Coil diagnostics

[jackbeagle]

G'day gentlemen,

Anybody know a good way to check if coils on a SOHC are good? I have power to the coils, and good resistance across the terminals, but since the plug wires are hard wired (I think) to the coils how do you check resistance there? Or is that not a good test? The bike runs (sort of) but I keep getting a random cylinder not firing. It varies by day - sometimes Cylinder 1, then 3, then 2 and so on. So I think it must be an electrical gremlin. I still have stock points and coils. I tried cutting a bit off each plug wire but the problem persists.

Any help would be greatly appreciated.

Jim

 

[amc49]

Compression test to make sure you're not chasing your tail. A waste of time chasing other issues if the basic motor is flawed. Weak cylinders pull all sorts of off-running tricks. If the points not set correctly then you will get those issues too. A kill switch resistance issue can greatly drop spark too. I have drilled into those 'permanent plug wire' coils to replace the plug wires before...........

 

[jackbeagle]

Yep, been chasing my tail for awhile. Compression is good, and re-did the points & timing way too many times, and no kill switch any longer.

Which kind of brought me around to the coils being shot. Normally you would test resistance between different plug sockets, but when I tried doing that at the spark plug caps instead because they're attached to the coil I didn't get any readings at all.


What do you mean about drilling out the plug wires?

 

[Dave]

When "Bench Testing" the coils you would remove the plug caps first as they are "resistor type" plug caps. The plug caps themselves can go bad as well.

I also "re-did the points & timing way too many times" until I just replaced it all. New coils, plug caps, and a pamco electronic ignition. Best money I ever spent on the bike.

 

[amc49]

Some of the lower level coils do not have screw on plug wires, the wires are permanently molded ('hardwired') into the coil body. You have to probe for where the connection is then yank wire and drill up the hole to free the old wire then replace with new soldered onto coil leads and epoxy it all back together. I've 'rebuilt' a coil/wire package like that to have it then go years longer. In my case the wire condition was the random miss, new wires fixed it, the coils themselves per se were fine.

I tend to 'butcher' parts like that to get them going again rather than buying new overpriced but often still crap quality stuff. Got countless parts like that on my cars. The hundreds of dollars saved all over the place at individual costs of maybe $10 each add up to superbucks. Fixing a Ford Focus window regulator today ($150) for about again, $10 maybe in parts. New car parts are such utter crap now that it often pays a couple hundred to sit down and think of a permanent way around repeat buying of junk new factory parts. I buy (or at least did buy, no longer) Ford cars, they have moved to increasingly use their engineers to design not to have parts last, rather to make them break over a couple years, and preplanned to increase parts sales over the counter. The problem now is the cars last too long, the motors and transmissions no longer break, so now the cars are made to make owners eventually give up fixing them due to all the nuisance items that will break to drive one crazy with parts buying every month when another one breaks.

Best test of an inductive type coil? Have known good plug wire and grounded plug on it with a wide gap bigger than your needed one and then power up coil with 12V and ground it by hand, then open the ground up. Coil will fire across plug or no good. On twin tower waste spark coils like these 750 use the two plugs have to ground to each other at plug metal bodies as well as the power source to complete the circuit.

On my 550F I went to parts store and bought simple old school cheap condensers for a smallblock points Chevrolet 350 and replaced the Honda condensers with them, mounting condensers up with coils rather than under the side cover, lots more room under side cover then and worked great.