Yes, the plug does make a difference after the engine is started. Not as big a difference as you are experiencing though. Check to make sure the throttle is actually opening all the way. Since you have recently switched engines, and adjustment to the linkage will likely be necessary. If you have allready done that, assume it is in second gear and tighten the adjustment screw a little. The pinch test for low-speed needle adjustment, must be performed after main needle is set. Main needle (high speed) adjustments make mixture changes accross the entire throttle range. Pinch test at idle will reveal mixture conditions at idle, but does not isolate which needle needs to be adjusted, unless the main needle is set first!