I know this is an old post, but I found the solution to this problem and since it was never mentioned in this thread, I’d like to add it.
I have this 5hp
Mercury motor, which is normally quite a good unit. After storing it dry of fuel last year, for the first time in many seasons, it demonstrated flooding when I first started it that spring. I removed the carburetor and examined the needle, seat and float; all was well. I reassembled and the
engine ran fine for the season. This year I experienced the exact same problem. I took the carburetor apart again, and again the needle, seat and float were fine. But this time upon reassembly the
motor continued to flood – what gives?
After disassembling again, I examined the fuel feed and the bowel more carefully. The fuel pumps into the carburetor body through an opening in both the bowel and body and into the needle seat. What had happened was that the bowel
gasket had dried out right at the point where the fuel
passage goes through. I believe what is happening is that the leaking
gasket allows the fuel to go into the bowel directly, bypassing the needle valve, thus flooding the engine.
Using a small knife, I scraped some
corrosion off the carb body and the gasket. The gasket is rubber and scratching it just a bit swelled the rubber a little. Upon reassembly, the flooding was fixed. The motor is running fine now and I have a new float bowel gasket on order, which I will install when it arrives.
The problem is not the needle, float or seat. It is the gasket. Take a close look at it.