Which Walbro WB Series Carburetor is Best for my Raket 120 Engine?
The Raket 120 engine stalls at idle because the balance of air to fuel ratio of the WG8 carb is not proportionate during idle so it has to be made proportionate depending on the engine it is mounted on.
I also noticed that since the stock WG8 carburetor has a FIXED High speed jet, a low pop-off pressure setting will cause the engine to run rich with subsequent loss of power and without an easy way to correct the air:fuel mixture at high engine RPM.
Please consider that if you are not comfortable fiddling with carbs, and it ain't broken, then don't mess with it; bring the carb to your local lawn mower/chain saw shop for service.
The fuel is discharged primarily through the pressure difference between the atmospheric pressure and the pressure in the carburetor bore, not by the amount of air rushing by. With this in mind, chose a carb with a large enough throttle bore and venturi diameter to allow maximum airflow while promoting a high-speed signal to the carburetor. If the bore and venturi are too large, the pressure will be too weak at low RPM & idle and the fuel will NOT be effectively pushed into the discharge jets. The opposite of this is true when the bore and venturi are too small for the engine's airflow demands at higher RPM.
Here is a formula to calculate the range of the throttle bore diameter for your engine. Remember, however, that the ideal carb is found by trying and error, the formula is just the first step.
D = K x square root of (C x n)
D= throttle bore diameter, in millimeters
K= constant between 0.65 and 0.90 (determines the smaller and the
biggest diameter respectively to be tested for the specific engine).
C= cylinder displacement, in cubic centimeters: the Radne 120 has
n= peak RPM/1000. My Radne's peak RPM is 9400 RPM (Prop: 53.5" x 22"), so n = 9400/1000 = 9.4
Therefore the functional range of the throttle bore diameter:
D= K x 33.4
Dmin.= 0.65 x 33.4 = 21.7 mm minimum throttle bore diameter
Dmax.= 0.90 x 33.4 = 30.1 mm maximum throttle bore diameter.
For soaring purposes, I value the comfort of a slow and steady idle to explore the thermal I'm in so, initially, I chose the WB-42 carb for its small venturi as it sends a greater low-pressure signal at idle.... this compromise was at cost of reduced maximum air intake at full RPM. But once I discovered the throttle plate modification for
slow and smooth idle,
then I did not have to compromise any more on small venturi or throttle diameters at cost of power, so I purchased a WB-39 carb (28.3 mm venturi and a 30 mm throttle bore diameter); I had to add a choke to it.
Note that the carburetor's plastic mount flange, its gasket and the cylinders intake bore have to be enlarged accordingly in order to match smoothly the larger throttle bore diameter surface area.
Air Flow Requirements:
The Radne 120 engine (118.8cc) at wide open throttle “ingests” an air volume of about 15.1 CFM (Cubic Feet per Minute). The WB carb series were originally designed for kart engines of up to 150cc displacement so their large bored carbs should be able to handle a maximum air volume of about 23.4 CFM.
Here is a spreadsheet indicating specific Walbro carb models that you may try:
Carb models to try.
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