250xs Idle and Low RPM Misfire

[JustAGuy95]

2005 250XS S/N: 0T998699 - Roughly 550 Hours

So I recently purchased a boat with this 250XS on it. It has a random misfire at idle. If I give it any gas at all it gets worse and more constant, it hits a wall at around 1000-1200 RPMS and struggles to get on plane for about 10 seconds but once it gets over around 1200 RPM it runs fine! Not knowing what kind of maintenance has been done on it I decided to start replacing some items on it and see if anything would help the misfire but I've reached my limit of knowledge with this engine.

What I've done:

-Replaced fuel/air/tracker diaphragms
-Replaced spark plugs
-Replaced ignition coils and wires due to out of spec for resistance
-Sent reed cages to CCM to have rubber coating machined off because the coating was deteriorating and new reeds installed.
-Fuel pressure 90 psi running and air pressure 80 psi. (Both held well after shutting engine off and did not bleed off quickly)
-Checked compression, 119-122 across all cylinders.

Not sure if the previous owner ran quickcleen or quickcare in it but I have ran it for about 8 hours with both in every tank.

Can anyone recommend anything to look at or check before I take it to a shop? Only thing I know to do is to send fuel/air injectors off to be cleaned and tested.

Thanks!

[Savage]

Motor needs Diacom testing performed on the water.

[JustAGuy95]

Thanks for the reply Savage.. I found where I could rent the software and cable to perform Diacom diagnostics myself. I'm new to outboards but I feel like I'm above average as far as mechanical and computer/software stuff goes. Is the diacom stuff pretty easy to use and understand or should I just get a shop to do it?

On another note I left the batteries turned on last week and they drained so I went and got a charger and got them back charged to 100%. I took coil, coil driver, air/fuel injector, crank position, ect. connectors apart and applied contact cleaner/conditioner on them. Also took off main power connections at the motor and cleaned up the terminals good and it still had a little stutter and misfired a little getting up on plane but it did much better than it has in the past.

[JOE54]

80-90 psi fuel and air psi checked with glycerin filled gages ????? both are low/ 95-109 14lb differential needed , remember fuel follows air = low air psi = low fuel psi

[JustAGuy95]

So I just rechecked it because it’s been awhile since I did and it was 85. And no not technically using the correct gauge for it. Just trying to figure it out with what I got. I guess it really needs to be checked with the correct equipment.. Question though.. if I had a weak compressor and I revved the engine slightly to around 900 RPM, would I see an increase in pressure?

[Savage]

In the Mercury service manual is detailed instructions for testing air and fuel pressures.