Coil tests okay but no spark

[bda4130]

Hi all, I have just fitted a dyna-s due to my points being pretty worn and bike not running right.

Bike is a 1977 gs550
I have done the coil relay mod and have good voltage to the coils. I'm not getting spark on spark plug 2 and 3. Did the primary and secondary ohm test on the good and bad coil,both were the same ( can't remember, but I think it was something like 4.8 primary and 26k secondary). Swapped over the black and white wires to eliminate it being a dyna-s ignition fault.

Tested the plug caps... 3 of them had 4.8ohms and one was a dud. Replaced that but still no spark at that coil, trimmed down all leads to eliminate corroded leads etc. Tested the earths and all connections.... every thing fine.

So next, I tried a coil off my bandit motor and hey presto! I have spark at 2 and 3!


So anyway, here's my question: coils tested fine, plug caps good, voltage to coils all correct.. But no spark. Can a coil test as fine,as this one has but still fail to produce spark? I'm no electrical guru so im gonna replace the coils anyway but was wondering if there's some test I have missed that would have shown me what is/ isnt going on with this coil as when tested as above it seemed fine?

Cheers.


Here's some shots of the bike in case anyone's interested:

[tom203]

Sure can , as you've found out. The ohm test uses a small current, while the actual ignition blasts it with about 3 or 4 amps- any defect in winding would show up ,i.e. no/poor spark.
Nice pics, is that a basically open exhaust?

[Guest]

How old are those tires? they have the right look but I would be hesitant on the age.

[bda4130]

Sure can , as you've found out. The ohm test uses a small current, while the actual ignition blasts it with about 3 or 4 amps- any defect in winding would show up ,i.e. no/poor spark.
Nice pics, is that a basically open exhaust?

Cheers, that makes sense. Will be ordering a new set of coils today. One other thing I forgot to mention is that when I had the original points on it, it did produce spark, but very weak... I guess the dyna-s sends so much more power to the coil that it can't handle it and just doesn't work.

The pipe is a cycle works 4 into 1, it has a baffles up inside it ( just looks like open pipe but without the offensive noise!)

[bda4130]

How old are those tires? they have the right look but I would be hesitant on the age.

Those tires are 1 month old. They're Firestone deluxe champions, brand new reproductions of the original.