weak spark, what to check? UPDATE, problem identified

[gltriker]

I agree that the compression numbers are low. Anything under 90 psi is most certainly going to affect performance. I'm also curious why you removed the resistors from the plug caps. Not that I'm much of an ignition master, but I've read that the resistor helps 'shape' the strength and duration of the spark.

Several years ago, Whiskerfish had mentioned this phenomena, but the results I found from a "search" for the details of spark shaping lead to something way over my head about a subject involving 'plasma'.
My aged, yet still inquiring mind, needs to know less about plasma and more, beyond addressing Radio Frequency Interference, about supplemental resistance enhancement in a Kettering ignition system. ( I guess that's what I'm needing to understand? )
Can you please direct me how and where you found this info., Track T? Thanks!

[gltriker]

Sorry, I'm an idiot.

My crusty memory was digging up differential test numbers. Straight up compression (can't find where I recorded the exact numbers right now) was more like 150-155. I did a full valve job during restoration, If I got 70 I would have failed miserably.
IMG_1955.JPG
Disregard this question. I just reread your post #1 and found your compression numbers edit!
elwood
what is that dual 0-100 psi pressure gauge set-up utilized for? Is this your cylinder leakdown tester?

If you are pulling plug wires and there is no change at idle then odds are very high it is not idling/ hitting on all 4. Start it up from cold and if you have an IR gun or access to one use that on the headers. If not use a mist spray bottle of water and check the header temps for evenness.

I regularly use my IR gun to check header temps and they are always within 20-30 degrees of each other. I agree though that it seems like its not hitting all four, but header temps and spark plugs all read the same.

When I pulled plug wires, the results are the same for ANY wire I pulled. I pulled all four, one at a time, and none of them seemed to make a difference I could detect. Only after I pulled a second one off could I notice a difference.

[elwood440]

elwood
what is that dual 0-100 psi pressure gauge set-up utilized for? Is this your cylinder leakdown tester?

Yes. I borrowed it from an airplane pilot/mechanic friend of mine. He called it a "differential pressure" tester, which I think is the same thing. The dial on the left shows the pressure of the source air going into the cylinder (80) and the right one shows the pressure in the cylinder (77). The needles in the picture look a little off from those numbers, but it's because of the camera.

[gltriker]

I always try to make these things short and sweet, and then I look up and I've written a novel. Sorry about that.

Yeah, but it's an interesting novel!
From the observations I've gained over 21,000+ miles with my "75 GL as a trike......

My 1975 trike isn't a good example to compare your 1979 engine concerns to, and I can't comfortably (won't willingly) exceed 85 mph.
But, when my engine is, in general, running correctly, there isn't a perceptible smooth/not-smooth scenario occurring at a steady speed.

What ground speed and gear position combination are you sensing this concern around?

Although, when my trike's carburetors weren't fully synchronized, it would 'couple' when there was a very light engine load and very small throttle opening.
When I dynamically synchronized them, and performed an idle drop on all 4 carburetors pilot adjustment screws towards a richer setting at an 1100rpm hot idle speed, the 'coupling' disappeared and its engine, in general, was much happier.

I could easily get carried away, with more thoughts, but this may give you a little something to look at.
Done.

[gltriker]

elwood
what is that dual 0-100 psi pressure gauge set-up utilized for? Is this your cylinder leakdown tester?

Yes. I borrowed it from an airplane pilot/mechanic friend of mine. He called it a "differential pressure" tester, which I think is the same thing. The dial on the left shows the pressure of the source air going into the cylinder (80) and the right one shows the pressure in the cylinder (77). The needles in the picture look a little off from those numbers, but it's because of the camera.

close enough!

[Whiskerfish]

I do not understand spark shaping enough to be able to explain it well. I do know and understand that some "terminal" resistance is required to end the spark and do other things. The biggest advantage I see to this is that you do not completely drain the coils. Basically due to the resistance you are having the spark drop off suddenly when it's usefulness is over rather then to slowly subside. The same principal applies for the initiation of the spark. You want it to be at full charge when it starts not weakly and then building. The added resistance of a component together with the resistance of the actual air gap (a variable due to compression and fuel mix) ensures that it will not initiate weakly. You can provide that terminal resistance through the wires, the caps, or the plugs themselves but it is important that at least one of those components have some resistance. The other 2 added benefits of terminal resistance are the reduction of Radio Frequency Interference (RFI) and potentially damaging feedback charge. In a stock system the capacitor also help to protect the points from this feedback charge as the spark ends. I do not know how it is done in the various electronic systems.
Hope this make some sense.

