Any ideas on if a small crack in the balance tube on a 470-50 could caused the #6 cylinder to have both plugs to run rich? The CHT on this cylinder averages 150+ degrees low and the EGT's average 300-400 degrees lower than the rest. Both plugs on the #6 cylinder are black and
sooty and the rest of the cylinders look great. The engine is still in break in with only about 15 hours on it. Compression on this cylinder is currently at 76 psi.
Everything had been going great with the break in, only used half a quart until its first oil change at 5 hours, changed it again at 10 hours with no oil burned. Today I had to turn back to the airport slightly after trimming out into a cruise due to the engine started running very rough, seemed to smooth out when adding power but became rough again when power was reduced to 23 sq. Carb heat had no affect, switching mags made no difference. The only issue I found so far is a small crack in the weld on the balance tube that is leaking.

Ive had a bit of trouble with my P-ponk, IS It cold where you are running your plane? 20 degree's or less outside. There is a chance the balance tube is freezing. I zip tied a piece of fuel line fire sleeve over my balance tube to help prevent this. If the crossover tube is cracked it should be repaired or swapped out for a good unit. I would think a crack would lead to and air leak and a lean cylinder not a rich cylinder. All of my issues were from running to lean and a small intake leak that I never really did find. I took everything apart and re-assembled. and now it runs fine. Too rich is the opposite problem I had, maybe none of my info is helpful. Hope you sort it out. This stuff can be a nightmare.

I do love the Pponk and appreciate the work Steve did to make a 520 “economical” to install in in a non fuel injected Cessna airframe.

That being said the intake tract is a bit of an Achilles heel, there’s good reason continental starting injecting these engines as they got into the large displacements with long intake runners.

Careful attention to intake leaks is paramount in these engines to keep them running right. There’s also been much swapping of forward crossover tubes over the years with most of us settling on the smallest diameter tube offered by continental as correct.

Steve was famous for spending hours during the build up process for mixing and matching intake castings to find a set that lined up properly. I attempted to do the same when building my engine, it’s hard to do.

All this is to say yes, an intake leak can do weird things to a Pponk.

Sam is exactly right on intake leaks.

I was having weird issues with my PPonk a few years ago.

We tested for induction leaks and fixed a few by overhauling the carb and some other fixes. Made a huge difference, and I've had 100+ problem-free hours since then.