Carburetor adjustment on the Minari 180/200
by Had Robinson
Proper adjustments takes patience and time. The info here assumes that the carburetor (Walbro WB37) has been rebuilt within the year or is new. If there are any doubts, go to the Walbro WB page for specifications, midrange problems, and location and identification of parts.
It is ALWAYS safer to run this engine too rich than too lean!
A. High speed needle adjustment
If you are careless adjusting the high speed (HS) needle, you can burn up your engine. You should install a CHT on the engine in order to help prevent disasters. Normal operating temperature are 200ºC-220ºC. When a cooling shroud is installed the operating temperatures drop dramatically, often to under 140 ºC. Minari allows operating temperatures of 220ºC-260ºC but I do not recommend it. It is so easy to overheat these engines and ruin them.
After you make the high speed adjustment and have flown the engine a short time in the air, Minari (and this mechanic) recommends that the spark plug be removed and checked for the color of the last 5mm of the center electrode. If the color is a light brown or white, increase the fuel/air mixture by turning the HS needle 1/8 turn CCW. If the color is black, turn the HS needle 1/8 turn CW. Always decrease or increase a needle adjustment by 1/8 turn but no more. Then test the engine in the air to see if further adjustment is necessary. The HS needle is VERY sensitive and 1/4 turn CW could result in the engine overheating.
Sea level adjustment
This table gives the safe range for the approximate setting of the HS needle at the respective ambient temperature. The needle is turned CCW after being fully seated CW. The turns are expressed in minutes where 60' = 1 turn. This table is from the Minari user manual. It is always better to set the needle at the maximum for safety. After the basic adjustment is done, the setting can be fine-tuned.
Temp Cº -10 0 10 20 30
MIN 65'
60' 60' 55' 50'
MAX 90' 85' 80' 75' 70'
Close the main jet by 1/32 to 1/8 turn and fly the engine at full throttle briefly. If the CHT remains at or below 170º C, you have leaned out the fuel mixture sufficiently to gain a small amount of power output and smoother running. Depending on the CHT, you may have to make the mixture leaner or richer, as required.
High altitude adjustment
You must have a CHT installed to adjust your carburetor at high altitudes. It is also a good idea to install a tachometer at the same time. Miniplane-USA has all of these items. Follow these instructions for doing the basic adjustment of the carburetor. Adjust the low speed for your altitude and then close the high speed needle 1/8 of turn, fly the engine, and observe the CHT. If the temperature is safely below the maximums, close the needle another 1/8 of a turn. The basic purpose is to achieve a smooth running engine at all speeds. It is always better to run too rich than too lean.
B. Low speed needle and idle speed adjustment
The Minari manual specifies that the low speed needle be set to the same number of turns as the high speed needle. This is not particularly helpful. This page has the information that pilots need to correctly adjust the low speed system. It is easy to do. DO NOT INCREASE OR DECREASE THE LS NEEDLE FOR MAXIMUM RPM. If you do this, the engine will stall when you apply throttle quickly. In any case, snapping the throttle open in 1/2 second will always stall the engine, especially clutched engines. It takes about 1.5 seconds to open the throttle fully without causing stall. Keep this mind when launching.
C. Burned up engine
Here is an example of a pilot who adjusted the high speed needle incorrectly and burned up his engine. The chrome plating turns blue around 900 degrees F. The exhaust gases right at the exhaust port were probably 1,300ºF - 1,500ºF. That the chrome plating turned blue so far down the exhaust pipe demonstrates just how hot everything got. The piston seized and the entire top end of the engine was destroyed. Do not be careless or ignorant when adjusting your carburetor!
This photo is of the same engine. The lack of the right kind of oil and lean running condition got the lower connecting rod bearing (red arrow) so hot that the bronze rear side-bushing began to melt and disintegrate. The engine will have to be completely rebuilt – an expensive and unnecessary mistake made by the pilot.
