MA4-5 Idle mixture adjustment

[Stan Bahrns]

Somewhere I found a very detailed method of adjusting the idle air mixture on The Marvel Schebler MA4-5 but can't find it back now that I need it. Would anyone out there have some good instructions? I just purchased a rebuilt carb from Spruce & want to set it properly. It idles & runs just fine but have had a little rich problem with my old carb & want to make sure it is right. It is possible that I don't have to do anything but if I can get good instructions I will check it.
Thanks in advance
Stan Bahrns
RV7-A 350 HRS
RV12 Ready for first flight

 

[az_gila]

This may help -

http://www.eaavideo.org/video.aspx?v=86367876001

 

[Mike H]

The idle mixture adjustment screw is a lage knurled screw on the outside of the carb body and is easily adjusted. The MA4-5 has in internal "enrichment circut" that can be adjusted if you are experiancing a stumble during acceleration.

To set the idle mixture you need to have the engine at operating temp, preferrably after flying. Then with the mixture knob full rich stabilize at idle of 1000 RPM or less for at least 30 seconds then retard the throttle all the way to idle. Once stabilized at idle quickly pull the mixture to cut off and watch the tach. If there is no RPM rise when the engine shutts off the it is too lean. If the RPM rises more than 50 than it is too rich. Adjust the mixture screw clockwise to lean, and counter clockwise to richen the mixture. Try for 25-50 RPM rise at shut off.

 

[CamTom12]

Question for the smart folks:

This process was designed for mags running fixed timing at low advance, like 20-25* for most engines I think. I used the technique above to set my idle mixture but I have pmags and was probably around 33-36* of advance if I had to guess. It still seems like I’m running rich at idle. Should I pull their MAP signal (fixing timing to 25*) and try it again?

Thanks!
Cameron

 

[lr172]

Question for the smart folks:

This process was designed for mags running fixed timing at low advance, like 20-25* for most engines I think. I used the technique above to set my idle mixture but I have pmags and was probably around 33-36* of advance if I had to guess. It still seems like I’m running rich at idle. Should I pull their MAP signal (fixing timing to 25*) and try it again?

Thanks!
Cameron

advance should have no real bearing on proper idle mixture adjustment. retarding the timing will just drop the idle RPM, but that has nothing to do with proper mixture adjustment.

Larry