More Carb Problems

[CubCrazy]

Seems that the general problem with carbs is running rich, my problem is running lean.
Engine runs fine and idles well (O-320 A2B) this only came to my attention when I pulled the plugs to do blow by`s. This is a new engine that was locally overhauled, and has around 35 hrs on it. On rear cylinders - 3&4 the plugs have a distinctive white residue, the forward cylinders 1&2 have a normal grayish/Brown residue .I have done the normal visual checks for intake leaks on Clamps , hose condition ect also tried Supercub MD suggestion with the grease on the intake joints but nothing obvious . there is no primer system fitted ,and have checked the blanks fitted for possible leaks , I have been running full rich in this condition ,Our field elevation is around 5000ft which also means that this engine should be more on the rich side . Carbs is the MA 4SPA but the pn is 10-3678-11 Seems it should be a -12 could this be my problem? Is there a reason for the rear cylinders running leaner than the forward cylinders?

Thanks Ben.

[Fortysix12]

There are two things I would check.

1. Check the jet size if applicable.
2. Check for cracked intake tube flanges and varify the fit with the couple adapter and gasket. The seat area can wear so that the flange and gasket no longer seal. Start with the two leanest cylinders. Shake the intake tube with a firm grip back and forth. It should be tight.

When idling at 1000 does your engine rpm rise 100 rpm when leaning to stop engine. Lycoming said this should happen all the way to full thottle. Could it be something simple like the travel on your mixture cable. just some ideas.

[T.J.]

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