Even on my oldest SC/ASP .12 there is still a pinch around TDC that is easily felt with the plug out. These engines are true ABC and run fine on straight synthetic fuel too.
With a second hand engine you never know how it was treated, but you could check for a pinch around TDC with the plug out, or leave the plug in and check how long it holds the compression at TDC. On a cold ABC engine there shouldn't be any leakage at TDC at all.
There are many other possible sources for poor running. You could check the head gasket (or add one if there are none) and the rear crankcase gasket too.
On my best running engines I have added an extra shim under the head to avoid over-heating. Even on 10% nitro I think the stock compression is a little too high for props around 7x4.