40 hp mercury idle problem

[Daviet]

I have a 1996 40 hp, 4 cylinder, that will stall unless you keep using the choke.
It has 125 compression on all cylinders, spark is great and new plugs.
I removed the carbs, they looked clean, checked all the passages and replaced all gaskets, made no difference.
It will keep running if you keep pushing in the key. The fuel pump has 2-3 lbs pressure.
It almost acts like there is not enough vacume to pull fuel out of the carbs.

The engine did overheat enough to start to melt the plastic water pump housing, but did not discolor the engine. Replaced the water pump and it cools properly.

Any thoughs as to what the problem might be.

 

[sarantis]

Re: 40 hp mercury idle problem

I have a 1996 40 hp, 4 cylinder, that will stall unless you keep using the choke.
It almost acts like there is not enough vacume to pull fuel out of the carbs.
.

You look much more experient than me however my thought is that air is intruding into the carb not the proper way (either from the fuel line or not) spoiling the mixture ratio.

 

[Daviet]

Re: 40 hp mercury idle problem

I have checked all fuel lines and connections, all tight.

 

[sarantis]

Re: 40 hp mercury idle problem

I would do a last step before thinking about worse scenarios.I would connect a small transparent elastic tube between the carb inlet and the end of the fuel line to get sure there is a good flow of gas, clean of airbubbles.Sometimes air can intrude the system without noticing gas leaking.If it was ok and if i was sure about the carb the next thing i would suspect would be mating surfaces that got distorted due to the overheating.I woud spray round the intake soapbubbles or something else (i ve heard about something like airgas or CO2 in spraycan that changes the rpm if absorbed in the cylinders) to lelp me find the airleak.In addition a compression test with the throtle open and closed may give a conclusion.Good luck!