Carburetors

In the February '98 issue of RCMB, John Finch has a good article on picking a carburetor for your engine. I've known John since the early '70's as club members in the Old Dominion Model Boat Association. He has gone on since then to become one of the more respected authorities on R/C model boats especially in the mono classes where he specializes.

John suggests the following carburetor bores to be used for the specified engines.

Suggested Carburetor Selection Chart

Engine	Bore (id.)
.20	Up to 0.355 in.
.40	0.355 to 0.430 in.
.60	0.430 to 0.470 in.
.80	0.450 to 0.482 in.
.90	0.470 to .510

This is a good start , but I think that we need to know more in order to make a better selection. For instance, how long or deep should the barrel be, what is the correct depth for the needle valve body assembly?

For the answers to these questions I turned to Gordon Jennings book "2-Stroke Tuners Handbook" referenced in my Formula page. The general rule for sizing the carburetor bore and length is that the length of the barrel from the opening to the center of the needle valve should be 1.5 to 1.6 times the bore diameter. The bore can be calculated with "Carburetor Throttle Bore Diameter" formula on my Formula page.

    D = K x SQRT( C x N )

    D is throttle bore diameter, in millimeters
    K is a constant ( approx. 0.65 to 0.9, derive from existing
      carburetor bore)
    C is cylinder displacement, in liters
    N is RPM at peak power

    i.e.; 80 cc = .08 liters
             1 ci  = 16.39 cc

For this example let's say that we have 7.5 cc (.45 ci.) engine which produces it's maximum power at 20,000 rpm. Therefore:

C = .0075 liters
N = 20,000
K = .65

D = 0.65 x SQRT(.0075 x 20000)
   = 0.65 x SQRT(150)
   = 0.65 x 12.247
   = 7.96 mm
   = 0.3134 in.
If we use K = 0.9 then D = 0.433

Pretty darn close to John's suggested sizes huh? But now you know how and why, not just at random, he chose those numbers. Using the ratio I quoted above, the depth of the bore to the center of the needle valve will be approximately 0.470 in. for a bore of 0.3134 and 0.65 in. for a bore of 0.433.

Guess your wondering how far from the center of the needle valve to the bottom of the carb we should be? Me too! The only reference to this I could find is that maximum air flow will be obtained with the carb located close to the intake port window, but it also gives the poorest mixture delivery. Locating it further away from the port reduces volumetric efficiency, but provides the best mixture-strength stability.

At this point I should point out a phenomena that occurs inside the carburetor. The air drawn into the carburetor actually passes the needle three times. Air passes the nozzle moving into the intake tract, then reverses direction as the result of the pulse generated when the intake port chops shut, and passes the spray nozzle a third time as the next intake period begins. You can actually see this if you look carefully at the intake of your carburetor when the engine is running at medium rpm. You will see fuel trying to exit the carburetor.

The end of the spray bar should be located exactly in the center of the carburetor barrel.

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