Question 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

 

This started out as a hypothetical scenario. The fear that what I say can and will be held against me. But we all know that I am talking about my aircraft.

I have keep our local IA in the loop and he has supervised me in this process. The airplane goes into annual inspection October 1. That said, I have been talking to several people about what RPM they use to test the fuel mixture setting at idle/shutdown. I have found that the responses are varied as to what RPM they set before moving the cockpit mixture control to cut off. Some say 1000 RPM, others say Idle.

When I preformed the test at 1000 RPM I have not noticed an RPM increase. The RPM just dropped
dead.

Yesterday I did two shutdowns at Idle (600) RPM. As soon as the mixture control hit the idle stop, the engine felt like it got smother. The RPM had an increase. Not much. It felt like the engine's lungs took one last dying breath and exhaled.

Poor dead Cherokee. I shall bring it back to life and a week or so. Right now I feel satisfied that the carburetor mixture setting is at least close to correct. 2.5 turns from the stop. The airport elevation is 1000 ft MSL and the temperature was about 85 F.

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 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

Let us suppose that the new A&P on the airfield is fresh out of the military. Almost all of his/her experience is on modern helicopters with turbine engines. This airfield is at sea level. A new tenet on the airfield has just acquired a PA-28 180C from an previous owner who hangared the Cherokee in Denver Colorado.The new owner reports to the A&P that his EGT is off the peg most of the time and it doesn't take much leaning to peek the needle. The owner also says that when shutting down the engine there is no rise in RPM as the mixture is pulled to cut off. The assumption is that the Marvel-Schebler MA-4-5 Carburetor needs its mixture reset to sea level.The new A&P says he needs to study up on carburetors. What advice would you give to this mechanic? What study materials would you point him too?