The pilot's failure to declare an emergency following partial engine ignition failure contributed to subsequent track deviations while operating on an IFR flight plan in IMC.

Narrative

Shortly after takeoff; Tower turned me over to Departure; where I was climbing as instructed from 2;000 FT to 3;000 FT; when I had a magneto failure. I informed Departure and requested a return to ZZZ. Departure instructed me to turn to a heading of 360 degrees; descend to 2;000 FT and asked if I was declaring an emergency. Although the aircraft's engine did not appear to be making full power; it was operating smoothly and seemed to be developing sufficient power to fly safely; so I declined to declare an emergency and turned to 360 degrees. Shortly thereafter; I was instructed to turn to a heading of 070 degrees. I turned to the new vector and shortly thereafter; I spotted the runway environment at ZZZ at about 3 o'clock position; through a broken layer in the clouds and requested an immediate VFR descent direct ZZZ. I was feeling quite stressed because of the magneto problem; the power loss and because of the variations I had seen in ZZZ's ceilings. (ZZZ's ceilings were quite ragged and had been reported at 400 FT; just before takeoff; but just off the west of the end of the runway I reported to ATC they were 1;000 FT on climbout.) Departure informed me that there was an aircraft on a 5 mile final and again asked if I was declaring an emergency. Again I said no; but asked if I could descend below the cloud layer on a right VFR downwind to ZZZ; which I could see through the broken layer. Shortly thereafter; I found the aircraft in a nose high attitude with speed bleeding off; and I abruptly pushed the nose forward; lost about 300 FT and leveled off at about 1;650 FT. Departure instructed me to contact Tower; who informed me that the aircraft on approach was going missed and cleared me for a visual approach to Runway XX; where I landed without further problem. About 2 hours later; I received a telephone call from TRACON. During this follow-up call; the caller asked me about my flight and I shared with him the problems encountered and the return to ZZZ. He then politely informed me that I had deviated from the vector assigned by Departure and suggested that after reviewing the tapes; that I should have declared an emergency. He said that the opinion of the TRACON was that I was under substantial stress and that because of the emergency; failure to follow the vector would not be reported. I stated that after having reviewed the situation upon landing; I clearly felt that I should have declared an emergency; and in any such future situation would not hesitate to do so. In the future; I will declare an emergency immediately; to avail myself of the help and resources available when I face such problems and will focus more intently on flying the airplane.