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Author Topic:   90hp Mercury Bogs Down at High RPM
soshea posted 06-15-2015 01:20 PM ET (US)   Profile for soshea   Send Email to soshea  
I have a 1985 Mercury 90-HP outboard engine. Right now, when the engine runs at 4,500-RPM it goes for approximately 45-seconds, bogs down to 3,000-RPM, and then tries to fight its way back to a higher speed.

This occurred after replacing my lower unit. I first thought it was a fuel-restriction, so I checked everything, working backwards from the fuel pump. Pump looks good. Cleaned the filter. Replaced fuel lines. Replaced fuel. Added Seafoam. No luck. Sprayed Seafoam all through the carburetors and cylinders. No luck.

Moved onto thinking the cause might be electrical. Spark plugs are new. I replaced the stator. Still no luck. A mechanic told me I needed to check if I have a bad cylinder. Told me to pull the wires one at a time at WOT and see if it affects the performance. My buddy got a nice shock and lost a pair of pliers overboard, so we stopped that test.

I did notice yesterday that when I primed the fuel primer bulb close to the point of flooding, the duration in which the engine ran at high speed was longer than 45 secods, maybe two minutes, which has me thinking the cuase is again, fuel restriction.

Any suggestions?

jimh posted 06-15-2015 02:33 PM ET (US)     Profile for jimh  Send Email to jimh     
Reduction in engine power caused by lack of fuel generally results in deceleration that comes on slowly and returns slowly. Electrical problems tend to manifest as being on or off suddenly.

Regarding your diagnosis of the fuel system, you said "pump looks good." That suggests you just made a visual inspection of the fuel pump. I am not certain that a problem in the fuel pump, other than a leak, will be observable visually.

To check the spark, insert an in-line spark gap tester in the spark plug wire. Run the test after sunset so you can see the spark more easily.

Try pumping the fuel line primer bulb when the engine begins to decelerate at full throttle. If the engine speed resumes to normal as you pump the fuel into the engine with the primer bulb, the fuel pump in the engine may be failing.

Typically a fuel pump from a 1985-era engine would not have been made with material that was expecting the fuel to contain ethanol. In any case, it's a 30-year-old fuel pump. Perhaps time for a new one, with ethanol resistant components, if available.

soshea posted 06-15-2015 03:09 PM ET (US)     Profile for soshea  Send Email to soshea     
I will check to see if the pumping of the bulb makes a difference. When I was originally troubleshooting I would push in the ignition key to prime it, but that didn't do anything.

In regards to the fuel pump: I inspected the filter and cleaned it. I inspected the diaphragm: no holes or tears. The sediment on the pump filter led me to the deterioration of the lining of the fuel lines, which I have since replaced. I have to check, but I think the fuel pump was replaced in 2007 to 2008. --Shaun

jimh posted 06-16-2015 08:36 AM ET (US)     Profile for jimh  Send Email to jimh     
I am not sure what action occurs with a 1985 Mercury 90-HP outboard engine when you depress the PRIME button. It is unlikely to produce more fuel flow if the cause of the problem is insufficient fuel pump pressure or a restriction in the fuel hoses.

If the fuel hoses on your boat date from 1985, they are very likely not rated for use with fuel that is an ethanol-gasoline blend. Use of rubber fuel hose rated for alcohol-gasoline blended fuels did not become common until a few years after 1985.

jimh posted 06-16-2015 07:33 PM ET (US)     Profile for jimh  Send Email to jimh     
Rubber hoses intended for use as fuel hoses with ethanol-gasoline blended fuels are clearly marked as being of that type. Check the fuel hoses for such markings. The epoch when rubber of this type and marking came into use is c.1987.
Qtrmeg posted 06-17-2015 07:38 AM ET (US)     Profile for Qtrmeg  Send Email to Qtrmeg     
You should check all of the engine wiring, especially the ground wire,(black wire with yellow stripe).

There was no reason to touch any of this while replacing a lower unit but things happen.

DauntyDave posted 06-17-2015 11:10 AM ET (US)     Profile for DauntyDave  Send Email to DauntyDave     
Reading this thread I have a few comments that might help. You say you replaced the lower unit, how is the water pump? Any possibility you are over heating? Do you have good water discharge from the tell tale?

Hitting the primer button will not help if there is a fuel supply problem as the primer circuit gets its fuel from the fuel bowl that in the case of a fuel supply problem has no fuel to use for priming.

Visual inspection of the fuel pump parts may also not reveal pinhole leaks in diaphragms or check valves that have the tiniest bit of crud in them or are just worn out. A fuel pressure gauge installed between pump and carburetor might be a better indicator as you would expect to see fuel pressure drop off if you are running out of fuel at high rpm. The fuel pressure could also be expected to drop off if there is some fuel system restriction upstream of the fuel pump such as an in tank pickup screen or a plugged fuel tank vent.

These are just ideas from an old guy, hope they help you find your problem.


soshea posted 06-17-2015 09:05 PM ET (US)     Profile for soshea  Send Email to soshea     
Spent all day working on this, still have the same problem.

Checked all the floats in the carbs. They are all okay. Found a small air leak in a gas line going from the fuel pump to the carborators. Replaced all zip ties with 5/8 hose clamps to secure the hoses and eliminate the air lead. Didn't fix the problem, but did reveal when squeezing the primer bulb at WOT when it starts to bog down, the problem stops. So, I replaced/rebuilt the fuel pump. Didn't fix the problem.

When I replaced all the fuel lines, I did not replace the primer bulb because I thought I had a spare in my shed but turned out to be a dud, so I just put my other one back on. I am going to replace that next before pulling off all my carbs and going through them with a fine tooth comb.

As for the lower unit. Water pump was replaced. Stream is strong.

Talk about frustrating. My gut is telling me that it is something simple that I am just overlooking.

tedious posted 06-18-2015 09:01 AM ET (US)     Profile for tedious  Send Email to tedious     
First off, forget about further work on the carbs - your experiment with squeezing the bulb proved that if you can deliver fuel to the carbs, they work fine.

Either your fuel pump is weak, or you have a fuel restriction upstream of it. Since you already worked on the fuel pump (by the way, can you clarify what you did?) then I'm betting the primer bulb itself is bad or you have another restriction upstream of the primer bulb.

What are you using for a fuel tank? I am assuming this is a Montauk with a portable tank - have you checked the vent and the fuel pickup?


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