BF5A "chugs" when I put it into gear

I inherited a BF5A when I purchased my trawler last year. After running well on our first trip out, it suddenly starting chugging (running rough, unable to achieve full rpm) when I put it in gear. I replaced the fuel with non-ethanol with a stabilizer, replaced the carb, fuel filter, spark and changed the gear oil in the lower unit. It starts easy, idles well, and if I defeat the limiter on the throttle when it's in neutral, the rpms run up easily with no chugging. However, as soon as I put it into gear, it starts chugging and struggling to keep running. Any clues?

Good suggestion. I pulled the spark plug and the prop turns easily with one finger when in neutral. When in gear it takes quite a bit more force, but not more than I would expect to turn the engine over. No grinding or clunking. No difference between forward and reverse. Any other ideas?

No fishing line on the prop shaft, which makes sense given that the prop spun freely in neutral. Getting ready to pull the prop shaft. When I drained the oil out of the lower unit (I replaced the oil last week), the oil had gone from a nice honey color to dark and essentially opaque. I'm thinking that's got to be telling me something.

I pulled the prop shaft assembly and in spite of the discolored oil, the ring and pinion gears look good. No corrosion or other evident wear. There is one tooth on the reverse gear that had a little burr on it, but nothing significant. The o-ring on the seal was soft and pliable and no nicks, although I may replace it anyway. Since the lower unit looks good, spins freely in neutral and the engine runs great in neutral, and the drive shaft is always spinning when the engine is running, I think that eliminates anything having to do with the drive shaft or lower unit. This really feels like an engine issue.

You say that you replaced the carburetor. With a brand new one? This sounds like it COULD be a fuel starvation problem. It takes a good bit more fuel to idle in gear than in neutral.

Does your fuel tank primer bulb stay hard after you run the engine for a brief time? If not, that could be a clue.

Try running it with the cap on the fuel tank loose to ensure complete tank venting.

And remember....even if the carb IS new....it could still have a mis-adjusted float right out of the box.

N.E.W.
That CAN mean Never Ever Worked.

Something else that comes to mind that is NOT fuel related is the flywheel being on tight. When the outboard is placed into gear, the added load will cause the crankshaft to "thrust". If there is too much end play in the crank due to worn thrust bearings...OR if the flywheel is not seated properly on the crankshaft....then the relationship of the ignition magnet and ignition "module" will be altered and your spark could be affected. Not a "usual suspect" but just an out of the box thing you may want to consider if all else fails.

Good luck.

The rest of the story: It turns out that the little wire that connects the choke knob to the choke lever on the carburetor had a kink in it. Instead of sliding freely back and forth, the wire was bending upward right behind the choke handle. So, while it looked like the choke was completely open, it was actually never more than half open. The fuel starvation suggestion above was close, but it was actually an air starvation problem. The fix involved straightening out the wire so that it runs smoothly back and forth. No more chugging.

At least I now know more about that particular outboard than I did before.

Thanks so much for all of the advice.

Alright! Good catch! Thanks for the detailed update and goodonya for figgerin' it out.