posted 07-30-2010 10:01 AM ET (US)
Re the possible cause of the stalling being due to an ignition spark coil or the drive belt:The engine stalls when the fuel-air mixture is no longer combustible. There are three elements to the combustion:
--spark
--air
--fuel
If you can verify that you have good spark, you can eliminate the spark as a source of the problem. To test for spark ignition at each spark plug, insert an in-line spark tester into the spark plug wire lead. If there is a loss of spark when the engine throttle is increased, it should be noticeable while observing the in-line spark tester. The presence of good spark would tend to eliminate an ignition spark coil as being the cause of the problem.
The "drive belt" operates from the engine crankcase and drives two important auxiliary devices, the automotive-style alternator and an air compressor. The air compressor is critical to operation of the OptiMax engine, which relies on the Orbital Combustion Process, a technology developed by ORBITAL, an Australian company, and used under license by Mercury in their OptiMax engines. The air compressor supplies air at an elevated pressure to a distribution system which feeds electrically operated injectors that push the air into the combustion chamber. If the air compressor is not able to produce air at an elevated pressure, the Orbital Combustion Process will not work properly. This could lead to stalling, as it affects the fuel-air mixture.
If the "drive belt" is not operating the air compressor properly, the drive belt could be the cause of the stalling.
To proceed further in testing and diagnosis of the problem, it would be helpful if you had a factory service manual, and any specialized test equipment required. Mercury engines tend to use a proprietary digital diagnostic test tool, which is expensive and often difficult to obtain.