Mercury XR6 RPM Loss, Fuel Lines?

[SteuryLover]

I recently installed a Mercury 150 XR6 on my fishing boat. Ran great at 5200 RPM with a full load turning a 21p Lazer prop. Last time out I was only able to achieve 4600 RPM with a little lighter of a load. I checked my plugs, they look good. I was checking out my fuel filter and separation filter and noticed the fuel line from the water filter going to my bulb then to my fuel pump is all 5/16". I have read it should be 3/8" above 75HP going to the fuel pump. I typically do runs for an hour or so each way at 4k or so. Is it possible I already damaged something because of this? Is it possible this is the main problem? The motors starts right up and feels and performs well. I know the oil is injected after this hose so I am hoping the cylinders still got oil and are ok. Any info would be appreciated.

 

[j_martin]

Re: Mercury XR6 RPM Loss, Fuel Lines?

I recently installed a Mercury 150 XR6 on my fishing boat. Ran great at 5200 RPM with a full load turning a 21p Lazer prop. Last time out I was only able to achieve 4600 RPM with a little lighter of a load. I checked my plugs, they look good. I was checking out my fuel filter and separation filter and noticed the fuel line from the water filter going to my bulb then to my fuel pump is all 5/16". I have read it should be 3/8" above 75HP going to the fuel pump. I typically do runs for an hour or so each way at 4k or so. Is it possible I already damaged something because of this? Is it possible this is the main problem? The motors starts right up and feels and performs well. I know the oil is injected after this hose so I am hoping the cylinders still got oil and are ok. Any info would be appreciated.

What hurts em is the fuel leaning out at high output raising cylinder temps to the destruction point. It makes no difference at all how much oil is in the fuel. In fact, too much oil can exacerbate the problem.

If it were mine, I'd rig a fuel pressure gauge on the carb fuel rack and watch it at WOT. Poor fuel delivery probably causes the demise of of more of these engines than all other causes combined.

I have found that these items can cause problems
Carb setup, especially float level
bayonet fittings aftermarket plastic cannot deliver enough fuel
fuel lines -- small, or some older ones will shell out the inner tube.
fuel fittings -- air leaks cause problems
primer bulb
fuel pump
fuel filter
fuel pickup tube in the tank
tank venting
debris in tank, sometimes an almost invisible gel

Have at it. It's a fine motor worth caring for.

John

 

[SteuryLover]

Re: Mercury XR6 RPM Loss, Fuel Lines?

Thanks for the reply, I do think I will go through it all. Do you recommend going to 3/8" fuel line to the pump? The motor was used very little before I got it a few months back, the previous owner had the shop do all the work on it before I got a hold it. It did run perfect spinning 5200 RPM previous to last week. Do I need both inline filter and water separation filter? Can I delete the inline filter safely to help with pressure? I also have a Lowrance fuel flow meter after the bulb and before the pump, think this can be the problem? Lowrance claims it adds .5 PSI to the fuel pressure, could this be a bad thing for this motor? Any suggestions on a fuel pressure gauge to install? Thanks for the quick response.

 

[j_martin]

Re: Mercury XR6 RPM Loss, Fuel Lines?

Man you have a lot of questions. Just for reference, I've been there, done that. That's where I compiled my list.

The engine I acquired with WFS (WOT flameout syndrome) was also worked on by a "mercury certified mechanic"

The fuel pump had warped rubber valves in it, indicating it had not been overhauled in years, or very old kits were used.

Both parts of the fuel bayonet as well as the fuel line and primer were Atwood. None of those components will flow enough fuel for a V6.

Gasohol had seperated in the tank over years and the sludge formed was repeatedly plugging the fuel filter.

The carbs looked like they had been adjusted while intoxicated.

I replaced all the internal fuel lines with tygon. I removed the bayonet and ran the new mercury fuel line and primer bulb directly to the oil mixer T. Please note that tygon will not work for the fuel pump pulse hose. Ask me how I know.

I removed and discarded the internal fuel filter. I installed a 60 GPH Racor Filter/separator between the tank and the mercury fuel line. The key is the fuel lines are all new and in good shape.

I now burn 92 octane Non-oxygenated gasoline. What I'm burning now is 6 months old. (Stored in tight drums with StaBil) I have no fuel problems.

Adding 1/2 psi to the suction is like adding 1 foot to the lift. Shouldn't be a problem. Judging from the out of context clip you supplied from their instructions, it sounds like it should be installed between the fuel pump and the carbs.

The gauge I recommended would be like swiping the gauge off'n grandma's pressure cooker, or any other 10 - 15 lb gauge, hooking it up to the fuel rail with a plastic hose and putting it in your lap, just for troubleshooting. None of that install would be either kosher or safe. A permanent fuel pressure gauge would need to be electric, or have a pressure coupler, which is a device with a membrane in it. The line to the motor is filled with gasoline. The line to the gauge is filled with glycol. That way a ruptured line under your dash doesn't dump fuel on your feet.

hope it helps
John

 

[SteuryLover]

Re: Mercury XR6 RPM Loss, Fuel Lines?

Good info, thanks for elaborating. I did go this morning and purchased new 3/8" hose and replaced the 5/16" from the water filter to the T where the oil line meets the fuel including the fittings and bulb. I also replaced the water filter and purchased a can of Quicksilver power tune. I will keep you posted in the results once I get it out. Thanks for your time.