View Full Version : fuel problem!!!

06-30-2010, 11:57 AM
crank boat and runs great while running around the lake.. come back to dock and let boat sit for a while.. then when you go back out for another run there is no fuel in the in line filter.. boat will crank and run til the bowl on carb runs dry.. prime the bowl up by pouring gas in by hand and the filter will fill up and boat runs great til you shut it off again and let sit for a while.. you can shut off and crank right back up and it does fine.. seems to only do it after you make some rounds in lake and shut it off for 30 min or so.. this is a 79 mastercraft with the 351w.. i have changed the fuel pump, filters, and the needle and seats in the carb.. any ideas would be helpful....

06-30-2010, 08:18 PM
You can check the fuel pump by removing the fuel line after the pump and putting it into a container to collect the fuel, then disconnect the coil wire to prevent it from starting. Crank the motor and fuel should come out of the line. If it doesn't then the fuel pump is bad. If it is not the fuel pump, check for vacuum leaks around the carb by spraying starting fluid at the base of carb while the motor is running. If it revs, it has a leak.

06-30-2010, 11:18 PM
When you say "in line filter" is this the little Fram canister filter after the tank but before the mech pump? If equipped, that's a pesky little rascal. If air gets into the filter hsg thru a loose top bolt or misfit gasket, the tank will loose it's suction/prime and you'll experience the symptoms you describe. Take it apart and replace the filter, you should get new gaskets, etc. The fuel pump will not pull the fuel from the tank if this filter is sucking air, actually mine ('76 351W) won't pull the fuel from the tank unless I establish a siphon/suction from the tank using a hand bulb pump.

Alternately, you may have water in the filter and/or tank.

07-01-2010, 12:46 AM
This issue is relatively close to mine; although your carb and my fuel injection operate differently. See my post labeled "Fuel Problem Mercruiser Scorpion 330HP"j at this link - http://www.mastercraft.com/teamtalk/showthread.php?t=36340.

I would start at the tank and check the fuel pickup tube to ensure it isn't cracked. Then I would pressure test the fuel line to the pump for leaks. Then check for secure connections and tighten clamps at all connections. It soulds like you are losing your suction as Napagary indicated above. If you have a mechanical pump, the only reason it wont prime on its own would be a (major) air leak in the fuel line on the suction side. Look for signs of fuel leakage after an hour or so to establish if your fuel line has a hole. My fuel problem came from a combination of the heat of the engine and negative pressure in the fuel line. The result was fuel boiling in the suction line and the pump not being able to prime itself caused by a check valve in the tank outlet barb that required 3+ lbs of negative pressure to open. Your issue is probably not as obtuse as that, but keep it in mind while troubleshooting your issue.

Hope this helps, Please post your findings when you find out what's up.