I have a Mercruiser 305 with a Mercruiser 2-barrel carb. I replaced
the fuel pump last fall as it left us in the middle of the lake. I
took it out today for a test run. Dang thing would not start worth a
darn. Then when it did, it would gradually run slower and slower until
it quit. About the only way I could get it started was to have the
throtle lever all the way forward. I turned up the idle a little which
helped keep it running. It ran great at anything above idle. Pulled
well, etc. My thought is that the idle mixture may be too lean. I at
least want to try that before taking it in to the shop. Your thoughts
would be appreciated. Thanks.

If it runs above an idle then the fuel pump is ok. You can play with
the idle mixture if those adjustments are on that carb. Start by
lightly bottoming out each mixture screw and then backing them out a
couple turns. You want to adjust by turning the screws in until it
stumbles and then backing out until it smooths out. Stop there. If
you have to turn the idle speed up much to keep it idling then the idle
mixture is not going to appear to make any difference. If the idle
screw is opening the plates up much then it will be idling off the
jets.

I'd suspect the carb its self though. Water in the gas is common on
boats and usually fouls up the carb after a while. Have you been good
about periodically draining your fuel filter? Low voltage can also
cause idle trouble but that's less likely. Carb rebuild would be the
first thing I'd try if the mixture screws don't help. Unless you want
to pop for a decent aluminum intake and a 4 barrel :-)

JJVB wrote:
I have a Mercruiser 305 with a Mercruiser 2-barrel carb. I replaced
the fuel pump last fall as it left us in the middle of the lake. I
took it out today for a test run. Dang thing would not start worth a
darn. Then when it did, it would gradually run slower and slower until
it quit. About the only way I could get it started was to have the
throtle lever all the way forward. I turned up the idle a little which
helped keep it running. It ran great at anything above idle. Pulled
well, etc. My thought is that the idle mixture may be too lean. I at
least want to try that before taking it in to the shop. Your thoughts
would be appreciated. Thanks.