[Track T 2411]

Cliff, the info WF posted above was pretty much what I remember reading, and it was discussed in a thread I believe was started by ericheath. I don't recall if Eric had any more specific info or sources. I based my question to Elwood on the premise of eliminating the simple stuff, like a deviation from a 'typical' or stock setup, rather than any specific knowledge of the need for a resistor.

[gltriker]

Thanks, WF and Track T!

Done.

[Track T 2411]

I re-read through this thread, and still have a question concerning the resistance. In your original post, you state you tested the coils for resistance, and they were within specs. Did you take into account the non-resistor caps in that observation? If not, running resistor plugs would increase the resistance beyond spec. Just a thought...

[elwood440]

I re-read through this thread, and still have a question concerning the resistance. In your original post, you state you tested the coils for resistance, and they were within specs. Did you take into account the non-resistor caps in that observation? If not, running resistor plugs would increase the resistance beyond spec. Just a thought...

I tested the coils, wires, and caps separately. I know when installed, its easiest to go from cap to cap and get the total resistance of the loop but I was building up new wires and measured all the parts on their own. So the Dyna DC1-1 coil primaries measured somewhere between 2.9 and 3.2ohms, the secondaries measured somewhere between 13.5k and 14.5k, the wires measured zero ohms, (copper core) and the caps were 5k new, and then zero ohms after converting. I converted the caps because I am running resistor plugs. The Iridium plugs only come in 5k (and maybe 7k?) but not 0 ohms like the standard plugs. I think I may have measured one of the plugs at least, but maybe not all of them.

1.JPG

So instead of having 5kohm caps with 0ohm plugs, I have 0ohm caps with 5kohm plugs. Should be normal.

I will try to re-measure them tonight and report back.


.

[elwood440]

I pulled the plug wires and measured from cap to cap I'm getting 14.3k on the aft cylinders and 14.5k on the fwd cylinders. I forgot to measure the plugs, I'll do that tonight.

So any ideas what else I should check that might be giving me a weak spark??

[gltriker]

Ideas?
Scroll all the way down to the 5 Similar Topics at the bottom of this page?

[desertrefugee]

As a long time EE (don't let that lull you into thinking I'm an ignition expert), I'll give some purely theoretical observations. First, the bit about resistor wires/caps/plugs and "shaping" of the spark is probably irrelevant with regard to your described problem. A resistive load is not reactive and will not "shape" anything. It'll load the impulse, but will not alter it's profile other than attenuating it. That's sort of how it reduces (doesn't eliminate) RFI radio noise. And that's really the main reason for resistors in the secondary.

So, really, a weak spark boils down to two basic things: 1) voltage developed in the secondary circuit and 2) plug gap. The secondary voltage correlates directly to the primary voltage - and that's where I think you might want to focus. Assuming solid switching (Dyna in your case), the connections and wiring path between the switches and the coil primaries is critical. As is the primary voltage itself. You've verified all connections. I would suggest maybe wiring a temporary DC voltmeter onto the handlebars and watch it like a hawk while you're riding. If your poor performance happens even while the system is putting out 13.5V, it's not battery or alternator.

After that, monitoring primary voltage to the coils gets a little trickier as its duty cycle will not allow a DC voltmeter to be much help. You'd have to do a static check there.

I've babbled enough and really haven't added anything to the discussion. The only other thought is that (and don't be offended), maybe your spark is just fine. "Weak" is pretty subjective unless you can somehow quantify it or compare to a known good example on a similar machine.

Would suck to find out you have a fuel delivery issue after chasing ignition gremlins for a month...

[elwood440]

UPDATE: I got no real leads here for what I can check, so I went to a friend of a friend who has a bike speed shop with a dyno to do troubleshooting. I was close when I was suspecting spark. It turns out it's not weak so much as a bad intermittent coil. On the dyno rpm chart, the rear two cylinders fired VERY erratically, but the front two were very smooth. That points me directly to a coil. So now, should I bother trying to contact Dyna about returning/trading them, or should I just buy a new pair and call it done?

[Sagebrush]

I would contact Dyna, they have been good about replacing other components in their ignitions systems.