69 Johnson 4hp mis-fire at high speeds (weak spark on one cylinder)

[777funk]

I put a neon light spark tester on a 69 Johnson that runs rough (as in low power) at medium to high speeds and noticed that one cylinder lights the bulb up solid and bright, but the other I can tell is so so (kind of dim). The coil tests ok (ohm meter) but I realize that the dielectric could be broken down. The coil doesn't look super old though I bought the engine used so I have no history on it. By looking at it, I'd say it was replaced in the past 5 years.

What should I look for to fix the weak spark on the one cylinder?

 

[Bosunsmate]

spark plug, weak lead possibly more, ive never seen one opened up

 

[777funk]

I swapped the 2 plugs and the weak spark continued on the same cylinder.

 

[sutor623]

Pop the flywheel off her and check out the situation. My guess is that its time for fresh points and condensers. Pretty inexpensive and real easy fix. I'd at least take a look at em with the flywheel off and go from there.

 

[777funk]

The points and condensers are new (that I can see for certain). Whoever had it before me must have tried that first before selling.

 

[sutor623]

Well thats good. Hopefully the gap is set correctly? Also, I have had the bottom cylinder drop spark on my 4horse because the cover latch wore into the spark plug wire.

 

[racerone]

Look at the botom of the coil.---They are known to burn through and arc to ground.

 

[schematic]

agree with above. These coils leak to ground quite frequently. Replace both if original.

 

[777funk]

The coils look like they were changed (not original but not sure the age). I don't think they are OEM. Are there bad new coils?

 

[Chinewalker]

Might be as simple as cleaning and gapping the points. Do that before messing with the coils (particularly if they appear new). Gap the points at .020 with the cam rider aligned with the keyway on the crankshaft. Then spray a business card with carb or brake cleaner and run it through the points to remove any film on them.
Also, make sure the flywheel isn't rubbing on any wiring.

 

[777funk]

I set the points using TDC timing marks and an ohm meter (something I saw in a video). Is that an ok method?

 

[racerone]

Using the timing marks on the magplate and the 2 marks on the flywheel is the best method.-Gets you the strongest spark exactly 180 degrees apart.