LAX84FA304

The plt of a quickie Q-2 had returned to the arpt & made 2 low passes, then began a normal apch to rwy 25. When the Q-2 was on a low final, the plt of a Mooney acft turned onto the rwy & began taking off. At about that time, a ground witness heard the following transmission over the UNICOM freq, 'thanks a lot mr. Mooney, next time you do that I'll chop your tail off.' The Q-2 was then observed to make a very abrupt left turn to reenter downwind - to-base. The Q-2 plt was then stated (on UNICOM) that the eng was sputtering & quitting & that he was 'going in.' During an attempt to land on a construction bluff, the acft collided with large rocks, then burned. No preimpact structural failure was found & an eng teardown revealed no internal problems. However, there were several eng modifications that had been made by other than factory reps. The throttle cable bracket was made of non-standard material & was deformed by heat & wear. A throttle cable clamp was missing. Throttle full on, but carb housing closed.