Why not try pulling the mixture well back when you get to cruise. Pull it back until the engine is running rough and then push richer until is smooths out. That is what we did years ago before EGT gauges were common. Check your plugs later if you have no roughness problems.

Could it be a leaking primer system?
 

Recently I experienced a badly leaking primer system that caused my newly-overhauled carburetor to seem to run overly-rich. Fuel was continuously being leaked into the cylinders (bypassing the carburetor).

Perhaps this is your root cause but why it would take a half hour to manifest itself is beyond me. Mine was continuous and I could troubleshoot it on the ground.

--
Joe

Yes true, but I was trained (40+ years ago) that no leaning is necessary below 3000'.

Maybe things vary depending by how a carb was last overhauled. I'll clean up the plugs and try flying it using aggressive leaning, though that has never been needed in the last 1100+ hours.:confused:

I don't have a primer on the airplane.

Another old wives' tale that regularly shows up as false when put to the test.

If I ran full rich at 2000' i'd be burning 30% more gas than I need to and not going much (if any) faster.

On my other aurplane I installed a fuel flow gauge and sensor. I discovered that I could reduce the fuel burn by 25% with no loss in power at low altitude by leaning. That system cost me $500 but paid for itself in no time and probably reduced the amount of crud in my engine. So leaning can always be done for cruise regardless of altitude.

Yes but in my case it's so rich the plugs foul up in less than 1 hour and the engine is running so rough it's not safe to fly. This is something completely new, thus my concern. At only 2000' the engine should run perfectly fine at full rich without fouling.

Yes can do this, but this has never been necessary in the past, especially at a low altitude, thus my concern.

Thanks for the suggestion. I don't know about collapsing, but the filter is very old (but kept cleaned and oiled). Maybe it's worn in such a way as to restrict air flow enough to cause a too-rich mixture. I'll put a brand new one on and see what happens.

Possible Cause?
 

?. You said you recently changed fuel pumps! Check your fuel pressure is not
over 5 or 6 psi, This is definitely a possibility, as those carbs are very sensitive
to fuel pressure. Thanks, Allan..:D