ss did not have them and they had only 49bhp instead of 52bhp . so it did something to the flow of the carburration! something to think about ! yours Anna j

Submitted by laurent_husquin on Tue, 29/05/2012

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Hi Dominic,

I know from my last experience (using a 30 mm Amal MK2 on my 99) that Chris and Gordon are right. Going back to a 28 mm MK1 gives a great improvement of responsiveness at all ranges of rpm, without reducing the top end power.

To me, it is not due to moving from a MK2 to a MK1, but to fitting the more appropriate size of carb (28 mm = just a bit more than the 1" 1/16 recommended for 99s).

Hope it helps

 


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