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)
Items I have Not yet checked and possible causes?
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.
- 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.