Yamaha 2T mid to high speed surge

[670eddie]

Hello all I'm new here, I'm a DIY "tech" I'm having some trouble in resolving a mid to high speed surge that I believe to be cylinder 5 intermittently firing on an 1985 Yamaha 150 v6 on a bass boat, I believe its cylinder 5 due the the visual condition of the spark plug, the plug is appearing mostly clean and wet(with gas) with a dark electrode porcelain and ground strap, where as all others are brown. the water pump was new 2 years ago and never overheated. this is what iv done/checked so far (not in order)

• Compression tested, hot engine & cold - all 110 psi (all within 2% of each other)
• Rebuilt fuel pump (minus the stainless steel one way valves)
• Pressure and vacuum tested fuel pumps
• Tested fuel pump pressure and vacuum at WOT
• Set timing pointer and initial and advance with a dial indicator as per manual, checked with light to confirm
• Rebuilt carbs and set float height per spec in manual
• Replace the Yamaha control unit due to strange ohm readings
• Replace the spark plugs
• Resistance tested the entire ignition system
• Replaced 2 ignition coils due to failing resistance test
• DVA tested the entire ignition system at idle and @1500 rpm on ear muffs (I know no load but didn't have a 2nd person to drive well testing under load)
• Replace the primer bulb and all the fuel lines and connectors and checked the tank pick up tube for obstructions (also tested with a new aux tank)
• Installed a brand new ignition coil onto cylinder 5
• Tested underway in water with the green wires disconnected from the rectifier/regulator

Items I have Not yet checked and possible causes?

• Reed valves
• Cylinder head warpage
• Cylinder head crack
• Cylinder crack
• Crank shaft cylinder seal ring???

Would a hair line crack in the cylinder head or cylinder be enough to suck water through and cause a misfire but not show up on a compression test? I feel like this is an issue due to over fueling of cylinder 5 or ignition miss fire that is happing intermittently under load as the spark plug looked washed clean and some of the piston top as well, however I have not been successful in finding any evidence of water in the cylinder, plug looks wet an looks and smells wet of fuel I have picture of the plugs I can post as well as DVA test results that all exceeded the spec in my service manual.

[670eddie]

I was doing some spark gap jump testing with an adjustable unit and what I found was cylinder 3 was struggling(intermittently) to jump a 5/16" gap where all the rest would do it just fine, I'm thinking that under cylinder pressure this is where the misfire is coming from, after swapping a new coil to that cylinder and it continuing to misfire I was trying to isolate the problem to either the cdi unit or the trigger. not knowing which trigger fires cylinder 3 I began to remove one trigger wire at a time and retest hoping to find the one that would totally kill spark on cyl 3, however I found that it seem to stay intermittent through all the wires until I reach the last wire, white with red trace. to my surprise when I removed the wire/red trace trigger wire the spark on cyl 3 became very strong and would easily jump the 5/16 gap, this left me with more questions then answer as I did not get the test results I was expecting.. how is cyl 3 spark better after disconnecting a trigger? the cdi shorted internally or somehow that trigger being bad dragging down output and back feeding through the other trigger wire? the system has 3 triggers 2 wires on each.

[670eddie]

Does anyone know which trigger wire fires which cylinder? there are 3 triggers(pulsar coils) 2 wires on each trigger, each trigger can fire 2 cylinders independently by the cdi sensing which flywheel pole (north or south) passes that trigger first, ie north pole passes trigger 1 first signals cdi to fire cylinder 1, south pole swings pass it next it fires cylinder 2

[670eddie]

does anybody have an idea? I'm leaning towards the CDI as the triggers are pretty simple devices and I would think it be weak on 2 cylinders if a trigger was at fault.