Mercury 500 drops from full to idle

[BarryN]

I have a Mercury 500 - 50HP outboard. It has the blue lines and from my understanding it is a 71-74. Anyway, when I punch it the boat takes off fine. Right before she planes out the engine drops down to idle speed. If I wait about 5 to 10 seconds she picks up and goes again and then does this over and over again. The engine never dies, doesn't stall, doesn't backfire, and doesn't kill. It just drops down to idle speed. It almost seems like the float bowls are emptying and can't keep up. When I pump the fuel line ball while some one else is driving the thing will run fine. I know it is a fuel issue, but I don't know what to do. I have replaced the diaphram and gaskets around the diaphram. I have replaced the entire fuel line and ball that runs from the tank to the engine. I have replaced the fuel lines that run from the fuel Pump to the carbs. Still I have this issue. Need help. The Fuel line that I replaced from the fuel pump to the carbs is not an after market replacment. I used standard fuel line, with some fittings that I got from the hardware store. The fittings have opening that are a bit narrow compared to the ones on the stock line. Would this decrease in diameter cause this issue, perhaps fuel can't flow fast enought though the line now. I am not sure if that could cause the issure. This problem was happening before I replaced the fuel line and is still happening. Since then I have changed the diaphram, which was in pretty bad shape. I don't think that the fuel line would cause this problem since it was happening before I replaced it. Not sure.<br />The engine also runs great at idle and all the way to 99% full throttle, once it hits full it is done. This is a huge message, I hope some one can help. <br />Thanks.

 

[Merc805]

Re: Mercury 500 drops from full to idle

Have you run any old fuel latey, that could have "Gummed" up the carbs? A very common practice recommended on iboats is to remove, clean and re-set the carbs. You may want to try a different bulbs on it first in case that is restricting fuel delivery.

 

[achris]

Re: Mercury 500 drops from full to idle

Check the lines that run from the block to the fuel pump. They alternate between pressure and vacuum. One of those could be collapsing and stopping the pump from working properly.<br /><br />When you had the pump apart, did you check the valves?<br /><br />Chris........

 

[BarryN]

Re: Mercury 500 drops from full to idle

Brand new fuel. Boat was sitting for 6 years. Maybe carbs? In most cases I would agree, but it runs great all the way through, until I go full throttle. Dirty carbs would usually cause an engine to run poor during idle as well as intermediate speeds. This engine runs great at every speed until you punch it. If I go full throttle and then back off just slightly the boat will plain off and run great all day long. The second I push it to full it will take off and then drop down to idle. If it was dirty carbs wouldn't it run like garbage at that point where it is just about running at full thottle?

 

[BarryN]

Re: Mercury 500 drops from full to idle

Chris,<br />Checked valves and every thing seemed to be cool.<br />One of my next steps was going to be to replace the fuel lines that you are refering to. They are pretty much the only ones left to replace. I figure they are either collapsing on themselves or they are old and air may be getting into the mixture.

 

[achris]

Re: Mercury 500 drops from full to idle

These lines don't carry fuel, just differential pressure to move the diaphram in the pump............

 

[BarryN]

Re: Mercury 500 drops from full to idle

Good to know. I will replace those lines and see what happens. It is pretty much all I am left with.

 

[BarryN]

Re: Mercury 500 drops from full to idle

Replaced the Entire fuel line from the tank to the motor, Bulb too. No change. Installed new check valves in the fuel pump, no change. Installed new Plug Wires, No change. Stumped. Problem has gotten better. Now when you apply constant pressure to the Fuel bulb when at WOT the motor will run all day. No question that it is a fuel issue. The carbs can't keep up with the demand of the engine. Why? If anyone can help that would be great.<br /><br />Thanks.

 

[BarryN]

Re: Mercury 500 drops from full to idle

Help?

 

[mellowyellow]

Re: Mercury 500 drops from full to idle

if it runs fine when u pump bulb, i suspect fuel<br />delivery to motor isn't working. have seen many<br />"new" defective primer bulbs...what kind of tank?<br />is it vented properly? have a portable tank or<br />another tank you can try?

 

[BarryN]

Re: Mercury 500 drops from full to idle

I have tried two separate tanks. Both are vented properly and it doesn't make a differance which one I use, the motor runs the same. I know it is a fuel delivery problem, but I don't know where else to look, or what else to try. I know the primmer ball is not defective. The motor ran the same with the original one, also the same with a replacment I installed, and also the same with the New one that I installed which also included a whole new fuel line. Do you think that an air leak in the fuel line from the motor input to the fuel pump could cause this type of problem? It is the only line that I havn't replaced. This line is not leaking any gas, and I find it hard to belive that a small air leak could cause this issue. I don't have any other ideas on how to fix the problem though.

 

[mellowyellow]

Re: Mercury 500 drops from full to idle

bad quick-connect?<br />what type/size fuel line are you using?<br />a clear hose from pump to carb will show air<br />bubbles is it's got air infiltration maybe.<br />also, pull cover off pump and make sure gasket <br />fits properly. I once installed gaskets that were<br />just a tiny bit diff. and had sim. problem.<br />good luck,<br />M.Y.

 

[BarryN]

Re: Mercury 500 drops from full to idle

I have already check the gaskets. They are fine. I will try and change the quick connect along with the line from the the quick connect to the pump. Don't have a clear fuel line. Using standrd 1/2 inch fuel line.

 

[mellowyellow]

Re: Mercury 500 drops from full to idle

persistance breaks resistance.... <br />fuel lines can degrade from inside-out, so achris'<br />may have already given you the fix above. <br />other remote things? <br />spun prop hub<br />clogged exhaust<br />faulty power-pack/ lack of consistant spark...<br />if u pull off a plug wire and RPM's don't noticibly<br />drop, that is dead cylinder...

 

[achris]

Re: Mercury 500 drops from full to idle

Mr Merc 50,<br /><br />Have you checked/replaced the pressure/vac lines from the block to the pump? If so, try removing the fitting from the end of the fuel line (from the tank) and the fuel line in the engine that runs from the fitting to the pump. Push the end of the fuel tank line onto the pump and seal it with a cable tie. Test engine and report back with the results.<br /><br />Chris...........

 

[BarryN]

Re: Mercury 500 drops from full to idle

Will do. <br />By the way there are only two lines that run to and from the fuel pump. One line is the fuel supply line that runs from the tank to the Fuel pump, the other line runs from the fuel pump to the carbs. There are no Pressure/vac lines.<br />I am going to try and replace the line from the quick connect on the motor to the fuel pump tonight. If that doesn't solve the problem I will try to bypass the quick connect using the method that Chris suggests.<br />Hopefully that will do it.<br />Thanks for the help Mellowyellow & Chris.

 

[BarryN]

Re: Mercury 500 drops from full to idle

Fixed the Problem. Replaced the Fuel line from the Quick connect on the motor to the fuel pump. The line was in pretty bad shape on the under side. Also replaced the quick connect on the motor. The Engine runs strong at WOT now and goes all day. <br />Thanks MellowYellow... Quick connect was part of the problem.

 

[mellowyellow]

Re: Mercury 500 drops from full to idle

glad you worked it out <br />safe boating,<br />M.Y.