And you would be amazed at how many new thermostats are no good, either stick open or closed or dont open or close enough. Get a new one again.

Try a merc 140* T-stat, if that doesn't work, try a 120* and if that doesn't work, start trying restrictor plates. You could also drill some holes in one of the T-stats.

Are you sure the engine is not running lean. Running lean can make it heat up in a hurry under load.

Could you be sucking the hose flat?

I would also try a different thermostat.

Good Luck
Bulldog aka Ronnie

I had a problem that drove me nuts like this on a bravo 1. On the transom plate look at the water inlet that is held in by 2 bolts. Looks like a small t-stat housing. Take that off and check for any obstruction. Often, the housing will corode constricting the plastic retainer.

overheating
 

I've had bad brand new thermostats before that have been defective. I now test any new thermostat by putting it in a pot of hot water to make sure it functions properly before I install it. I did have a recent problem however that you should be aware of....and that is pieces of debris from previous raw water pump impellers clogging the oil cooler on the intake side. Its a tough location to get to, and some guys drain the hose on the front side of the cooler and think that clears the system, but it doesnt. The input side of that cooler is in the transom end of it, and its hard to get to. The pieces will sometimes jam up the tubes that run thru the cooler, requiring you to push them out from the front with a coat hanger or welding rod, and sometimes they will just swirl around until they eventually clog up the flow tubes temporarily.... then change their position next time you start the engine. Do yourself a favor and check it....the results may be surprising. Good luck... Glenn

Probably a T-stat problem. Try getting a good temp thermometer and pot of water. Put the pot on the stove with T-stat in it and see what temp the T-stat opens and make sure it opens all the way and closes when it cools. New T-stat could be bad. Otherwise just try a new one if it was me. Way easier than checking for something else. Were you running the cross-over prior to rebuild? Any temp trouble prior to rebuild?
Just my .02 Good luck. Check the easy stuff first I always say.

If you call "Brownie" at Stainless Marine he will tell you that you must drill three 5/32" (I think; therefore call Brownie) holes at the appropriate places in your stock thermostat if you are using a crossover system. That guy has forgotten more than most of us will ever know put together. Just skip the opinions here when it comes to something as important as cooling and go to a PRO.!! Later --- Jer

Cooling problem fixed!! My local baja marine dealer gave me a noncrossover style T-stat, looked like a typical car big block, when i talked to my performance marine shop they asked if I had a crossover or not, since I do have one then I need a T-stat with bypass holes in the top. Installed new T-stat, cools great, runs 155-165 '. BIG wieght off my shoulders, thanks for all the responces, its boating time! Derrick