Weak spark from one spark plug

[danielvanalphen]

I've restored a 1966 johnson 6hp. I've completely replaced the whole magneto system so coils, condensors, points, ht leads and spark plugs. I've set both points using a volto meter and ohms using the points on the flywheel. The bottom cylinder sparks perfectly and starts first pull and runs smooth on one cylinder but the second cylinders spark is weak and won't run bu itself. I've switched the magneto parts between the two cylinders and still the same cylinder has little to no spark. I'm so confused and can't seem to figure it out

 

[Crosbyman]

it happens :-(

concentrate on the bad plug...cct. the other one works !

could be condensers ( hard to test without a high voltage test box... VOM tests are not conclusive unless a solid short exists.

did you swap wires to the bad plug why not install all new cooper core wires 7mm and be done with that portion...

what is the ohms value boot to engine frame ?? is it a solid 4-8 KOHMS ??... plug wires break in the bends

 

[danielvanalphen]

Yeah I've replaced both plug wires with 7mm copper core. There is some what spark as in when running the engine with the faulty lead connect to an external spark plug it gives spark through my tester and spark plug at around 800 rpm but once it goes over I'd say 1.5k rpm there is no spark at all. It's very confusing as I've set everything up correctly to my knowledge and it gives spark at low rpm but cuts out at any rpm higher than idle

 

[Crosbyman]

measure the boot to frame ohms in the slow/idle position of the throttle

keep measuring the ohms as you rotate the trhottle and the mag plate truns all the way to fast....
ohms should be rock solid all the way from idle to fast
did you change any condensers or coils
clean the points ...again !!! wipe with carb cleaner or acetone

post results !!!!

 

[danielvanalphen]

Hopefully this video I took helps explain it easier. The engine will start and run on the one problem cylinder but at a very low rpm for full throttle. Then i connect the spark to the good cylinder and as shown it runs well but only on one cylinder due to the fact that the spark of the problem cylinder has gone shown by when removing the bottom cylinder spark the engine dies.

 

[racerone]

You have to pull the flywheel.------Points need cleaning or coils are cracked / burned through shorted to ground.-----Note exact position of coils.----Then take out screws and look at bottom of the coils.----Very simple magneto.-=-----Lots of info on-----leroysramblings-----All you need to know on this wonderful / reliable / simple / cheap to maintain magneto.

 

[danielvanalphen]

You have to pull the flywheel.------Points need cleaning or coils are cracked / burned through shorted to ground.-----Note exact position of coils.----Then take out screws and look at bottom of the coils.----Very simple magneto.-=-----Lots of info on-----leroysramblings-----All you need to know on this wonderful / reliable / simple / cheap to maintain magneto.

I've pulled the flywheel and replaced every part of the magneto which is why I'm so confused

 

[kbait]

Did you swap condensers to see if the weak spark follows?

 

[Crosbyman]

so you did replace / swapped most parts ... the big variable is current flow to induce a strong spark via the HV secondary that begins when the stored magnetism collapses and the primary coil tires to sustain itself & generates a current flow which ends up in the condenser as the points open up.


did you burnish thee point surfaces and clean then afterwards
....sorry to ask!

at this point may as well work that area ..even new condensers can be bad and the only way is to test them with proper high voltage test eqpt.

make certain all connections are clean.. scrape paint of the coil surfaces

and... post results of your OHMS measurements !!!

btw.. I had a similar issue on a 4hp last year.... did all that swapping thing...and suddenly bingo it cleared up. I never found out why so don't feel bad

patience brings joy

 

[danielvanalphen]

so you did replace / swapped most parts ... the big variable is current flow to induce a strong spark via the HV secondary that begins when the stored magnetism collapses and the primary coil tires to sustain itself & generates a current flow which ends up in the condenser as the points open up.


did you burnish thee point surfaces and clean then afterwards
....sorry to ask!

at this point may as well work that area ..even new condensers can be bad and the only way is to test them with proper high voltage test eqpt.

make certain all connections are clean.. scrape paint of the coil surfaces

and... post results of your OHMS measurements !!!

btw.. I had a similar issue on a 4hp last year.... did all that swapping thing...and suddenly bingo it cleared up. I never found out why so don't feel bad

patience brings joy

I feel like such a rookie right now. Swapped condensors and boom runs well. I know assuming is bad but I assumed that new out of the box condensors wouldn't be bad but I was evidently wrong. Thank you so much for the help I genuinely wouldn't have thought it would have been them due to the fact they are brand new and would have just kept overlooking them

 

[Crosbyman]

happy ending... Great !! thanks for your feedback as it helps others with similar head scratchers .

depending on your longer goal to work on oldies here are some articles on condensers and how the fail and can be tested/repaired

https://wrcoutboards.org/wp-content/uploads/2020/04/Part1_Condenser_Construction_Failure_Modes.pdf

https://wrcoutboards.org/wp-content/uploads/2020/04/Part2_Condenser_Testers_And_Testing_Correctly.pdf

https://wrcoutboards.org/wp-content/uploads/2020/04/Part3_Sizing_Condensers_Correctly.pdf

https://wrcoutboards.org/wp-content/uploads/2020/04/Part4_Repairing_Broken_Condensers.pdf

see article MR.Mohat can send you his diagrams for buildinga test box ..the simpler model is easy to build and works great