Water temp rising and falling

[ghittner]

Broke in a new 498 blower motor today with a Bravo One with no ciculating pump, a crossover, and new impeller in the raw water pump. Timing, fuel ratio, oil, fluids, all look good. Here is my issue. I run a Merc 140 degree thermostat, same as I did on the last setup which was the same motor except for the heads and a few cubic inches. Now the temp goes from 150 to 200 and back to 150 in about 15 seconds. It then stays at 150 for about 5 minutes and then starts the process all over again. Thermostat? Sender?

[DUCKY]

What type of water neck/t-stat housing are you using, and how is it plumbed? If you are running a t-stat, you need a "4-port" type t-stat housing so the water continues to flow even when the t-stat is closed. If you are just using a "jet boat style" water neck with 2 dump fittings and a t-stat, you will build up a large amount of pressure in the block when the stat is closed and have "hot spot" where the water is starting to boil. I can't explain why this problem never occurred before...Just an opinion.
Just for a quick test, you may consider drilling a few bypass holes in the outer rim of the t-stat and see what happens.

[ghittner]

Doing the holes now, will report back.