1980 mariner 25hp DVA test

Motor will not exceed 3k rpm when under load. The last lake test the rpms started around 3k but slowly climbed to ~4100. Once I throttled down it would never exceed 3k again. My theory is the performance degraded once the motor warmed up.

Compression is 120 on both cylinders. Starts 1-2 pulls and idles perfectly.

Rebuilt the carb and replaced all fuel related components. Strong spark jump 7/16 when tested. All resistance test on the coil, pulsar, lighting coil are within spec. Flywheel magnets look good. No cracks.
replaced switch box cdi
new spark plugs and boots


Rechecked the timing and even went as far as 1-2 degrees each way. No change then went full advanced with no measurable change.

New next step is to build a DVA adapter to test the pulsar trigger. What is the correct voltage for the pulsar trigger? I truly believe the trigger signal is off since the timing had no effect on the WOT performance.

Did this problem develop suddenly ?

6.5 to 8.5 V

Unless the motor is skipping at WOT, you're wasting your time checking electronic components.

I suspect your rig is simply over propped and struggling.

Jeff

racerone said:

Did this problem develop suddenly ?

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problem has been present since day 1 of purchase. In the beginning it would act up intermittent which led me down alternate paths. But I have had the boat up on plane 5500 rpms with me and the wife. Now the it will not exceed 3k with me alone. Lower unit seems to be functioning as it should. Changed the lower unit oil twice. No signs of water. No abnormal vibrations or sounds, shifts into gear without any issues.

boat is a 14ft Dixie Malibu skiff
prop 9 7/8 X 10.5

I suspect you have a failing coil that's skipping though you might not hear it. Suggest you change them out--not too expensive.

Jeff

fastjeff said:

I suspect you have a failing coil that's skipping though you might not hear it. Suggest you change them out--not too expensive.

Jeff

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Part lookup is calling for a 95188T. None currently available.

Would this work?

I highly doubt its the coil or trigger pulsar... I have seen the exhaust tube come loose and cause similar problems. Coil and pulsar can be tested with 12V timing light. Check coil either lead at RPM and see if it drops out. Pulsar is triggered by flywheel magnets, its either works or it don't. Test by verifying TDC on flywheel. You should see timing move as magneto plate moves. It really sound like its over fueling like high speed jet is damaged/wrong or missing or a problem with emulsion tube.

faztbullet said:

I highly doubt its the coil or trigger pulsar... I have seen the exhaust tube come loose and cause similar problems. Coil and pulsar can be tested with 12V timing light. Check coil either lead at RPM and see if it drops out. Pulsar is triggered by flywheel magnets, its either works or it don't. Test by verifying TDC on flywheel. You should see timing move as magneto plate moves. It really sound like its over fueling like high speed jet is damaged/wrong or missing or a problem with emulsion tube.

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TDC does line up with the mark on the flywheel. I have tripped checked the timing and adjusted a few degrees each way with no change. Haven’t checked with a timing light. But I did disconnect the throttle linkage to the carb and rotated the stator manually. The rpms did increase to ~1500-2000 by advancing the stator only.

I rebuilt the carb recently and replaced the float and inlet valve and seat. Cleaned the carb and high speed jet. I could tell someone had some struggle getting the main jet out previously. But I don’t see it affecting the flow. Thoughts? I don’t see a large amount or really any raw fuel coming from the exhaust. Not sure if it would be noticeable or not.

It’s funny you mention the exhaust tube as I was wondering that myself recently. I do notice exhaust gas coming from the prop while it’s on the muffs. But I have also seen some fuel/oil(it’s black) leak out of this area after running the motor. It’s usually a drop or two on the garage floor when the motor is trimmed down. This got me thinking maybe there is an exhaust leak or blockage.

I definitely would replace the jet for giggles(Yamaha) and drop unit to check tube...……...

faztbullet said:

I definitely would replace the jet for giggles(Yamaha) and drop unit to check tube...……...

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I plan on dropping the lower unit this weekend. However where exactly would the exhaust tube come loose? Doesn’t the exhaust travel through the housing and out the prop? (Blue line)

Bottom of powerhead...

fastjeff said:

I suspect you have a failing coil that's skipping though you might not hear it. Suggest you change them out--not too expensive.

Jeff

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purchased a used coil and now the motor accelerates up to 3800 and slowly climbs to 4800. However I noticed a small fuel leak around the upper crank seal near the fly wheel. Could this be causing the unusual performance ?

also the lighter color plug was top and the darker plug was bottom. Could the fuel leak be causing a slightly lean condition? Or the used coil not igniting the bottom plug properly.