When was the last time you re-torqued the head studs????
If you evaluate and find its NOT an occluded vent, do a compression test *after the engine reaches full operating temperature*. Your signs and symptoms to me would indicate (at worst) a head gasket that is beginning to 'blow' (usually between #3 & #4). The indication here would be that under full load the engine wants to stall but under 'no load' (trans in neutral) you can reach max rpm.
Alternatively if you have *any* dirt on the emulsion tube in the carburator (an approximately 1" long tube with closed end and a few 'holes' drilled into the side of the tube) you can also result with erratic performance similar to that which you report. The emulsion tube is responsible for 'metering' the correct air/fuel mixture in the carburator. Any rust that develops in the fuel delivery system usually gets 'caught' on/in the emulsion tube. Many times when you change the 'fuel filter' you can break small particles of rust free which then get 'collected' on the small holes of the emulsion tube.
Usually the tank vent line will terminate in a small 'clamshell' that incorporates a fine monel or stainless screen (spark arrestor) and is mounted somewhere near the stern of the boat .... spiders LOVE to build their nests in these clamshells and the spider nests in the vent externals will block the vent.
hope this helps.
Last edited by RichH; 07-29-2007 at 02:33 PM.