Sunday, October 2, 2011

How to Feed Fuel to the Privateer

Back when my engine was new the factory supplied an AMAL 27 remote bowl carburetor with it.  As time went by, AMAL ended production of the 27 and the JAP factory started specing the AMAL 932; mostly because of availability and an increased intake size.  It seems, after a quick look at what is available, most carbs are a series of compromise choices that may or may not work.  So I decided to better define what I was looking for in a carb.

This maybe a dream but I wanted a simply design that was easy to tune for either street use or track use.  Will not really track but maybe some high speed work.  It would be reasonable in cost and had spares available and most importantly I wouldn’t have to buy it new.  Yes again I’m cheap.  In the years since my engine was new there has been lots of innovation and improvement in the area of carbs.  Or as they say, "Everything old is new again".

So I decided to do a bit of research to understand the process of taking fuel and mixing it with air to feed to the engine.  Research would hopefully give me some criteria for making a carb choice.  So to the library shelf I went.  When I need to understand the basics of mechanics I always go for my Father-in-laws, Charles ‘Chuck” Waltari’s copy of Audels New Automobile Guide, copyright 1949.  May seem strange to use an automotive book to understand a motorcycle but really the basic principles are the same. 

What I found was a step by step discussion of the principles of mixing air and fuel, with illustrations; I like illustrations.  What stopped me in my tracks was the part on efficiency and the venturi principle.  Sorry for the poor scan.  But the up side was that I learned something.  So I went to do some more research.  Oh the Audels guide says that a carb with a venturi throat shape at the fuel inlet jet is the most efficient design for obtaining, “Homogeneity of mixture “.  You have to like that!

So with this in mind I did more research into which carbs would provide that design; or something very similar.  It seems that modern carbs are built to a price point and with a design that is adequate for the job but not ideal.  Well except for one.  I reacquainted myself with a carb called the Lectron.

This carb is a third or fourth generation of the original flat slide carbs of the 60s called the POSA and the Lake Injector.  Besides being a flat slide carb it does have a true ventuir shaped throat for the fuel air mixture.  Well that’s what I believe the design provides.  Considering that on a 32mm air intake it tapers 2mm to 30mm at the fuel jet and then tapers back to 32mm on the head intake side.  Go here to read what Lectron says about their own carbs.  Bottom line for me is that it should provided what I’m looking for.  We shall see.  I would appreciate any comments from the experts that may read this.

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