"Sputter" at High RPM

[andywb79]

Hi all. Having some issues at top end. Motor runs great until it hits around 5500 RPMs then it'll sputter/hesitate. Current top speed is around 62-65MPH. Motor starts up and idles perfectly and will get on plane without issues. The following has been addressed. Any next steps would be appreciated.

1. Fuel line from tank to fuel pump (new bulb).
2. Rebuilt fuel pump (new diaphram kit).
3. Replaced VST fuel filter
4. New spark plugs (IZFR6J) gapped at .042.
5. Compression test
1 - 115
2 - 111
3 - 115
4 - 115
5 - 116
6 - 110




1. 1B809497 (389 engine hours)
2. 2011 Optimax ProXS 250 on a 2011 Ranger Z521
3. Tempest Plus 25P (w large PVS plugs)
4. 5500 RMP (max)
5. Smartcraft Gauge

[EuropeanAM]

Air/Fuel Pressure Tests

OHMS test the plug wires, and check for any corrosion on the coil towers.

[andywb79]

Thanks, Don. I’ll report back.

[andywb79]

Quick update:

I found several coils with corrosion and several of the plug wires had erratic ohm readings.

I replaced all of the plug wires and several of the coils, but the issue remained. At this point, I felt it was best to take to a local mercury technician.

Here's what all he found:

1. Performed G3 diagnostics
2. Rechecked all compressions. All were at 120psi.
3. Both fuel and air pressures were low. (Fuel was below 90. Don't recall what the reading was for air).
4. Upon inspection of compressor found some damage.
5. Used borescope to inspect cylinders, reeds, and other internals. All looked good.
6. Temp on engine would not exceed 90 degrees at idle.
7. Replace thermostats.
8. Replaced compressor with new out of box compressor. (came with a nice discount)
9. Cleaned/Inspected VST and found high pressure pump rubber sleeve was split which was allowing fuel PSI to leak down after shutoff.
10. Took new readings at idle: 95PSI for air/105 for Fuel. Temp readings at idle are back up in the 120-130 range.

I will be taking out this weekend for testing.

Thanks for your help and knowledge, Don.

[JOE54]

spent many days in a boat like you have exactly,,,, best prop we ran on it was 24 FURY 3 BLADE

[andywb79]

@Joe54, really good info. I have not been satisfied with my tempest.

[andywb79]

Final update. We've located the issue after more troubleshooting.

Here's some more things we did before finding the root cause.

1. New plugs. (After an hour of running, the plugs were extremely fouled)
2. Replaced the tuner gasket. (found a tear in the gasket)
3. Replaced MAP sensor. (Readings were inconsistent)
4. Pulled the reeds and found several that were chipped and a few that were loose allowing them to "chatter".

After all of this, the hesitation at WOT (it's more like a cylinder miss) was still there. While inspecting coils, we found #6 coil plug had a bent connection which prevented a good connection to the coil.

IMG_7202.jpg

IMG_7198.jpg

Thanks Don for all of your help. This truly is the best forum!

[EuropeanAM]

Hang on to that mechanic/tech. Someone willing to go the extra miles to chase down what turns out to be MULTIPLE problems is hard to find.

[andywb79]

Hang on to that mechanic/tech. Someone willing to go the extra miles to chase down what turns out to be MULTIPLE problems is hard to find.

Agree 100%! He even took time out of his day to take a test run on the lake.

[lpugh]

Quick update:

I found several coils with corrosion and several of the plug wires had erratic ohm readings.

I replaced all of the plug wires and several of the coils, but the issue remained. At this point, I felt it was best to take to a local mercury technician.

Here's what all he found:

1. Performed G3 diagnostics
2. Rechecked all compressions. All were at 120psi.
3. Both fuel and air pressures were low. (Fuel was below 90. Don't recall what the reading was for air).
4. Upon inspection of compressor found some damage.
5. Used borescope to inspect cylinders, reeds, and other internals. All looked good.
6. Temp on engine would not exceed 90 degrees at idle.
7. Replace thermostats.
8. Replaced compressor with new out of box compressor. (came with a nice discount)
9. Cleaned/Inspected VST and found high pressure pump rubber sleeve was split which was allowing fuel PSI to leak down after shutoff.
10. Took new readings at idle: 95PSI for air/105 for Fuel. Temp readings at idle are back up in the 120-130 range.

I will be taking out this weekend for testing.

Thanks for your help and knowledge, Don.

Dont run till fuel pressure is up to 109 PSI

[EuropeanAM]

Good catch, Leon. Or at the very least, swap gauges to confirm it was the gauge that skewed the readings.

Needs 14-15 psi differential (14.7 technically).