Topic: Coil Performance test: Update

[JBMorse]

This past weekend I did another tuneup on the bike.  I used a timing light for checking timing, and I discovered a sporadic spark on cylinders 2&3.  Fearing a bad coil, I took it off and tested it according to the manual.  The resistance across the two leads read a healthy 4.8 ohms.  Is there another test I can run to determine whether the coil has failed?  
The bike has been running a little rough lately, enough to vibrate a few fasteners loose.  The wires and caps on the 2&3 coil are pretty crappy, so I am very tempted to toss the coils in favor of new Dyna coils and wires.  I'm planning a long trip this summer and wouldn't mind redoing the ignition, but I feel kind of stupid doing it if there's nothing wrong with my current system.
Also, could the timing or point gap settings, or condition of points and condensers be bad enough to cause erratic spark?  
Thanks!

[TwoTired]

Unlikely the coils themselves are bad.  If the HT wire ends aren't making firm contact with the boot screws, that is a problem.
Are the boots themselves showing proper resistance?

This past weekend I did another tuneup on the bike.  ...
Also, could the timing or point gap settings, or condition of points and condensers be bad enough to cause erratic spark? 

Yes.  But, I am confused.  Addressing points and timing is part of a tuneup.  What did you think a tune up consisted of?

Also, your timing light itself may be reporting erratic flashes because of it's trigger design.  Your spark leads have alternately, positive and negative pulses depending on the wire you select.  Your T light trigger may only reliably detect positive pulses.  If the T light was made primarily for cars, it is cheaper to make a trigger that is polarity sensitive.
Test equipment isn't infallible, and it is important to know if it it giving you correct info.

[JBMorse]

Thanks TT,
First off, the timing light is made for cars, but I think it's working fine as it flashes normally on cylinders 1&4, then erratically on 2&3.
Second, my tuneup started with a valve adjust, filter clean (UNI foam filter in stock airbox), cam chain adjust, plug gap, and finally timing.  Stupidly, I did a carb sync before timing because I had the carbs off for cleaning and bench synced them again, but I figured I could re-sync after the timing was complete.
I have seen posts indicating that the coils themselves rarely fail.  My problem may only be with the plug wires and ends.   But, even if the ends are bad, wouldn't the coil still fire normally?  Or do the wires have to be part of a complete circuit, with ends and plugs all hooked up?
I don't know about the boot resistance, I'll have to check it.  What should it be? 
Thanks for the help.  Unfortunately I am away from the bike until the weekend, so I'll recheck everything then.

[dave500]

5kohms in the plug caps,if ones bad ,replace them all,the spark can kind off "back up"and fool the timing light with bad caps.disconnect and clean the little bullet conectors aswell.,if the carbs are out of sync much itll vibrate,do you have a vacuum guage set?

[TwoTired]

When the coils are triggered to fire, they increase output voltage until there is enough to jump the gaps.  Bigger gaps mean a higher voltage requirement.  It is possible to make the gaps larger and require more voltage than the coil has energy to jump the gap.  It's not just the spark plug gaps, it is ALL the gaps in the circuit.  If the HT wire ends are near, but not contacting the cap connections, you have increased the gap of the entire circuit.  Similarly, if the plug cap resistors are open, another gap to jump is created.

The coil energy flows from one lead to one spark plug, across the gap to the strap, through the head to the other spark plug strap, and then through the other plug to the other HT lead.  All gaps must see enough voltage potential in order to create spark at each gap in the circuit.

The spark plug caps for the 500-550 are usually found to be 10KΩ.  They will work 5K-10KΩ in each cap +/- 10%.

You could also swap the point condensers side to side.  If the intermittent firing follows the condenser, then you found a bad one, probably leaking or have lower than required internal resistance siphoning off power to the coil.

Do re-sync the carbs at the end of the tune up check list.

[Deltarider]

This past weekend I did another tuneup on the bike.  I used a timing light for checking timing, and I discovered a sporadic spark on cylinders 2&3.

Did you remove dust, dirt and grease from the HT leads before you attached the triggerclamp? You know about polarity, as TT explains?  'This side towards plug', it says on the inductiontriggerclamp but with our bikes on two of the four HTleads the clamp should be the other way around. Have you found out which ones?
Tip: let your engine idle in the dark and closely look for sparks flying between caps, HTleads, and head. Sometimes they're tiny, but in hours of darkness we, humans can see the light. Some spray some watermist.

[JBMorse]

Thanks for the help guys!
This weekend, I'll take the coils off again and test impedance of the end caps.  I'll also clean the plug wires really well and try playing musical condensers. 
The ends of the 2&3 plug wires are pretty crappy.  I feel like they're not making a positive contact with the plug caps, like they're just too short to reach into the contact on the plug end.  I wonder if the previous owner tried refreshing the ends by clipping them, and took too much off. 
Thanks for the suggestions, I'll repost if I have more questions.

[Deltarider]

A possibility often overlooked is cutting the lead to where it's healthy and connecting some new cable by using the excellent NGK J-1 cable coupler. Cheap and effective. No need to go Dyna with their ridiculous promise of 36.000 Volts. As if your plug does not already spark long before 10.000 Volts.

[OldSchool_IsCool]

+1 on watching you wires in the dark. Sparking from wire to head is pretty common.

also polish the faces of your points with a clean white non-glossy business card soaked in brake cleaner.

[TwoTired]

If it is a wire-too-short problem, then the NGK J-1 coupler and new wire should be a good fix-up.

I do have a coil where the internal core of the wire broke near/inside the coil.   This was found by check the resistance HT wire end to HT wire end and getting about 15KΩ or ∞ intermittently, spending on how the wires were moved about.  I guess the wires got so stiff that moving them around stressed the inner wire core until it broke.  Once broken, the internal wire gap will only get worse, as the arcing will erode the wire ends shorter and shorter, increasing the gap and making the insulation deteriorate until spark is unreliable.  If the breakage occurs as described here, coil replacement is the easiest remedy.

[JBMorse]

Thanks everyone for your help with this spark problem!  I played around this past weekend and figured out that the coils are fine.  I think my spark problem was simply a bad connection between the plug wire and caps on the inner cylinders.  Also, I took a good look at the points and they were pretty well pitted, so I've ordered an OEM points assembly (got lucky, there was one on ebay pretty cheap!)
I'm also going to get new wires and ends, using the couplers recommended here.  I am wondering what ends I should get?  After searching the forums I think these match the stock ends:
Inner: http://www.z1enterprises.com/detail.aspx?ID=427
Outer: http://www.z1enterprises.com/detail.aspx?ID=425

I realize these are possibly a lower resistance then the originals, but I did some searching and it seems these will work just fine.
However: I really don't like the angle of the inner plug caps.  Could someone recommend a longer/taller end to replace these? Or maybe use the 120 degree caps on the inner cylinders?  Sorry, I'm away from the bike for the week so I can't just look at them and judge for myself. 
Thanks everyone!