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Checking Mixture

CubCrazy

SPONSOR
Bentonville AR
I suspect my Cub is runing lean with the mixture control at full rich ,How much should the RPM Increase when the mixture control is moved to cut-off and at what RPM should this be checked at ? thanks in advance

Ben
 
When I last calibrated mine, we idled at 1000 rpm for a minute, then pulled the mixture to idle cutoff. RPM rose to 1100 and then dropped as the engine quit.
 
are you refereing to idle mixture or running mixture? Also what altitude are you at? Idle mixture is checked as stated above, running mixture can only be set on a flow bench and is usually checked about 1800 to 2000 RPM. Altitude makes a big difference there. At sealevel you may get a 50 RPM rise where at 6000 ft you get about 250 RPM.
 
I Am at 5000 ft Elevation and the engine is the 0-320 . When pulling the mixture the RPM is increasing barely 25 RPM at 1000 RPM . I have tried adjusting the Idle mixture screw but this does not seem to have any effect . When leaned at 1800 RPM the result is much the same. I have checked for and repaired an intake leak and I am pretty sure that it is secure and had the carb looked at at a overhaul Facility . wich they modified from a 10-3678-12 to a 10-3678-32 . this has made no difference . The only reason that I suspect this is that the Plugs are burning more white than brown /Grey when running at full rich otherwise the engine runs and idles fine

Thanks
Ben
 
idle mixture

Ben: The idle jet(bleed hole) is it the upper throat of the carb. the throttle valve edge must be near this bleed hole(almost closed) for the venturi effect to draw fuel from it. Once you advance the throttle valve opening away from the bleed hole it no longer is the controlling force for the fuel mixture. You must check your idle mixture at Idle RPM. Not at 1000. Try it around @550-600 you are looking for 25 to 50 RPM rise....

P.S. The 1000 RPM check is for injected engines that use a different type of idle fuel metering system.
 
Thanks

I will go and recheck during the week , I am perhaps just being a little cautious as I don`t have a EGT

Ben
 
CubCrazy said:
I Am at 5000 ft Elevation and the engine is the 0-320 . When pulling the mixture the RPM is increasing barely 25 RPM at 1000 RPM . I have tried adjusting the Idle mixture screw but this does not seem to have any effect . When leaned at 1800 RPM the result is much the same.

When not leaned at 1800 rpm you should be able to get about 200 rpm by leaning the engine. this would show your mixture is fine for running. The plugs being white could be the type fuel you are using.
 
I am running on avgas 100LL , I have heard that it is not as easy to "read" the plugs with this type of fuel as with the older leaded fuel .
 
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