[Valiente]

Also make sure that your cable is running smoothly and that you are positively in gear. It make sound obvious, but the forward "detente" on an A4 needs a push to fully engage, and cable tweaking is often necessary, as is cable lubrication.

Of course, if you have crew, they can physically pull the gear lever with their hands (pick a crew with short hair!) pretty easily. If the problem persists, you need a transmission adjustment. If it doesn't, check the cable linkages.

 

[Valiente]

That could be a partially occluded tank vent (like with spider webs). The tank can't vent properly and eventually overcomes the mechanical fuel pump and the carb is starved. Because it's only a partial blockage, tank pressure equalizes under little load. Dissemble the gas tank vent and blow it out. If crap comes out, that's the likely answer. Switch to a larger diameter hose (I use 5/8" ID) and get a fine mesh vent screen.

If it doesn't, change the fuel/water separator or fuel filter. You should do this anyway.

Lastly, crud in the tank itself can gradually occlude the pick-up tube. Same deal to fix that...

 

[Valiente]

Good to hear it. The A4 is a simple beast, but being simple, it is easy to blame for things not necessarily its fault. Some TLC and a bit of knowledge will keep it running for years in the typical sailboat regimen of "motor out, head to weather, sails up, engine off....repeat in reverse coming back". A low-compression gas engine (properly ventilated with a blower, of course) is a better choice by far for the typical sailor who uses the engine for only short periods. A diesel is best left on, once on, at a cruising RPM for hours, such as a motorsailer would do.

You might want to delve into local lore regarding prop treatments and bottom paints in your area. The brackish conditions seem to be very challenging in the Chessie, from what I can gather, and it may be that the best solution is simply a weekly scrape.