Weak Spark: 1975 70Hp Evinrude

[classiccat]

Ignition gurus,

I'm getting weak-spark on all-3 cylinders of my 1975 70Hp Evinrude (70573B).

I'm about to drop some coin on additional ignition component(s) and I'm hoping to get your feedback on what else to check to isolate the problem(s). So far I know I need a new rectifier and i'm questioning the powerpack...or is a bad rectifier enough to cause weak spark on all 3 cylinders (hitting 550 RPMs during spark check with a freshly charged starting battery & no LU).

Quick background on the motor. I just purchased it and only heard it run briefly when with the seller...I had him cut the engine when I learned that the water pump wasn't changed in 2 decades...The LU is on my workbench getting a re-seal. Compression is good (139, 132, 138 cold) and it had 3 cracked ignition coils which I swapped out for new.

Spark test:

• 550 cranking RPM
• zero spark at 7/16" gap
• weak blue/white spark at 1/4" gap

Rectifier (Hosed):

• Yellow->Ground & Ground -> Yellow: Both Open (bad)
• Yellow/Grey -> Ground & Ground -> Yellow/Grey: Both shorted (bad)
• Red wire insulation is badly frayed but still connected

Powerpack (not sure):

• disconnected terminals 6 & 11; reverse-biased is open (good) but forward biased I'm getting 1.39Meg-Ohm (not sure)

Ignition Coils (new)

• brand new and getting 2kOhm across each secondary

Sensor coil (good; spec is 8.5 +/- 1Ohm)

• 7.9 Ohm & no path to ground
• 7.8 Ohm & no path to ground
• 8.1 Ohm & no path to ground

Charge Coil (good; spec is 630 +/- 50Ohm)

• 626 Ohm & no path to ground

Stator (good)

• Yellow-to-ground: open
• Yellow/grey-to-ground: open
• Yellow to Yellow/Grey: 1Ohm

 

[classiccat]

Re: Weak Spark: 1975 70Hp Evinrude

I'll add that all measurements were made with a Fluke 75.

I don't own a DVA but it is definitely on my short-list.

 

[guitman32]

Re: Weak Spark: 1975 70Hp Evinrude

Ive found that output tests are more telling than the resistance tests especially when it comes to the timerbase, stator and powerpack...id say get or make a DVA then test the output I think it will give a clearer (thats a laugh) idea of where the culprit lies. Timerbase tells powerpack when to fire power received from stator...also dont forget a stephens load adapter to test the output from the powerpack coils (at least thats what is called for for my '89 175). In the CDI literature they say you can do the test with just the DVA, but i just opted for the load adapter just to be on the safe side.

 

[classiccat]

Re: Weak Spark: 1975 70Hp Evinrude

Ive found that output tests are more telling than the resistance tests especially when it comes to the timerbase, stator and powerpack...id say get or make a DVA then test the output I think it will give a clearer (thats a laugh) idea of where the culprit lies. Timerbase tells powerpack when to fire power received from stator...also dont forget a stephens load adapter to test the output from the powerpack coils (at least thats what is called for for my '89 175). In the CDI literature they say you can do the test with just the DVA, but i just opted for the load adapter just to be on the safe side.

I probably have all of the components to make a DVA in my garage (diode, resistor and a 0.47uF cap).

good suggestion on a load adapter! resistance of the ignition coil primary and some sort of inductive load?

 

[Blacksting]

Re: Weak Spark: 1975 70Hp Evinrude

Ignition gurus,

I'm getting weak-spark on all-3 cylinders of my 1975 70Hp Evinrude (70573B).

I'm about to drop some coin on additional ignition component(s) and I'm hoping to get your feedback on what else to check to isolate the problem(s). So far I know I need a new rectifier and i'm questioning the powerpack...or is a bad rectifier enough to cause weak spark on all 3 cylinders (hitting 550 RPMs during spark check with a freshly charged starting battery & no LU).

Quick background on the motor. I just purchased it and only heard it run briefly when with the seller...I had him cut the engine when I learned that the water pump wasn't changed in 2 decades...The LU is on my workbench getting a re-seal. Compression is good (139, 132, 138 cold) and it had 3 cracked ignition coils which I swapped out for new.

