1980 mercury thunderbolt 80hp. Low speed problem when warm.

[dleece]

When the motor is first started it runs great. After it warms up it will not idle, I have to take it out of gear throttle it up to keep it from stalling. It also runs very poorly at low speed often stalling. Exceleration is very sluggish. Once it is above 1/4 throttle it runs great again. Problems only occur after several minuets of running.
Any ideas what the problem cold be?

 

[quicktach]

could be lots of things. what have you done so far....basics ...fuel system, plugs etc ??

 

[Texasmark]

Put your hand on the top rear of the engine block. How hot is it? Your engine has no thermostat but on thermostated engines, the stat temp on mine was 143 F for the opening temp. 140F is the temperature of a domestic household hot water heater when in the Normal position for a reference. If you are hot, your engine is not getting proper cooling at idle. My first suspicion would be time for a water pump rework including a new impeller. Otherwise can't think of anything on that engine that fits that envelope. I had the 85 hp predecessor to that engine. Running dude it was and I loved it.

 

[dleece]

Texasmark- There appears to be plenty of water pumping out but I will check.
quicktach- new plugs, new fuel filter and a slight increase in idle speed is about all that's been done to this point. Was contemplating throwing a rebuild kit in the carburetors.

 

[Texasmark]

Texasmark- There appears to be plenty of water pumping out but I will check.
quicktach- new plugs, new fuel filter and a slight increase in idle speed is about all that's been done to this point. Was contemplating throwing a rebuild kit in the carburetors.

Don't see a clogged carb and a temp related rpm problem co-existing. If you were in need of more fuel based on a stingy carb, you'd need it when the engine is cold, not warm......combustion efficiency improves with temperature, that's why engines are equipped with thermostats.