I have a 1999 Johnson 250 v-6, carbed outboard Model J250PZEEN that will run fine from idle up to 2000 RPM and from about 3600 RPM to 5600 RPM. Between 2000 RPM and 3500-3600 RPM, the engine feels like it is not running on all 6 cylinders, it is very rough, sounds like it is missing, it is definately down on power and if the wind is right, I can smell raw gas. The engine does not set any alarms while this is occurring. The system check horn sounds when I initially turn the ignition key on, and the system check gauge illuminates all the warning lights, so I know that it is working.
I have checked the plugs and they do not look fouled. I use Champion QL78 YC. The power pack is good  I had a brand new one that I carry as a spare which I installed and the problem is still occurring. The stator was replaced with a CDI unit about 2 months ago when one of the magnets came off the flywheel and wasted the original one. I replaced the magnet and checked that all the other ones were properly attached and located. I have also checked for spark  I have good spark on all cylinders with a 7/16 gap. I re-checked the connections between the power pack and stator, timing base, and out to the ignition coils. All those connections are clean and good and in the proper locations, with no damaged wire insulation. I tested the shift circuit switch per the OMC manual and it is good. I also checked the blocking diode for the S.L.O.W. system and it checked out satisfactory per the manual. The connections at the battery and the starter are clean and tight.
I have new fuel line from the tank to a Racor filter and from the Racor filter to the fuel pump. The primer bulb is genuine BRP, not an aftermarket cheapie. The hose clamps are genuine BRP plastic ones, not the worm gear type. I replaced the fuel lines a while back because they looked weathered in some spots.
I rebuilt the carbs to try to solve the issue - found nothing in them, no dirt, no varnish, nothing. Made sure the floats were set right and that the carb body to throttle body o-ring did not get pinched when I reassembled them.
Tried squeezing the primer bulb at the RPM band in question to see if it was a fuel pump issue - no change. Tried pushing the choke - bog down. Also completely by-passed the pump and ran pre-mix straight to the carbs - no change.
Tried bypassing the shift switch to see if it was grounding the cylinder bank - no change, reconnected it. Also by-passed the S.L.O.W. input on the power pack. No change - reconnected it. Swapped the ignition coils and plug wires with known good ones - no change.
Performed a link and synch with the exception of WOT throttle timing.
Also took a compression check -
1- 110 psi 2- 105 psi
3- 115 psi 4- 110 psi
5- 110 psi 6- 105 psi
Placed an inductive timing light on each plug wire while in the rpm band to see if I was dropping a cylinder. I am not dropping any cylinders.
Lastly, I noticed the engine is not turning full RPM like before. Initially would turn 5600-5700 rpm but on my last outing, it would only turn 5100-5200 rpm with same loading, prop, hull and sea conditions.
Any suggestions with respect to what else I can check would be appreciated!<br /><br />
Modified by Br1dgemann at 1:12 PM 5/18/2011
Modified by Br1dgemann at 8:52 AM 5/19/2011
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