Brand New Mercury Reman 300XS (just out of break-in) Surging/Skipping on the Top End

[Savage]

Brand New Mercury Reman 300XS (just out of break-in) surging/skipping on the top end.
No Alarms.The powerhead was replaced due to an oil leak.
The WOT RPM's hit a wall at 5400 RPM, when the boat setup used to turn +6000 RPM. I took a quick look at the plugs late last night and they all appeared dry and normal. Next on the list is fuel pressure tests.
I'm leaning more towards a fuel issue because surge/skip can't be felt at slower cruising RPM's, but i'm no locked in on that theory.

My list of possibilities not in any order:
TPS
MAP
Compressor reeds
Fuel pump(s)
Coil
Spark plugs
???

Can't lake test again till Friday to get on the water fuel pressure tests, so I'm seeking out other items to check while its sitting inside my shop.

SN: 1E053598

[JOE54]

TPS and map sure are possibilities, seems comp reeds would show up before 5400 as would faulty coil , I know a man of your knowledge of these engines will spot it in a matter of minutes ,strange for me to even make suggestions for a mtr your looking at repairing

[KJK]

Following! Hope you get it figured out.

[Savage]

Joe I appreciate that, but I think I'm going to need you and Don on this one!

I took pictures of the plugs, one photo with the light colored side facing up, and one picture, with the dark side up. The #6 plug is a little darker than the rest, when the dark side is viewed, but its nothing to right home about.

Spark plug gaps were all way off. I reset all the plugs to .044 and reinstalled.

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[Savage]

Tore the VST completely down since I can't water test until Friday, and didn't find anything wrong.
Checked float level.
Checked rubber grommet.
Checked for debris
Checked wiring connections.
Checked screens.


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[Dan21XRS]

... Dan

[lpugh]

Savage something have done before to check for fuel starvation up to the pulse pump is rig a automotive electric to the hose that feed the motor and run the outlet back to the tank so that it is running under maximum flow rates and keep and eye on it to see if flow slows down. Could also rig a Vacuum/pressure gauge to the suction side as well. Just an idea to easily eliminate that area in the shop
My current 250prxs had a similar problem when I replaced the old 225 optimax, ended up having to get rid of all the excessive hose length and the splice, as well as new bulb to get above 5800. Was fine with the 225 but not good enough for the 250
Those plugs look pretty good to me esp. if it was ran through breakin with them, 6 looks a little oil rich

[Savage]

Thanks for the suggestion Leon. I have a Holley "Red" pump that I can run that test on. I use it for draining fuel tanks, and it it is easy to see the fuel start to aerate, when the tank gets low and starts to suck air. Easy test to try tomorrow.


I'm also wondering now if the MAP sensor got oiled down, while the motor was running double oil, during the break-in period ??????

[JOE54]

very possible/ vst looks brand new

[EuropeanAM]

You probably already checked this, but it would be a good idea to stick an INCH lbs. torque wrench (set at 70 lbs/in) on the belt tensioner. It should click BEFORE the tensioner contacts the flywheel cover retaining stud (stb side).

If it does not- the spring is worn (or improperly indexed). This will kill the top end, severity depends on how far off it is.

[Savage]

You probably already checked this, but it would be a good idea to stick an INCH lbs. torque wrench (set at 70 lbs/in) on the belt tensioner. It should click BEFORE the tensioner contacts the flywheel cover retaining stud (stb side).

If it does not- the spring is worn (or improperly indexed). This will kill the top end, severity depends on how far off it is.

That's on my list for tonight! Sent you a PM.

[Savage]

Tracker valve diaphragm not in the best shape. It's not torn completely through.....yet.

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[EuropeanAM]

Yeah... but it amazes me how little it takes for fuel to migrate at those pressures.

[Savage]

You probably already checked this, but it would be a good idea to stick an INCH lbs. torque wrench (set at 70 lbs/in) on the belt tensioner. It should click BEFORE the tensioner contacts the flywheel cover retaining stud (stb side).

If it does not- the spring is worn (or improperly indexed). This will kill the top end, severity depends on how far off it is.

Tensioner clicks off at 70 lbs/in. I tested it at 80 lbs/in and it clicked off also.

[EuropeanAM]

[Savage]

Yeah... but it amazes me how little it takes for fuel to migrate at those pressures.

But is it the smoking gun??? I'm not so sure since it didn't have a hole completely through. I have to drive one hour, one way, to get one tomorrow morning, so I can continue to test.

[EuropeanAM]

It is certainly a strong possibility- and an obvious problem. If this were anyone else's engine, we would both be telling them to replace the Tracker Valve and test run it before doing ANYTHING else.

[Savage]

It is certainly a strong possibility- and an obvious problem. If this were anyone else's engine, we would both be telling them to replace the Tracker Valve and test run it before doing ANYTHING else.

Agree 100%!

[Savage]

Tracker valve replaced and ran the motor on the muffs.

Low Pressure fuel pump pressure at the VST @ idle 23 PSI.

Fuel/air Pressure @ idle: ........Fuel 104.. ...Air 91
5 seconds after shut down ..............96..... .....86
10 seconds after shut down .............94......... 84
15 seconds after shut down............. 92..........81

Air compressor pinch off test the pressure went from 91 PSI to 110 PSI. Shouldn't this pressure go higher than this during the pinch test??? Manual states 120 PSI, or higher.
HP Fuel pump pinch test the pressure went from 104 PSI to +160 PSI (pegged the gauge).

[EuropeanAM]

The later air compressors are not quite as tight as the earlier versions- most will make it to approximately 110 psi (seldom will one achieve above that). Don't fail one strictly on that unless it is EXTREMELY lazy getting there.

If swapping gauges provides similar results, I would recheck the pressures at 1000-1100 RPM's and make sure the differential (delta) is within spec.

Otherwise not anything I'm concerned with from those numbers....