Merc 90 Power Loss only at full throttle

[Ocean Dancer]

?Huckleberry Finn?
Merc 90 RPM/Power Loss only at full throttle
Serial Number:
Merc 90 2003 or 4 carborated. consistently runs perfectly at 7/8ths throttle 5500 rpm, but moving to wot drops back to 5300. No sputtering or similar irregularities of any nature at any speed, except perhaps idle which is satisfactory. New racor filter installed (old filter had some water and noticeable rust-like particles (note poly in-vessel 23 gal tank). Installed new filter in water sep on back of engine(old filter discolored with small amount of water only). Clean non ethanol 93 octane fuel with stabilizer added . Note that just prior to this loss of power issue, 87 octane fuel with ethanol was used once-the only time ever- and the power loss issue followed this within 30 days. Power issue repeats like a sunset.

Installed new plugs: LFR5A-11 NGK Spark Plug Stock No. 6376 Quicksilver # 33-859495Q (old plugs-same as replacements- indicated good service, and stored for mechanics observation). NOTE: after new plugs and Merc fuel additive provided by merc mechanic, performed sea trials for approx. six hours (5 or so at max or 7/8 throttle) with no symptom change. Also, previously had to change ignition switch which works excepting choke does not activate when switch depressed (not an issue relative to the work in resolving). Squeezing fuel bulb while at wot effects no change (engine does not act like it reverts to the well running 5500 rpm at 7/8 throttle, but holds back to the same 5300 rpm).

Am clueless about linkage issues and throttle position and lube.

Ellen Clark

1. Verify Throttle Position Sensor TPS is Fully CCW secured.
2. Observe throttle cable adjustment on engine
3. Observe linkages
4. Spray linkages with white lithium grease
5. Replace fuel squeeze primer
6. Look at console speed/shift assy.

 

[Texasmark]

1. Squeeze your fuel primer bulb as you are going into WOT and see if it helps. Also bulb should be somewhat firm...indicating the presence of fuel, not solid like when you are priming for a start and not collapsed. If wrong symptoms, engine running out of fuel...pump or line restriction.....inside of tank to engine fuel line flakes.....time for a new line and clean up the tan residue.

2. Over rev limiter module if you have one could be bad. Don't have my manual any more so I can't tell you what color wires go to it. surely red or red/purple or purple (power) and black (ground) and maybe grey to get the tack signal to count pulses. If found, disconnect it and see if problem goes away.

3. Check butterflies and linkage and ensure that you don't have slop in it from worn or broken plastic pivot bushings on the linkage which move to an incorrect position at WOT due to the added pressure on it required for butter fly fully opening and spark fully advancing.

 

[Ocean Dancer]

Holy Smoke, S.M. Texas Mark! Powerful troubleshooting/recommendations here. I'm grateful. Ellen

 

[quicktach]

Check to see if the advance linkage is dropping back as you go to WOT too !