Fuel Pump? Half power at Full Throtle

[landolph]

I have a '99 Mercury 120 HP sportjet engine. It had been running fine until one time I throtled up, and it would only run at 3k rpm. I let it set for about 30 min and it ran fine until be got close to the docks, then it started acting up again. I went home and replaced the fuel filter (doesn't hurt), and readjusted the carbs, and pulled all fuel lines and pressurized to check for leaks (including the on in the fuel tank. Boat ran fine for about 15 min, then it start doing the same thing. So I filled it up with premium gas thinking old gas. She ran fine, unitl last weekend when it started acting up again. This time I notice it was just below 1/2 a tank of gas. So, I top the tank off andshe started to run fine again. I have check the electric fuel pump and it is working great.<br /><br />All thing seem to be pointing to the Mechanical Fuel Pump? I just need a second opinion.<br /><br />Thanks,<br />Lan

 

[funpilot]

Re: Fuel Pump? Half power at Full Throtle

Hi landolph, and welcome to the forum. Not too many sport jet operators here, but if you do a search on sport jet (or maybe sportjet too) in both the mercury and i/o sections, you will get some neat posts to review.<br /><br />The electrical systems on these motors includes a rev limiter which is pretty sensitive, and they do go bad. If it is a temperature related failure, it might be intermittent. When my limiter went 'south' it acted differently each time I ran the boat, but mostly it limited the revs at wide open throttle (wot) to the point the boat acted like it was unable to function at more than 3/4 throttle. Next time you have the problem, disconnect the yellow black wire from the rev limiter (it is the small black electrical module on the starboard side of the engine closest to the end where the carbs are) and see if the problem goes away. Be very careful running (especially from a standing start) do not over rev the engine. Accelerate slowly. As long as you don't aggressively jump a wake, the motor can only go about 5000 rpm when loaded with water going through the jet pump. But when first starting there is no load, and that's when over revving can occur.<br /><br />Good luck. It's worth a shot and easy to check.

 

[kenimpzoom]

Re: Fuel Pump? Half power at Full Throtle

You could have a fuel related problem, but I would suggest you start by checking spark on all three cylinders when it is having the problem.<br /><br />I had a bad trigger that would do the same thing. Run great for a while, then only half power. One cylinder was not firing because of the bad trigger.<br /><br />If you discover that spark is the problem, here is some info to help diagnose the problem.<br /><br /> http://www.geocities.com/kenimpzoom/KensSeaRayder.html <br /><br />Ken

 

[m_bickle]

Re: Fuel Pump? Half power at Full Throtle

It sounds like the rev limiter is kicking in, test all related components. Also get an external fuel tank and run off that to isolate the boats fuel sys (the half tank theory).