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1985 150 mid to high speed surge

6

670eddie

New member
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.
 
How about the park plug wire itself. Could it be loose at either end? Did you carefully inspect all ground points and wiring for fraying or melting? Sounds like you have thrown the kitchen sink at this one. Very frustrating no doubt.
 
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.
 
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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
 
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.
 
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Run with a timing light one cylinder at a time out on the lake.----Observe the flashing light.----Might help pinpoint the problem cylinder.
 
Todd H. said:
I am guessing you have looked at the CDI troubleshooting guide. It gives complete details about the CDI, testing, what fires what... your engine starts on page 163.

https://www.cdielectronics.com/wp-content/uploads/2016/12/7th-Edition-Troubleshooting-Guide_RS.pdf

I have a 1989 150 and have had to replace 3 ignition coils over the years, swapping them around can help locating the problem if the misfire / non fire moves with the coil, guessing you have tried that as well.
Click to expand...

Yes i did print off the cdi troubleshooting pages for the motor very helpful, and I swapped around coils as well.
 
racerone said:
Run with a timing light one cylinder at a time out on the lake.----Observe the flashing light.----Might help pinpoint the problem cylinder.
Click to expand...

I actually did try that with a timing light that has RPM gauge on it, it did not seem too clear to me as when the engine misfires its slows all cylinders down not just one and it may be happing so fast the digital rpm display is not really catching it. I feel like that technique would work a lot better on complete dead miss.
 
670eddie said:
Yes i did print off the cdi troubleshooting pages for the motor very helpful, and I swapped around coils as well.
Click to expand...
So, after going through all the steps in the CDI guide and using resistance and DVA readings, you concluded what? All values are as they should be?
 
my results

pulser coils spec. ohms 288-432, peak volts with load 1.6, no load 2. 7@1500 rpm

1. 348ohm 4.48 volts - 5.35 volts - 8.7@720
2. 341 ohm 5.37 volts - 6.90 - 8.7@720
3. 362 ohm 3.48 volts - 6.90 volts 8.7@720

charge coils spec.

low speed less then 2000 rpm. ohms 840-1260 peak volts with load 75. no load 85. 145@1500rpm

high speed 2000rpm+ ohms 19-29, with load 14. no load 14, 120@1500

low speed - 1046 ohm, volts 112 - 101 - 190@720 rpm
high speed - 23 ohm, volts 16.44 - 16.19. 156@1500

CDI output at the coil well running idle speed 125v min

cylinder # with volts

1. 191
2. 178
3. 165
4. 185
5. 180
6. 195

ign coil primary and secondary ohms all within spec

CDI output to the coils at the cdi box spec 58 peak volts - 120@1500RPM

cylinder # w/ peak volts

1. 99 - 208@1500
2. 101 - 194@1500
3. 95 - 208@1500
4. 101 - 208@1500
5. 98 - 208@1500
6. 95 - 208@1500
 
Todd H. said:
So, after going through all the steps in the CDI guide and using resistance and DVA readings, you concluded what? All values are as they should be?
Click to expand...

From what iv seen yes, as they should be or better, however this could be problem that only begins under load.
 
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