I thought the starter battery should be a 6 cell (7.2V) pack? This may help you as the starter motor won't spin so quickly during start.

The starter is turned off at about 35,000 RPM. If the engine does not flame out until 55,000 RPM then I think you have another problem. The only two flame-outs I've had with the 44 were caused by air in the fuel line. Check all your connections and have a look to see if there is air in the line after you have removed the starter.

You say the engine was spluttering. Was this just as the engine started or did it continue throughout? If it continued this may indicated air also.

Also check that your ECU battery is fully charged.

Let us know what happens.