View Full Version : Help with mercruiser motor problems
08-14-2009, 09:04 AM
I pump the gas throttle and the boat starts right up. It idles fine and runs great at very slow speeds. When i call for full throttle she will jump out of the water and go for about 20-30 seconds and then just shuts down. If you pump the throttle again she will start up and go at slow speeds. The motor is a 350 merccruiser on a 21ft speedboat. Any ideas to help solve my problem? Thanks-
08-14-2009, 09:10 AM
Try the fuel filter first sounds like you are running it out gas, might be the fuel pump but I think its the filter
08-14-2009, 09:49 AM
There could be more than a few things, could be water in the gas (replace the fuel/water filter). Then try the boat. Next is the fuel pump going bad and the boat is running out of gas in carb at higher rpm but has enough to run at slower speeds (I'm assuming it has a carb because you are pumping the throttle). If it is a carb model I would think it has the Thunder Bolt IV ignition so check the ground wires leading to the ignition as a lose one can cause the same problem.
08-14-2009, 10:29 AM
The check valve in the fuel tank is cobbed up... Very common to get plastic spirals from the fuel tank wadded in em... You can check this out by remove'n the supply line and adding a portable outboard tank with fuel or just remove the exist'n fuel line and try to blow air back into tank if it bubbles its NFG and needs to be cleaned out...
you didn't say the year but, if it's fuel injected, it has a high and a low pressure fuel pump, sounds like it could be the high pressure pump. usually if it's bad it is pretty warm to the touch. if memory serves me correct the low pressure pump is mounted on the fuel\water seperator, follow the fuel line up towards the motor and you'll find the high pressure pump- they both look the same but are completely different, i think they are carter pumps. some motors have a schrader valve on the injector line where you can put a pressure guage and check pressure at the injectors, i think it should be near 90 psi(call local dealer for spec.)
if it has a carb it still (newer models) has an electric pump located on the seperator but only 1 pump. you should hear the pumps cycle when you turn the key to the on position,the pump will run for a couple seconds.
yamadooed is correct, a very common problem, the check valve has a ball and spring in it that will get jammed with plastic or fuzz,
shift interuptor module bad
08-14-2009, 07:51 PM
Thanks for all the advice on my boat problems. I should have mentioned the boat is a 1986 and the mercruiser has a rodchester 2 barrel. Will be trying all the fixes tommrow and will let you all know how I do. Once again thanks for the help!
3rd agreement on the check valve in the tank
08-14-2009, 11:56 PM
Had the exact issue, Mine has a steel fuel tank. Rust had flaked off the tank and would collect around the fuel sump. After it starved for gas and died, the rust would fall off.
09-08-2009, 10:45 PM
yamadooed & jim -Thanks for all the advice with my boat. The problem was just what you guys said- Fuel line into the gas tank was stuffed with debris. Needed a coat hanger to reem her out !!
Check valve was fine because the junk never made it up the tube. John Dee groupies rule as that old Mercruiser of mine is back to whipping tubers and pulling skiers. Thanks all you guys,
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