When is it a spark plug & when it is more serious????

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When is it a spark plug & when it is more serious????

Postby Schoobytwo (Bruce) » Thu Jan 28, 2016 10:02 pm

Hi All,

Thought I would share a little challenge I've seen several times now and again tonight. If you were at the Get Together this past weekend, you may have seen me drive a fresh 502 refurbishment from the trailer. When it idled, I walked away and then just like that, I heard it quit. No biggie I thought as they can be a bit cold blooded.

So I tell people all the time who can't get a Diablo started and are sure they have fuel and spark, to change the plug. If that does not work, change the plug again. If that does not work, change the plug again. I've seen an occasional spark plug foul and if you have a crankcase full of fuel, it will foul another plug without even giving a puff. So after three plugs in this Diablo and it did not start, we pulled it back on the trailer and enjoyed the rest of the day.

In the shop tonight, I got down to it with that red devil. This Diablo would puff and backfire. It had spark. It had fuel. It ran perfect before, it was a screamer before I loaded it on the trailer. I layed in bed last night thinking about it wondering why it was acting like the timing was off. This morning, it popped in my head that the mechanical advance must be stuck. So off came the flywheel tonight and it was not stuck, but I took it off anyway and cleaned up the lever. Put it back together and the same issue.

I know of no good test without the proper equipment to test a condenser or ignition coil. I tell people all the time who want to throw a new condenser and ignition coil in their Diablo just because they want new, that the OEM condenser and coil are darn hearty and rarely, I mean really rarely do I find a bad one.

So out came the ignition plate. I tested the existing condenser and coil against new aftermarket parts with an Ohm meter. Same readings. Same ramp of continuity. It just can't be these parts I keep saying. But I had that sneaky suspicion. So I replaced them, put the machine back together and one pull almost sent it through the wall. I'm just always shocked when I find a bad condenser or coil. In this case, I had spark - good blue spark. But during the initial troubleshooting, I had the plug out and was testing the spark and I noticed just a slight abnormality in the spark. At first I just dismissed it. Now I know the cause.

I really can't offer much guidance on this. Sometimes you just have to take a chance and replace some parts.

Anyone else have any good thoughts on diagnosing bad condensers and coils?

Bruce
Too many Diablos and parts to count.
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Re: When is it a spark plug & when it is more serious????

Postby jayvee43 » Fri Jan 29, 2016 2:27 pm

I ran into that problem in the past also.
I believe there is a resistance between the spark plug electrodes. If you place the plug in the head on an engine with, for example, 10:1 compression, you are compressing 10 atmospheres into that gap, which adds to the resistance.

I had what looked like blue spark, but once inside the engine, no go, or back-fires to wake up the dead.

With the right equipment, you can measure spark duration, but I have seen a plug fire out of the engine, but not have ignition(measured through the plug wire or coil) because of a cracked or shorted coil that wouldn't fire under compression. As electricity takes the path of least resistance, it was easier for the spark to short in the coil, or short from the plug wire to the cylinder head than to actually fire the spark plug.

I'm a marine engine technician, I have seen this scenario several times with the older OMC engines of the day.

I hope that can help a little, (The cracks in the ignition coils were internal) not always visible. 8-)
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Re: When is it a spark plug & when it is more serious????

Postby Schoobytwo (Bruce) » Fri Jan 29, 2016 8:12 pm

I talked to a fella today who has some race sled experience and is a mechanic like us. His 503 is giving him problems and I mentioned this scenario. He said pretty much what you said Jesse and your explanation was excellent. Something I did not know and now explains why we thing we have spark but really don't.

This is something we need to remember!

Thanks all. The power of the forum comes through again!

Bruce
Too many Diablos and parts to count.
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Re: When is it a spark plug & when it is more serious????

Postby mike oreilly » Fri Feb 05, 2016 11:59 am

I had something similar a couple of years ago, including backfires that would wake up the dead and lying in bed at night trying to figure it out.

I dicked around with it for days, driving me crazy - finally I got it: when I had installed the new condenser I had tapped on the little center nub to drive it into the stator plate. A definite brain fade mistake, rather than use a socket to drive on the outer circumference.

I had spark, but not reliable spark, ie: not at the right time, thus the backfires.

So in goes a new condenser, and it runs great. In Bruce's case, having changed 2 components we will never know which one was suspect?,

I chased this and chased this: all new intake gaskets, larger main jet, etc. Nothing. Finally a friend says have you looked at the plug: answer: I've looked at everything, had the stator off, checked the coil, etc. and yes, good spark. And the plug looks OK. Anyway we change the plug for lack of anything better to do and voila! Cured.

Which drove me insane. As Bruce says, with an old 2 stroke, with shitty Tillotson carburetion change the plug, and change it again. Don't leave home without a pocket of plugs. But for a modern 4 stroke, with electronic ignition and a Keihn carb??? Can't argue with results, and my old friend (small engine mechanic) was gloating for weeks.

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Re: When is it a spark plug & when it is more serious????

Postby mike oreilly » Fri Feb 05, 2016 12:02 pm

Oops, paragraph 3 somehow got deleted: The last 2 paragraphs have to do with a Honda GC160 4 stroke that was giving me grief on a log splitter.
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Re: When is it a spark plug & when it is more serious????

Postby BillNH » Sat Feb 06, 2016 7:26 pm

I wonder if that's my problem, plenty of fuel, spark looks good, but, no start. Rest timing, changed many plugs, rebuilt carb, no start.
Now I'll be looking for a condenser.
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