1994 Mercury 25HP 2 Stroke - trying to improve top end

[JGeiger]

1994 Mercury 25HP 2 Stroke, Motor S/N 0G002366, black flywheel, mechanical timing advance.

I've been making several adjustments lately and trying to improve my top end. With just me in the boat, I have not been able to get above 5500 RPM. Squeezing the fuel bulb has no affect on this, so I wouldn't think there is any sort of fuel line issue between the tank and the motor. The bulb gets hard and stays hard. It seems that the fuel level in the filter stays high.

The idle could be a little smoother, but it seems decent enough (on muffs and the water). The motor starts right up every time.

I have rebuilt the carburetor, changed the fuel filter, installed new plugs, new plug wires, and previously changed the reeds.

One thing I have noticed is that the idle adjustment needle doesn't seem to have a significant impact. I can actually close it most of the way without the engine stalling. For some reason, my carb rebuild kit didn't include a new seat for the primer injector. I am wondering if fuel is leaking past the check ball and affecting the idle mix?


On my last trip out, my motor seemed to be limited to 3650 RPM (according to my small inductive tach - pickup wrapped around Cyl1 plug wire). The engine sounded good until I tried to go above that - it seemed to rev up and down a little. This limitation was very repeatable. Under 3600, smooth operation. Try to go above, crap.

Does the switchbox automatically fold the timing back above certain RPMs? I seem to recall reading something like that in a Mercury service manual. Since I don't have electronic timing advance, and there isn't a separate rev limiter, I was suspecting the issue might be from the switchbox or the pickup. Using a timing light, the timing seemed very stable when running on muffs (which is lower RPM and no load on the motor). I had considered that possibly the pickup was somehow double-triggering, but I couldn't notice that with a timing light.

I have not been able to confirm timing or stator output voltages on the water - no copilot with me :-(

Could someone also point me to the shaft spacings for normal starting adjustments for the timing and throttle link arms? I have attempted to tweak both of those and want to make sure they aren't way out of whack.

After I brought the boat home and was thinking over it, I probably should have double-checked to make sure that the throttle plate wasn't going past full open, and the mechanical timing advance linkage wasn't going "over center" and pulling the trigger back (retarding the timing).

I'm about ready to just put all new ignition components on it (stator, pickup, switchbox, coils, plugs, etc.). Thoughts on me using Sierra components?

[EuropeanAM]

Sounds like you're going to need to invest in a Factory (Mercury) Service Manual for your engine. All of the adjustments and service techniques can be found there.

Sounds like your original problem may have been as simple as needing to re-prop.

Before you begin buying components- make sure you try a fresh set of the CORRECT NGK Spark Plugs, gapped correctly. No substitutes.

[JGeiger]

I have an electronic copy of the service manual covering my motor, but I didn't see the link dimensions listed.

The motor had performed acceptably with the current prop, so I didn't think that the prop was an issue. There are a couple of small nicks on the blades, but perhaps mistakenly, I didn't think they were enough to have affected the prop.

I have always used the NGK BP8H-N-10 (as called out in my copy of the service manual). Gapped to 0.040"

Does the switchbox have the ability to automatically fold the timing back above certain RPMs?

[EuropeanAM]

Mercury does not offer Electronic Copies of Service Manuals for consumer use (thus any you might have found are likely bootleg/copyright-violations).

Section 2-C of the correct Factory Service Manual covers the proceedures of Engine Timing, Synchronization and Adjustment.

The switchbox on this engine model is just that. A simple "switching device" that triggers the ignition. TIMING is solely controlled by the trigger position.

[JGeiger]

Well that just sucks about my manual. I was led to believe it was a legitimate copy. It even had an access code that was required to open it.

Could I bother you to get the part number for the correct manual for my motor?

[EuropeanAM]

Should be Part# 435093.

[JGeiger]

I've looked for that manual. The one's I've seen appear to be dated around 1984.
I believe my motor is a 1993, it's a S/N 0G002366. I've seen the manual for the 1994 versions, and they show different throttle linkages than mine has.

[JOE54]

put a timming light on it and watch timming for retarding that box should not have retard feature or advance feature fixed position trigger systems can do that