Ive an OS50SX thats instaled in my Blackhorse CAP232. It was run in as per instructions on the ground first plus one tank and performed flawlessly on the trainer I used to run about a gallon of fuel through it in flight. The first few flights installed in the CAP were flawless as well.
Yesterday it was sort of surging a bit at full throttle, maybe a few hundred rpm. Today it was surging quite noticeably. On the ground it idles well, although I think the low end needs to be a tad (1/8 turn at most) leaner, but on applying full throttle it picks up well then after say 5 or so seconds, really peaks the RPM. I notice several bubbles in the fuel line from the needle to the carb ONLY, not from the tank side, during this first few seconds at high rpm then the bubbles remain but get really really small as the RPM peaks. On the ground it holds a constant RPM at full throttle but surges mildly at mid range to idle RPM's. Its almost as though it doesnt reach a good peak until tank pressure has had a few seconds to build up and push the fuel out. The bubbles and lag to hit peak RPM is repeatable by reducing to idle for about 10 secs and opening throttle fully again. The needle valve has been set at 2-3 clicks rich of peak RPM, to a full turn rich of peak in 1/4 turn increments, with no difference in-flight to the surging problem. Flying inverted passes doesnt show any difference to the problem so its not a hole or leak in the clunk line above the normal attitude level.
Ive read loads of threads here about different problems people have had and subsequently checked the needle valve for obstructions, NONE found, tightened all the nipples on the remote needle assembly, none were obviously loose, checked the fuel lines for leaks, nicks or cuts and found nothing but unscathed clean fuel line. The needle valve O-ring 'appears' unscathed and in good condition, its never been out until tonight. I am using a DuBro fuel filler, that lots of people say they had nothing but problems with and others say never had a problem with, I have NO bubbles from the tank side of things so Im assuming its not a culprit here. I can disconnect the pressure line from the muffler and the line from the tank to needle valve and blow through it quite easily so there doesnt 'appear' to be any significant restriction in that department.
Its running an APC 11x6 prop for run-in and still is, but have an 11x7 to try (another club member with same engine a simlar weight plane tried one and wasnt as happy with it as the 11x6) and an 11.5x7. I recently heard of club member learning the fix for a poorly running engine and tuned pipe setup, was a bigger prop to load up the engine more. Is it worth a try here ? No tuned pipe though so maybe not. OS recommend 11x6-10 or 12x6-9.
I have also noticed some/a lot of castor oil residue on the muffler, it appears to be coming from around the muffler to engine manifold area and also some residue on the head itself from around the glo-plug gasket area. The glo plug is in tight and gasket/washer appears in good condition and the muffler bolts cant be tightened anymore (or loosened for that matter, the hex head has stripped out on one bolt so it'll stay on until absolutelty necessary to remove with brute force). Possibly its from a spill whilst hot or ? ? Ill clean it off and see if it returns tomorrow, though I doubt this part of my problem.

So far I think Ive covered all bases to solving this but need to do more in-flight testing tomorrow but if anyone can think of something Ive not done PLEASE PLEASE let me know. Other than cruddy comments about OS engines in general and what I should do with it or put it, as Im well aware there's a large percentage of people here who despise OS engines.