I have an exmark 48 hydro walk behind. Not very many hours on it. I can't seem to get full hp out of the engine. Not sure what happened, but at full throtle it seems to be at maybe 2/3 power. Really bogs down. I thought it may be the plugs, but I changed them and it still does the same thing. It starts up fine, idles fine, doesn't make any strange noises. I checked the linkage and there is full travel in the throttle. Any ideas?