What rpms are you seeing?
On my engines the conrod moves too far back and wears against the backplate. With time the initially round conrod will have now have a flat part facing the backplate, and the backplate has a distinctive wear from the conrod position from about TDC and 45° down (in the direction of the running engine). I solved this by placing a thin, and bent, washer on the gudgeon pin, between the conrod and the rear part of the piston. There is still enough room to easily slide the conrod over the crankpin during reassembly.
With several extra head shims it should be able to handle a little more prop load than the 5x3, but a 6x3 is likely around the highest load (cox props also have larger blades compared to APC for instance).
Why do you say that it has low suction at the NVA?
1.5-1.75 turns on the needle is not much compared to other engines. But if you have a "suction problem" my guess is that it is not in really a suction problem but rather a fuel delivery problem. All of my engines (four red head .061 RC and one blue head .049 RC) have had the issues with the plastic nipple that've described earlier.
Last edited by Mr Cox; 01-08-2018 at 12:03 PM.