The Mustang I which the 309 flew was conceived and built by the North American Aviation to specification issued by the British Purchasing Commission. It was to be an escort fighter for the RAF bombers. However with the excellent airframe, the aircraft as powered by the Allison V-1710 liquid cooled engine, which performance and high altitude was insufficient. The same engine equipped with gear-driven superchargers gave great performance to other aircraft, notably P-38 Lighting.
On November 12, 1943, flying out of RAF Wellingore, S/Ldr M. Piotrowski carried a shipping recce out along a Dutch coast. About 40 miles from the British coast, his Mustang (AM211) experienced engine trouble. The engine started to vibrate and cut out, then it picked up and the pilot managed to climb to 800 feet where the engine speed was falling again. He jettisoned the hood, released his harness and prepared to bale out. On the engine suddenly picking up again he climbed to 2,200 feet. The engine cut twice again and the aircraft lost height and speed but again recovered. Piotrowski managed to reach the coast and was vectored to Ludham Airfield. Over the Airfield the engine again cut out but he managed to land without damage to the aircraft. After the engine examination performed by the technical Officer, a thick black deposit found on the sparking plug points was found. After all plugs had been changed for the new ones, the test of the engine proved satisfactory.Frequent spark plug change was not to be a simple solution to the problem. Many other 309 pilots continued experiencing constant engine troubles, and as a result, the operational flights were suspended until problems had been identified and corrected.
On the November 20, S/Ldr Tillman (Group Engineer Officer) accompanied by S/Ldr Durant (Station Engineer Officer Digby) and Mr. Johnson (Allison Engine American Representative) visited the Unit and conferred with S/Ldr Piotrowski and the Echelon Engineer Officer regarding engine trouble experienced with Allison engines. As a result of the conference it was decided that the cause of failure was lead forming on the plug points due to engines running at too low a temperature. Arrangements were made to secure blanking shields for the radiators, and tests to be carried out. Eventually Poles were given the Mustang III version, powered by Packard V-1650-7 engine, produced under the Merlin license.