Help ! . . . Johnson 8hp Dies After 20 Minutes !

[Kapena]

Whimsy had the correct post. You have a tiny piece of dirt in the main jet. this is a simple fix. Take the carb off then remove the bowl on the bottom. There is a brass metering jet screwed into the bottom. Unscrew the jet and hold up to the light so you can see through the metering hole. Clean the dirt out but don't use a hard metal pick. Pulling on the choke creates a vacuum that forces a little more fuel into the engine but it is a losing battle. This is a very common problem, more so with small hp 4 strokes as the jet is very tiny.

It isn't a cracked float...The engine would die but there would be fuel all over the engine from venting out the carb and pulling on the choke would always make it worse.
It isn't a fuel pump problem as pushing on the bulb is effectively pumping past (through) the pump. It will run several minutes for each push on the bulb.

[roger.waite]

Noting you have it running now, it might still be worth considering cleaning the cooling system with SaltAway or similar. You know the history of your engine, and whether it has run enough to salt up ...

I had a similar problem on a Mariner 15 HP, and when I stripped it down found the cooling path around the head was completely salted up, and the cylinder channels were heavily salted. Cooling water was reaching the telltale via a relatively narrow, salt encrusted channel next to the cylinders. So despite new impeller, the cylinders still overheated.