My 1987 470 stranded me in the lake last weekend. I pulled the kids in the tube for half an hour then stopped to swim. The engine fired without problem but stalled in gear when I applied any power. Out of gear it rvs fine but no power when in gear. My first thought was fuel delivery but the filter is clean. I discovered the 2 month old battery was down to about 25% of capacity. Could this be that the generator/rectifier is failing and can’t keep up with the higher spark demand?
Does the engine run at idle when in gear or does it stop dead?? Can you bump the throttle up in small increments when in gear and will it run while doing this?
You might have an accelerator pump problem in the carb. With the engine off, remove the flame arrestor from the top of the carb, hold the choke open and have some one work the throttle to give it gas. You should see two good streams of gas from the accelerator pump nozzles. If there is a weak or no stream this will cause the engine to bog when you open the throttle. There is also a fuel filter behind the fitting that the fuel line from the pump goes into. You have to remove the fuel line at the carb and remove the large fitting on the carb. Fuel pump pressure is 4.5 to 5 psi and should be checked. Most fuel tanks have a filter at the bottom of the fuel pickup tube and these can clog. As for the battery, I wouldn’t think it would cause the problem you describe, but you would do well to replace the engine charging system with an alternator kit as it seems to be working only somewhat, the kits are available on the site. There is a plate on the right side of the engine bolted to the exhaust manifold that has the shift cable from the drive and from the shifter attached to it. When putting the engine in gear the switch on the plate should NOT be actuated, that switch will kill the engine if it is actuated. Its actual purpose is to blip the engine off so the gears in the drive will unload so the drive can be shifted out of gear.
To add to what Stractor said about the shift plate. That switch will kill the engine for a split second when taking the drive OUT of forward or reverse, If that happens, the rearmost shift cable is the likely candidate due to binding
The filter in the carburetor was clean, I’ll check to see if there is one in the pick up tube in the tank. Hadn’t thought about checking the shift kill switch, I’ll check the adjustent. The engine would idle for a short period of time when in gear and then stall. Accelerator pump was working earlier, but I can verify once I get fuel back in the carburetor. What’s the best way to check fuel line pressure?