90 HP Mercury won't accelerate past 3000 rpm

[elhewbull]

1996 Merc. 90 hp. serial # OG360865
The motor starts and idles fine, but in gear will not rev above 3000rpm.
It will rev. to redline in nuetral
Compression checked out ok, 135-135-138 psi.
carbs were rebuilt.
tested electronics-
coils input terminals, (reading across # and- terminals) 1 coil-221v.,
2 coil- 214v., 3coil- 213v.
switchbox stop circuit-309v.
checked resistance between coil tower and neg. term. on coil, one of the coils read like an open circuit the other 2 showed resistance is this normal?
stator low speed-330v.
stator high speed 90v.
resistance test on stator read low (blue to eng ground-1300ohm, red to eng ground-32.5ohm) normal readings 3250-3650(1300) and 75-90(32.5)
trigger tested ok 1300 ohms, p. to b/w 7.4v, b. to b/w 6.6v, w to b/w 7.6v.
Other than the #1 coil resistance test not reading and the res. test on the stator reading low every thing else seems fine.
What do you guys think?
Bulb and fuel line were also replaced.

 

[James K]

Re: 90 HP Mercury won't accelerate past 3000 rpm

I am working thru the same problem. I have a 1996 Mercury Classic 2 Stroke 75HP. I have limited power till it max's out at 3,500 RPM. I have tested all!. The spark test was undetermined...I noticed a weak or missing spark several times on all cylinders. The coils, one was way out of spec..so I replaced all three (better safe than sorry). The trigger tested fine, but, the wire insulation looked toasted... so I replaced (I don't want to be with out power 16 miles out). The stator tested outside of CDI troubleshooting specs...Replaced (I obviously have money to burn). Compression test...130 lbs.+ on all cylinders (Great). Entire fuel system has been gone thru, including take down of carbs (Good). Rebuild Fuel pump for entertainment value only (only $12.00). The Switch Box is on the UPS truck as I type! I hope this is the end of my journey. The Salmon are in close and the Night Walleyes are not far behind...And my wife is more than I can stand ! I will update if my problems are solved...PS serial #0G404328 and I do Not have a Rev Limiter...

 

[phebus1]

Re: 90 HP Mercury won't accelerate past 3000 rpm

Have you changed props? Could just be over propped.

 

[CharlieB]

Re: 90 HP Mercury won't accelerate past 3000 rpm

The motors needs to make power are few,

Compression, you've nailed that one down, all looks fine as far as ring seal.

Ignition, still undetermined, DVA testing stator output and/or verifying spark on all cyls when the problem arises still needs to be done. A motor can rev freely UN-loaded, but as load raises so does the demand for a high intensity spark. DVA test the high speed windings of the stator AND also verify spark using an inductive timing light or an inductive tach while under load at the max RPM the motor will pull. If there is a failure on even one cyl that will show up. I'm am assuming that you have already verified timing, and that the timing lever moves freely all the way from idle to max timing.

Lastly, fuel demand with load. You have replaced the primer and hose, rebuilt carbs and fuel pump, but you still need to check both fuel FLOW and for the presents of any air sucking into the system, which could also limit flow. Disconnect the fuel line to the carbs, crank the motor and observe for a strong pulsed stream, temporarily replace that hose with a clear hose, start the motor and observe for any air bubbles passing thru indicating air sucking in somewhere before the pump. Also check the dip tube and check valve in the tank fitting for restriction or air leakage.

 

[elhewbull]

Re: 90 HP Mercury won't accelerate past 3000 rpm

I don't think the prop is the problem, on two occasions it accelerated to full speed and I did nascar laps around the lake hoping it would fix itself.
No such luck, as soon as I slowed down the motors problem came back.
I used a dva meter for all the voltage tests at 1500 rpm hooked up on the garden hose.
I guess the next step would be to test motor in the water to see if I get any voltage drops around 3000 rpm.
I will also try pulling off fuel line and see if fuel is pumping out.
Can anyone tell me how to check the timing?