Expert: Jeff Grigg - 12/21/2006

Question
Here are the results of my last series of tests done on the engine.All tests were done with the Clymers manual. Compression is 120 on all cylinders. Power pack isolation diode test shows both packs diodes are good (continuity in one direction only).The engine starts easy and idles pretty good. Rpm sounds low at WOT. Tach in dash is broken. I haven't diagnosed the problem but I believe it is  stator or rectifier problem, however all engine tests were done with a fully charged battery. The results of todays testing are as follows. The DVA shows the charge coil output of 185 vdc on the left bank and 155 on the right bank. Right bank is a little low but that may be a slight error in my multimeter. Sensor coil output is 1.6 vdc on the left bank in both positions and 1.4 vdc on the right bank both positions. My manual says reading should be greater than 0.3 vdc but doesn't say how much is too much. Now for the part that confuses me. The manual says for the power pack output test to backprobe the 2 wire female plug for the ignition coil. The reading should be 170 vdc. The reading I get is about 1.5 milivolts on all 4 pins. That could be induced voltage and not an output at all. The procedure I used was black lead to a ground point on the engine and disconnected the 2 wire connector to the ignition coil. I backprobed the female connector (from the power pack). I assume that I am doing something wrong for this test. It isn't reasonable that the plugs would fire with 1.5 milivolts powering them. Now the last test. I checked the ignition coils. primary resistance on all 4 coils is about one ohm. Secondary resistance on #2 and #4 is 275 ohms (well inside the good range. On the right bank, #1 had a reading of 450 ohms (about 150 ohms high) and #3 read infinity. I changed #3 ignition coil and now have a good reading. I believe there was a short in the plug wire that manifested when it was twisted at an angle to allow me to take a reading.

I still question why (if the problem is a powerpack) when I move the powerpack and both ignition coils from the left bank to the right bank and from right to left that the double firing plug would not move also to the right bank?
-------------------------------------------
The text above is a follow-up to ...

-----Question-----
My initial problem was that my boat would not plane out. My tach doesn't work so I can only guess that my rpm is low (less than 3k).I replaced the fuel pump and rebuilt the carbs as part of a tuneup but no help. The motor fires on all cylinders (steady and blue with spark tester set at 7/16"). Timing is dead on @28 degrees. All components checked good with a cd voltmeter except #3 coil. I changed it even though it was firing good. Changing it did not help. The only problem I can find is a double fire on #2 cylinder. Both powerpacks pass the diode check, and swapping both coils and powerpack from port to starboard and vice versa does not change the double fire from #2. A timing light on #2 shows #2 firing at TDC on #2 and #4. #4 shows to be firing correctly with a light hooked to it. Is it possible for the charge and sensor coils in the stator to cause this double fire symptom? Have I missed a test that would have indicated a problem with the stator assembly. Resistance and voltage are within specs?
-----Answer-----
Bill, why doesn't your tach work? Have you tested the rectifier and stator?
First of all you need to index the flywheel and double check your firing.
I suspect you have an internal problem in the pack on the port side. If all of the tests check out and you are still double firing replace that pack. It is possible that the isolation diode in the other pack, the one for 1&3 is bad as well.
I assume you are using a service manual with the proper values. Check the system again and detail me the values and sequence of testing so I can see if you overlooked something.
Do you know how to do an isolation diode check?
Post back.
Jeff

Answer
Bill, I suspect you really don't have a double firing problem. Most automotive timing lights do not work very well on outboard ignition systems. The problem is they pick up stray impulses and show as a double fire when in actuality the cylinder isn't double firing. Particularly since you are seeing this on #2. The pick up is probably too close to the other plug wire.
What does concern me is the voltage differences from the stator. I like to see no more than 5 Volts difference from one side to the other. If you pull your flywheel you will probably see some cracks or leaking potting material etc.
The motor may run perfectly fine at idle but at WOT where it really needs the voltage the stator is probably leaving you or at least not supplying enough voltage to fire the cylinders properly.
Pull the flywheel and replace the stator.
Jeff