1989 Mercury 90 HP Power Problem

[WildWillJAX]

I have a 1989 Merc 90HP oil injected, 3 cylinder,S/N OC162559 with a low/no power production from the top cylinder. During my last fishing trip the motor was hard starting,then shortly after throttling up on plane it lost power and began to stumble at higher rpms. Rough erratic idle condition also. <br />I ran some tests and found the following conditions.<br />Compression=125 PSI on all cylinders<br />Coil Output=Good on all 3<br />Plugs=Number 1 (top) has no carbon accumulation and appears not to have been firing. Disconnecting the no.1 plug wire while engine is running results in no loss of rpm at idle however disconnecting Nos 2 and 3 causes a 400 rpm drop (each).<br />Replaced plug wires and plugs=No change in running condition<br />De-carborized cylinders= During this procedure, spraying decarborizer in top cylinder did not cause the engine to bog down as much as nos 2 and 3.<br />Little to no affect on rpm change when adjusting air/fuel mixture screw on No1 cylinder.<br />Overall assessment is that the number 1 cylinder is not producing power.<br />What Next? Is this a carb symptom? Possible Reed valve failure?<br />Testing Reed Valve Advice and Any logical help greatly appreciated.<br />Thanks in Advance.

 

[WillyBWright]

Re: 1989 Mercury 90 HP Power Problem

Either you're not getting any gas thru that carb at all, or you're losing spark under compression. Even a restricted carb will let in enough fuel to burn and leave traces of carbon on the sparkplug. But if there's no gas at all getting there, then it could be a stuck float valve.<br /><br />If you remove the carb, you should be able to see the reeds. But again if your motor had a bad reed, it would still be able to fire enough to leave some carbon on the sparkplug. I'm leaning toward weak spark.

 

[WildWillJAX]

Re: 1989 Mercury 90 HP Power Problem

Originally posted by WillyBWright:<br /> Either you're not getting any gas thru that carb at all, or you're losing spark under compression. Even a restricted carb will let in enough fuel to burn and leave traces of carbon on the sparkplug. But if there's no gas at all getting there, then it could be a stuck float valve.<br /><br />If you remove the carb, you should be able to see the reeds. But again if your motor had a bad reed, it would still be able to fire enough to leave some carbon on the sparkplug. I'm leaning toward weak spark.

 

[WildWillJAX]

Re: 1989 Mercury 90 HP Power Problem

I have removed and inspected carbs, all are clean and like new inside and out. I also did a visual on the reed valves, all looked clean, clear, and fine. I gently opened the valves individually with a pencil using the soft eraser end and they opened and closed easily. I have ruled out the fuel delivery system.<br />Update:<br />When testing the trigger coil for resistance values the number one coil (violet wire) tested slightly low on OHMS 940 to be exact. Specs call for 1100 to 1400.<br />The voltage created by the winding is only .4 volts A/C less than the others(1.20 versus 1.6 Volts A/C on 2 and 3) under cranking.<br />Question?<br />Before I pull the trigger coil and repalce it, can this small difference in resistance and voltage be enough to cause the secondary circuit to fail when attemting to fire the plug under pressure? The spark looks crisp and blue at the spark plug lead when held near ground and the timing light works AOK when checking the timing using the same NO1 spark plug lead. Your opinion is appreciated.<br />Thanks<br />Wild Will