I have a Mercruisier 5.0 MPi. Alpha One.

I live UK, It was winterized by mercuriser engineer, he's been ill so not
been able to help me at moment but said should be ready to run.

Went to it the the other day for first time this season & it would not
start.
Turned over really well but not even a misfire.

Fuel OK had addatives, wriggled gears etc.

I changed fuel filter/water seperator (He had not done on winterization, was
going to do it now!)
I took off flame arrestor to check intake, noticed a cube of sponge (size of
2 sugar cubes) on small port beside large butterfly.
It did not really look like it should be there so temp. removed it, put
arrestor back.

After 5 or 6 trys it started & ran perfectly but there was a loud sucking
noise? Put sponge back & started OK & noise went.
I actually put the starting down to the replacement fuel filter as it seemed
to be half full of water.

I managed to speak to engineer few days later & he said it it was called a
muffler.

Went back yesterday & again it would not start! Loads of battery power.

Took muffler out and after 5 or 6 more trys it started (Not quite
immediately). Put back in after just a few seconds of running.
Started & ran all day no problem.

I did leave for 4 hours at one time, assume long enough to cool down
completley, started first click.

The muffler, (cube of sponge) is quite charred & crispy, but sure air could
get through OK. The pipe of this port goes into a housing where wires come
out as if some sort of sensor. Surprised it could stop it completely.

Any ideas.

Thanks I/A
Greg Reed.