1999 Johnson 70hp stalling at top speed

[nim713]

Looking for some advice. Took the boat out and when I go full throttle I'm up on plane fast, I get to 5000rpm and run wide open for a few minutes maybe 1/4 mile. Then it's like the throttle gets pulled right back and it stalls out. Ang suggestions?

 

[racerone]

Run with another tank and hose.

 

[oldboat1]

And monitor running temp.