Marvel-Schebler MA-4-5 Carburetor Setup

Hi Arthur,
There are a couple of factors at work here.
I interpret the service manual to say that the idle mixture is tested at 600 rpm. Pull back to that rpm, pull the mixture control aft–watch the tack for a slight rise. The catch is that our analog tachs are sloppy. I can hear that little rise better than I can see it. 

However, years ago there was a suggestion from Cessna and Lycoming that a momentary rpm increase to approximately 1500 rpm prior to shut down would lessen the amount of lead on spark plugs. That was before the lead in avgas was reduced, which created 100 Low Lead (LL). Today we don’t hear of many lead fouled spark plugs so that practice has been laid aside for the most part. 

As I mentioned before, until the rpm increases to around 1400 rpm the fuel metering is still done by the idle circuit in the carburetor. I tend to do my idle mixture checks are around 800 to 900 rpm. 

But I do lean the heck out of my mixture during start, taxi and during run up prior to taxi.

Best,
S

Hi Arthur;
The fuel flow through the carburetor at approximately 1400 rpm and above depends on the amount of low pressure (vacuum is a better way of looking at it) developed by the airflow through the venturi of the carburetor, so that’s pretty much fixed. 

A high EGT meter reading is that–a high EGT reading. It doesn’t necessarily mean the EGT is high, it just means it indicates hi on that meter. EGT indications are primarily used to determine the peak EGT for leaning purposes. 

If the operation of  the carburetor is suspect, you can send it to http://www.MSAcarbs.com. It will test it on its test bench for free. 

The adjustments that a mechanic can make are the idle speed and idle mixture. 

The method I use when setting the idle mixture on a carburetor is to screw the mixture adjustment knob in a clockwise direction until it stops–gently, gently. Then turn it counter clockwise 1 1/2 turns. Unless there’s something else wrong with the carburetor, the engine should run at that idle mixture.

The final idle mix setting is done while the engine is hot, The following is from the service manual:
“b. Draw back on the cockpit throttle control lever to obtain a reading of
approximately 550 RPM on the tachometer.
c. Turn the idle mixture adjusting screw (1) located near the rear of the carburetor,
clockwise, leaning the fuel mixture. Continue to do this until the engine begins to run
roughly, at which time the engine speed will decrease.
d. Turn the screw counterclockwise until the engine runs smoothly again. Continue to
turn the screw in the same direction until the engine begins to run roughly once more. At
this point, the fuel mixture will be too rich and engine speed will decrease again.
e. Now advance the screw to a midway position between the lean and rich fuel
mixture; the RPM of the engine will reach a maximum speed for idle mixture settings”

After this initial setting, adjust the idle mix knob CCW or CW to get a 50 RPM rise at idle RPM (550-650 according the the manual) when mix control is pulled all the way aft. 

The idle mix will probably need to be reset at least twice a year to account for seasonal changes. 

Please let me know what you find.

Steve