reaction1

I have Mercury 275 HP (3.4 litre) outboards. New impellers and thermostats, also checked timing; still overheating at idle. I'm thinking of removing the thermostats. What are the pitfalls in removing thermostats in a carburated outboard? If any?

Fuhgedaboudit

you should NOT remove the t-stats completely. you need some restriction to slow the water down to cool the engine. Better to take the stats out and replace them with stainless washers, or cut out the spring and pellet from your existing stats and put them back in. I run tripple carb. mercs like that with no overheat problems at all. Good luck

Fuhgedaboudit

Hey, whats your water pressure at WOT?

mcollinstn

You sure you haven't got old impeller pieces stuck in your block?

I like the stainless washers.

ScarabDriver

If the overpressure relief valve is stuck open, water won't circulate properly especially to the upper sections of the engine. Problem goes away when engine is turning faster. providing more water and pressure. By removing the thermostats the engine will not get up to operating temperature which can give sluggish performance and poor fuel economy. Is this happening on both engines or just one? What type of boat/ setup are they on?
BTW, I have the same engines

jlavallee

Eliminating thermostats revents the engines from reaching operating temperatures. Also--Water pressure exceeding 30 PSI will blow hoses. You probably do not have water pressure gages?
I have 454's and drilled 3 holes 3/16 diameter in the thermostats to relieve pressure. Temp should runn about 140 degrees. John in VT