Limp mode. Will not throttle up.

[Richard.Burns]

Hey guys, new to site. Thanks.
ive got a 1992 ProV 150 Yamaha (carburetors).
my first time out, it went into limp mode. The motor would idle, but not throttle up. The motor would shake and surge.
i went home and it did the same on the water hose. I changed the water pump kit, new thermostats, and cleaned the heat sensors. Also have new gas lines, and fresh gas. I have great compression, good fire to coils, and all wiring looks good. It starts this when it runs 5 minutes or so. Alarm does go off, so I shut it down. I believe it is a heat issue, but not sure?
thoughts?

[99yam40]

did it sound the alarm when it 1st went into RPM reduction?
Did you test the over temp switches to make sure they operate in spec?
did you measure head temps around where the temp switches are mounted to see if one or both are getting hot in those areas?
those infared guns are not too expensive, or use you hand to check if one is hotter that the other.

how did the cooling passages look when you pulled the stats?
does that old 150 have a PRV that can stick open and dump too much water at low RPM?