Manifold hisses

[redjmp]

So I took out my 78 898 for its maiden run on rebuilt motor and noticed a loud hissing sound coming from the intake manifold. Specifically it is comming from a tube that juts out a half an inch. I think it is used to heat the carb choke but this carb is electric choke...
It keeps hissing for quite a while after I shut down the motor. How can I fix this or is it normal?

 

[wire2]

Re: Manifold hisses

Yes, that's a connection for a thermally operated choke. That's exhaust gas you hear hissing at you. It builds up pressure due to the normal discharge being under water.
Remove the 2 bolts and the plate, make another plate to match (but without holes, of course) and bolt it on with a bead of silicone gasket.

OR...
Cut the tubing off, run an NPT tap in the holes & put plugs in.

 

[redjmp]

Re: Manifold hisses

Thanks! I thought so... But now this is how I got this boat. Could it have come this way from the factory or did the previous owner swap engines with an automotive one that uses that type of choke? Does Merc make them this way?

 

[Bondo]

Re: Manifold hisses

Ayuh,.... There are thermal chokes, 'n electric chokes used on marine carbs,...
Not really a Red Flag there...

It's obviosly not the Original,....

Personally,... I'd braze the holes shut,... course, I've got a set of Blue-tip Wrenches on the truck too...

 

[a70eliminator]

Re: Manifold hisses

Are you sure the hissing is comming from those tubes, they don't have any source of vacuum, it's nothing more then a hollow pipe bent into a U that is inserted into the exhaust crossover, the exhaust crossover just heats the bottom of the intake to facilitate quicker warm up on a cold start, the choke tube is in those hot gasses, I agree to just plug them off, or make a block off plate, but I think your hiss will still be there, unless it's an exhaust leak sound.

 

[redjmp]

Re: Manifold hisses

Turns out it was hissing from the pipe so that must mean there was a hole in it. I just dumped a blob of JB Quick on it. ~