Mercruiser 7.4 runs great at times and horrible 5 minutes later
 

Hi,
I have a 7.4 Mercruiser that is giving me fits. The boat sat for some
time and may have had bad gas. It will start and run great for 5
minutes and then will stumble and even backfire sometimes. I have
read that if water gets in the carbeurator it will not pass. Is this
a possible problem. Other possibilities are check valve (seems to be
OK) and fuel pump. I have changed the fuel separator and fuel filter
and still am having the problem. Any ideas??

Rick

Mercruiser 7.4 runs great at times and horrible 5 minutes later
 

Mercruiser 7.4 runs great at times and horrible 5 minutes later
 

wrote in message
...
Hi,
I have a 7.4 Mercruiser that is giving me fits. The boat sat for some
time and may have had bad gas. It will start and run great for 5
minutes and then will stumble and even backfire sometimes. I have
read that if water gets in the carbeurator it will not pass. Is this
a possible problem. Other possibilities are check valve (seems to be
OK) and fuel pump. I have changed the fuel separator and fuel filter
and still am having the problem. Any ideas??

Rick

It sounds like the fuel delivery is not keeping up. When you say runs for 5
minutes do you mean at idle or actually running as underway or trying to get
underway? Those symptoms also happen when overheating but 5 minutes would
be a pretty quick overheat. When was the last time the cooling system was
serviced? Do you have a closed or raw water system?

If it starts and runs pretty consistently for a short period then it doesn't
sound like water in the gas. If it starts stumbling when you try to get
underway then it might be water, lifting the bow of the boat can cause water
to move in the carb and cover the jet pickups. It can also cause water to
shift in the tank as well. Water in the fuel is also hard on the fuel pump,
it causes corrosion around the flapper valves in it and reduces it's ability
to pump. When you removed the old separator and poured it out how much
water was in it? If it was mostly gas then you have changed it in time. If
it was mostly water then you might have too much water in the fuel system.
Remove your new separator and pout it out and see what you get. Water in
the carb usually resolves it's self if you stop more water from getting into
the carb. If you just have too much water in the tank you may be able to
run it, remove the separator, pour it out and refill it with gas,
repeatingthis until you stop getting a lot of water. I use a large glass
measuring cup when I pour out the separator as then I can pour the gas back
into it and leave the water in the measuring cup.