The ECU's idle regulation is different cold than warm, and the fast idle thermo valve plays a part also. Cold idle is higher RPM and more aggressively regulated, with ligher power output at that RPM, so the flywheel weight isn't really as big of a deal.
Again, there isn't any reason that it can't idle just fine with that flywheel, it just has to have components that the ECU can use in stock form, or it needs to be tuned. If you want to drop a little money, a wideband O2 sensor might be a nice investment to really tell you whats going on when you make fuel system changes.