How did you setup your thermostat how many hole in them and what hole dia to be able to fill the block of water?
I have try a 140 thermostat with a 1/8 hole dia and the water temp is not stable, they are stable on throtle and when i back to idle the temp raise sudently to 200-220f i shut off the engine imediatly.
How do you make this setup reliable?

start off with a 3/16 hole in the tstat..i usually end up with 3 3/16 hiles in a triangle shape..are you running a bypass? the hose that goes to your t-stat housing to the crossover?

Yes i use bypass, with std thermostat (140F)with the hole that i have made in it (1/8'') it is look like that the cold water coming from the bypass affect the thermostat or they make some steam pocket for some reason ???

I would check your impeller and pump housing. If they are like new, then swap the thermostat for a Stewart high flow model. If you are using a stock thermostat, they won't react fast enough and flow enough when you drop off plane after a hard run. After running wide open and dropping off plane, my boat will rise as high as 190, but will drop quickly back to 160.

Also where do you drill the 3 hole?

Any help would be apreciated.

Thanks

i will try today the thermostat with 3 x 3/16 hole in triangle and aling the one hole on the top and the other two hole aling at the botom of the engine. I hope it will work!

You have other issues drilling holes is not going to help. You have an 80 degree swing. Your cooling system is not in control. You have a blockage somewhere that needs to be addressed.

ck your oil and P/S coolers for blockage..old impellor pieces etc..as well as the water pump itself...are testing in the water or on the hose ? sometimes they will suck a garden hose closed..