1992 Mercury 2.5L wont rev over 3,000 RPM under load.

[Hubjeep]

Howdy all,
My 1992 Mercury 200 (2.5L) carb won't rev over 3,000 RPM under load!

After about 30 minutes of 4,500 operation (on plane, engine running great, boat at 32 MPH or so), engine power shut down like a light switch (so I'm thinking electrical).

-Starts right up and idles like normal.
-Revs to 3,000 with a slight shake.
-Hits a wall at 3,000 under load.
-Compression 130 on all cylinders.

Forums say likely bad high speed windings on stator, so I replaced that with a new one even though the original tested OK.

I have a parts engine with newer electronics an some new parts laying around, so far swapped over:
-Power packs.
-Trigger (my wire insulation was mushy, pulling away).
-New plugs and wires.

Checked spark with timing light, all cylinders are firing at idle.

Is there some sort of "limp mode" switch on the engine to keep RPM low?

EDIT:
This guy had the same problem, one day it suddenly ran fine! Sounds like a loose connection.
https://www.screamandfly.com/showthread.php?254186-Motor-not-running-above-2000-rpms

Thanks,
John Hub

 

[faztbullet]

No limp mode...check spark at lake on trailer

 

[Hubjeep]

No limp mode...check spark at lake on trailer

Thanks. When on land the spark at idle looked great.

Now it's in the water, under load is the only time I can see anything is wrong.

-John Hub

 

[Hubjeep]

Is a timing light sufficient to check spark? I wonder if spark starts cutting out around 3,000 RPM.

 

[Hubjeep]

UPDATE:
-Picked up a DVA, all 6 coils read 170v-175v DC.
-Checked fuel pump outlet, good flow.
-Timing advances to 22* at full throttle.

Geez, this is frustrating. I tried unplugging the idle stabilizer box and saw no difference. When I first got the motor 10 years ago it wouldn't rev over 2,000, when I unplugged it it cranked up to 5,500 so I replaced it. Only problems since was the occasional power pack failure.

 

[Mikey N.C.]

Does it have the wmh carbs. with the thermal air valve in starboard head ?

 

[Mikey N.C.]

I'm thinking fuel. After electrical test you've done.

 

[Hubjeep]

I'm thinking fuel. After electrical test you've done.

That's what my Merc mechanic friend said too. Going to try running it on a portable tank to rule out a tank or line problem. He also suggested spraying gas into the carb with a spray bottle to see if power picks up.

Does it have the wmh carbs. with the thermal air valve in starboard head ?

Yes! What does it do?

 

[Hubjeep]

I just tried the portable tank, no change. It "sounds" like it "wants" to go (hear fluctuating RPM at lower engine speed), see video.

https://www.youtube.com/watch?v=0Qev2WN6wWw

 

[Mikey N.C.]

It restricts air flow to idle side , making a richer fuel mixture for cold start, when head reaches 100deg. valve open let's air in for normal metering. What is fuel pump pressure ?

 

[Mikey N.C.]

Watched video, if you didn't back out of it , and it fell like that . I'm thinking it sucked carb bowls dry.

 

[Hubjeep]

I backed out of it. It will run at 3,000 for as long as desired.

 

[faztbullet]

Sound like carbs are not fully opening.....and it defiantly has a ignition miss there at low speed in video. You can hear cylinder come and go.

 

[Hubjeep]

SOLVED! After fixating on the wiring, I removed the airbox and sprayed fuel directly into the carbs. Top carb was not getting fuel!!

Took carb apart and found the float stuck up due to the float seat somehow backing out (vibration?). Now it runs like usual!

Thanks for all the suggestions!
-John Hub