91 Merc 5.0 - >1500rpm coughing/sputtering on water

[chimmike]

5.0, 2bbl carb. Brand new fuel pump. New spark plugs. Fuel filter really wasn't overly gummed/gunked up at all. New fuel/water separator.
With the mouse ears on, on the driveway, boat hits 4000rpm just fine. In the water, anything above 1500rpm resulted in coughing/sputtering and the motor almost dying.

All new gas in the tank.
Risers are of unknown age.

My thought was the risers are bad causing water to get sucked into the cylinders at higher load blowing out spark?

Or is it a carb issue?

 

[chimmike]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Anybody?

 

[alldodge]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Sounds like you have a fuel supply problem. Running the engine up to 4000 on muffs requires a very small amount of fuel. Running the engine up to even 1500 rpm moving in the water requires much more. At 4000 your engine will require a full tilte buggie supply of fuel to keep running. When it dies remove the fuel filter and see if it has fuel in it. Put a tee in the fuel line between the pump and carb, you should have 4 to 6 psi. Remove the fuel line leading to the carb and turn the engine over an see it your getting good flow. Check the anti siphon valve at the tank they can get stuck.

Let us know what you have found.

 

[chimmike]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Well, there's a hard fuel line from the pump to the carb. Should I replace that with a normal rubber fuel line? Supply goes to the separator, then the pump and then carb.

I'll check the anti-siphon on the tank, but I do know when I try to blow on the fuel line connected to the tank, I can't get any air through it.

Maybe the anti-siphon isn't opening enough?

 

[alldodge]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Well, there's a hard fuel line from the pump to the carb. Should I replace that with a normal rubber fuel line? Supply goes to the separator, then the pump and then carb.

I'll check the anti-siphon on the tank, but I do know when I try to blow on the fuel line connected to the tank, I can't get any air through it.

Maybe the anti-siphon isn't opening enough?

Usually takes a bit of doing but the easyest why is to replace the hard line with rubber for the test.

Try this first, remove the fuel line from the tank and place it in a portable gas can. Get the boat running and take it out for a short ride, if it all works ok, it's either the antisiphon valve or a clogged pick up tube. If it still coughs

Remove the fuel filter and see if it's full. If it's full, replace the fuel line between the pump and carb with a tee fitting and try again measurin the pressure.

 

[chimmike]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Yeah, the separator is always full.

I'll do the first test for sure though, just using a regular gas can. Sure would be sweet if that was the issue, a nice simple fix, haha!

 

[chimmike]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Starting to make sense now that it may be the pickup or anti-siphon valve because unless it's an immediate restart after engine dies (like within 5 seconds) it takes a good bit of cranking......i.e. the pump probably having to work extra hard to draw the fuel in?

 

[alldodge]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

Yeah, the separator is always full.

I'll do the first test for sure though, just using a regular gas can. Sure would be sweet if that was the issue, a nice simple fix, haha!

With the seperator always being full this is lean me to pump issue. Now if the seperator has fuel but not full then antisiphon or a clog.

Starting to make sense now that it may be the pickup or anti-siphon valve because unless it's an immediate restart after engine dies (like within 5 seconds) it takes a good bit of cranking......i.e. the pump probably having to work extra hard to draw the fuel in?

Could be but same as above, is it completely full

 

[chimmike]

Re: 91 Merc 5.0 - >1500rpm coughing/sputtering on water

overall same cranking issue with the new pump as the old (probably original) pump, that's the odd thing.

I'll check out the anti-siphon and try fuel from a gas can on the water and see what that does.