Spark test:

• 550 cranking RPM
• zero spark at 7/16" gap
• weak blue/white spark at 1/4" gap

Rectifier (Hosed):

• Yellow->Ground & Ground -> Yellow: Both Open (bad)
• Yellow/Grey -> Ground & Ground -> Yellow/Grey: Both shorted (bad)
• Red wire insulation is badly frayed but still connected

Powerpack (not sure):

• disconnected terminals 6 & 11; reverse-biased is open (good) but forward biased I'm getting 1.39Meg-Ohm (not sure)

Ignition Coils (new)

• brand new and getting 2kOhm across each secondary

Sensor coil (good; spec is 8.5 +/- 1Ohm)

• 7.9 Ohm & no path to ground
• 7.8 Ohm & no path to ground
• 8.1 Ohm & no path to ground

Charge Coil (good; spec is 630 +/- 50Ohm)

• 626 Ohm & no path to ground

Stator (good)

• Yellow-to-ground: open
• Yellow/grey-to-ground: open
• Yellow to Yellow/Grey: 1Ohm

when I get good info I copy n paste it n save it . will these tests work on my '85 70hp johnny ???

 

[classiccat]

Re: Weak Spark: 1975 70Hp Evinrude

when I get good info I copy n paste it n save it . will these tests work on my '85 70hp johnny ???

These are really just disaster checks; coils are continuous & not hard-shorted to ground & diodes are behaving like diodes (in the case of my external rectifier, they're not). What I really need to see (and what guitman32 eluded to above) is to see the peak output voltages of various components. I could just swap out the questionable parts and hope for the best however finding root-cause is important; i don't want to be throwing a brandy-new powerpack to the wolves nor do i want it to decide to crap-out when I'm in the path of an ocean liner :lol:

Try to get your hands on the factory service manual. You're ignition looks different from the parts lists but whats "inside" could be similar/same (example; '75 uses a powerpack3 with screw terminals where the '85 is fully encapsulated with plug/harnesses).

 

[classiccat]

Re: Weak Spark: 1975 70Hp Evinrude

One example of why the FSM for your engine is important, my powerpack test was hijacked from another thread since the FSM doesn't have a quick-n-dirty disaster check for the PP3.

Looking closely at the powerpack-3 schematic from my FSM, I'm questioning whether or not I should have different readings by switching the test leads (it's supposed to be going across a resistor, not a diode!).

 

[Paddy60]

Re: Weak Spark: 1975 70Hp Evinrude

Bluefin,

I built the DVA when I was trying to solve my 1975 70hp Johnson issues. Power pack was bad but it would hit and run a few seconds. In the end I just replaced the main components except whats under the flywheel. I was happy I did it, she started right up.

When you have a motor with all these years on it, to make it dependable going with a bunch of new parts just makes sense. I think the power pack was only $100.

Good luck and happy boating

Patrick T

 

[classiccat]

Re: Weak Spark: 1975 70Hp Evinrude

Bluefin,

I built the DVA when I was trying to solve my 1975 70hp Johnson issues. Power pack was bad but it would hit and run a few seconds. In the end I just replaced the main components except whats under the flywheel. I was happy I did it, she started right up.

When you have a motor with all these years on it, to make it dependable going with a bunch of new parts just makes sense. I think the power pack was only $100.

Good luck and happy boating

Patrick T

Pulling the rectifier gets my blue spark to cross a 1/2 gap...I think you jinxed me blabbing about your rectifier being bad :tongue: :lol:

While I have ignition on the brain, some DVA measurements are definitely in order.

Thanks for the feedback and well-wishes!

 

[classiccat]

Re: Weak Spark: 1975 70Hp Evinrude

Update on the ignition Saga:

I'm getting a strong spark across 1/2" gap on all-3 cylinders with the new rectifier...however it seems to be intermittent on all-3.

I did purchase a new powerpack and before changing it, I wanted to check the Max spark advance with the original; it was hitting between 13 and 14 (17 minus 4 with the Joe Reeves method):


When checking the timing, the cadence of the firing seemed more consistent.

I made a bottom-feeder DVA this afternoon and decided to play with it a bit (at this point, Mrs. BlueFin16 gave me the 10min warning so that my daughter can get her beauty-sleep).


I went ahead and checked the charge coils with it; I'm reading +300V...well within spec (150-400V)

I started to check the trigger coils (while connected to the old powerpack) and was only getting ~5V...and the Admiral shut-down the operation; now I have all evening to ponder this result :lol: