2004 mercury 50 runs good at low rpm but not high rpm

[gwgautney]

I'm researching this for my dad. He has a 2004 Mercury 50. it cranks and idles fine and will run fine at lower rpms fine. when he gives it throttle and runs at higher speeds it will run at that speed for a few minutes and then drop back down as if you were throttling it back down. if you hit the choke it will pick back up and start speeding back up. also if you let it sit for alittle while and crank back up it will run fine for a few minutes and then do it again. Its almost like the fuel is being restricted its not likes its bogging or anything. I don't have sn or model number right now but I will add it later

thanks

 

[Texasmark]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Does the bulb collapse? If so, is his tank vented? From your symptoms it seems the engine is fine, just having a problem with a restricted fuel supply. At idle and low speeds, the engine apparently doesn't suck fuel faster than the system (tank and all) can resupply it's air so there is no vacuum developed and no fuel loss. Could be a clogged inlet filter in the tank. Could be a clog at the squeeze bulb.

If the bulb doesn't collapse, how about the fuel filter in the engine being clogged?

Mark

 

[gwgautney]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

thanks mark im going up there today so I will check those things out and see if that will work just curios about carbs at first that's what I thought it might be but not thinking that now

 

[CharlieB]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Alimited fuel supply and the motor is simply running out of fuel.

Pull the fuel hose from the carbs and crank the motor. You do not need to start it, you just want to see a nice fat strong pulses of fuel. Replace that hose with a clear one and start the motor. Now watch for any air bubbles passing thru.

You are wanting to decide if air is sucking in and not allowing full fuel flow, or if you have a weak fuel pump.

A weak pump can be rebuilt, air sucking in can be anything from a crack in old hose , to a loose hose clamp, or a failing O-ring in a quick disconnect coupling anywhere between the fuel pump and the tank.

 

[snowman48047]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

My 2001 50 was doing the same thing until I found out there were 2 fuel filters. One is near the pump and there is another inline. I had always changed the pump filter but the inline was buried and I never saw it until I took off the left side cover. Fixed my issue...

Just remembered there was a fuel pump issue also... something that just came up a couple years ago. Maybe a weak return spring. Check with a merc service guy. I replaced mine before the filter but the pump didn't solve it. You may have both issues or just one or the other.

 

[Texasmark]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Just remembered there was a fuel pump issue also...

I didn't think it was the pump because of the time element involved. My thoughts are that if it were the pump or sucking air it would be immediate whereas a restriction in the line would take time. BTDT I had a 115 tower I ran at 6000 WOT and I could run it out of gas too. My solution was to reroute the lines, but it had the first iteration of Merc's self contained oil system but it was in a tank between the gas tank and the engine, not engine mounted like it is in later engines. Lots of extra plumbing. Course running 500 rpms over the upper recommended limit didn't help. Grin.

Mark

 

[CharlieB]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Keep running it going lean like it is and you will soon be buying a new set of pistons!

 

[Texasmark]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Keep running it going lean like it is and you will soon be buying a new set of pistons!

I don't correlate lean mixture and running out of fuel as the same problem. His problem is a catastrophic event, not something that he tolerates continuously! Besides, to have a good hole shot, I don't see how you could possibly be running lean, even if only a small percentage of cylinders were running lean. But you have your opinion and I have mine.

Mark

 

[snowman48047]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Ya Texasmark I said the same thing to the Mechanic about the pump either is bad or isn't. He had just had a class and they went over the pump issue and he was surprised to see it so quick. I don't remember exactly but something like the spring was weakening and intermittently would not deliver fuel as usual or the piston would intermittently stick. I agree that it is a fuel supply problem.

 

[CharlieB]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

Or air leaking into the fuel system. The pump can easily fill the carb it an idle while fuel demand is low. Open the throttle the bowl is full the motor accelerates the boat, until the bowl gets low because the fuel pump cannot pump the full volume it is supposed to. The motor goes lean, temps raise, the motor bogs, so lean that it can no longer fire. Motor speed drops and either dies or the operator cuts the throttle back to idle.

Thus, check the fuel pump, fittings, hoses, etc.

 

[gwgautney]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

ok guys thanks going to work on it today will let you know what I find

 

[gwgautney]

Re: 2004 mercury 50 runs good at low rpm but not high rpm

turned out to be either clogged fuel filter or carbs I cleaned carbs and replaced fuel filter got it running good now. thanks everyone for the help