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I keep burning out spark plugs. They are clean and light brown.

Customer Question

I keep burning out spark plugs. They are clean and light brown. They are Harley Davidson Plugs and correctly gapped. The bike does not use oil. I had a short that burned out my stator and voltage regulator. Voltage spikes or something ruined three batteries in one year. I changed the stator and voltage regulator and now the battery lasts and about 14 volts is read when the bike is running. It is a 1994 evo. Each time a put in a set of new plugs the bike runs good for about a week. Then one cylinder starts to miss. The plugs look new with a nice brown dry look. I'm wondering if my ignition module is causing the plugs to fail.
Submitted: 3 years ago.
Category: Motorcycle
Expert:  .... replied 3 years ago.

Brian :

Good day and welcome to Just Answer. My name is XXXXX XXXXX X'X be happy to help you with your problem today. How old are those plug leads?... is it the same cylinder that misses each time & which one is that?... have you tried moving the plug leads to see if the issue moves with them?.... Have you confirmed actual loss of spark, or does it just miss?.... when does it miss; warming up, cold running, hot running, choke in, choke out?... when was the last carb service?...... & lastly, what ignition module are you using?...

Brian :

take your time & answer each question with as much detail as you can. I'll be in & out of this chat but watching it as you reply.....

Customer:

The leads have been changed since the problem started.

Customer:

The front cylinder is the one that burns out the plugs

Brian :

what do you mean "burns out the plugs"?....

Customer:

The plugs are faulty tested by an ohm meter. As well the problem is fixed by the change of the plug. Also Switching the plug to the rear cylinder causes that cylinder to not fire.

Customer:

Not sure what burning out means other than it quites working.

Brian :

testing a plug by an ohm meter?.. I've never heard of that.. what results are telling you that it is no longer good?

Customer:

It doesn't matter if the bike is cold or hot, it does the same thing.

Customer:

The one plug had infinite resitance

Customer:

Between the wire connection and the center elctrode at the bottom. I'm not refering to the grounded part and the center

Customer:

The carb was cleaned a couple of years ago.

Customer:

The problem started when the battery shorted out.

Brian :

You do realize that a spark plug is a giant resistor correct? I'm not sure checking ohms of a plug would net any results. Normally the plug is inserted in the plug cap, laid on the head & the engine is cranked & you visually test for spark with said plug. Alternatively you can use a timing light, or a spark tester.

Brian :

Plugs don't actually burn out and I personally think they are being cracked when installed. I've never like the HD/Champion plugs. They cost 2-3x as much as an NGK plug which I have much better experiences with.

Customer:

I know what you are saying but the real proof is that it causes the rear cylinder to miss when I swap cylinders. Also some plugs with too much resistance can fire outside the cylinder but not inside under compression with fuel.

Brian :

That is what I asked & you said no, what dod you swap cylinders with, just that plug?

Customer:

They aren't being cracked. I have put them in carefully with the proper tool. There is lots of room and no reason to put side load on them. Also they run good for a period of time.

Brian :

and also why I prefer a timing light in these cases.

Customer:

I hear you on that.

Brian :

cracked plugs will do that. & the HD plugs are noted for that, but irregardless you seem to have spark loss issue & a sadly neglected carb.

Customer:

I swapped plugs on a few occasions to see what the difference would be by changing them.

Brian :

That carb needs at least one service a year. I do mine every month & it needs it.

Brian :

and, I still need clarification on what was swapped to replicate the issue on the rear cylinder?..

Customer:

I have never seen that recomendation before.

Customer:

I swapped the plugs from front to rear as the front cylinder was not firing. The rear cylinder then did not fire.

Brian :

I don't care if you have or not. I work on these things every day & I pull out my nearly clogged pilot jet every week. I can assure you, they need frequent services, at least the cv carb does. The monstrous cavities on the S&S can goes years, it all depends on the carb.

Brian :

So if the issue was isolated to the plug, you found the issue & that would be a cracked or junk plug, if the plug is not fouled, but will not fire under compression, I would change plugs brands for starters.

Customer:

I have owned one pan and a few shovels all carbed. They all go for years without needing service. Also I called Harley just now and confirmed that. Why would a guy even buy a bike if you have to change jets every month. You must have a dirty tank and no screen.

Brian :

You don't have to change jets unless it is require by improper jetting & you are welcome to believe what you want to. I can assure after only 7 days of riding or 4 days of sitting my pilot jet is nearly clogged shut & gets cleaned weekly. The carb entire gets serviced monthly & my bike stays in top performing shape because of it. Neglect your carb at your discretion, that is not my bike. My tank doesn't get dirty because I run through 3 tanks of fuel a week, pingles all have screens & I run an inline filter. It isn't dirt. It is varnish from this junk fuel that starts evaporating immediately in your vented fuel system. Believe what you want to, but it is very difficult to diagnose a miss when basic services are not done.

Brian :

And since the issue was isolated to the plug, I'd simply switch off those junk HD plugs to either Autolite or the NGK as I prefer.

Brian :

at least it was just the plug.

Brian :

if it were in the ignition system the issue would not have moved to the rear cylinder, & that is good news.

Customer:

What if something was screwing up the front spark?

Brian :

Then when you put in the rear plug you would have the same issue.

Brian :

you know when you switched them, the issue would have come up on the front again

Customer:

I just figure that the front spark is the issue. The rear plug was working so moving it to the front caused the front to fire. I then changed the plugs each time.

Brian :

no you are think about it backwards. The front plug was bad & when you installed that in rear cylinder the issue stayed with the plug. Putting the good rear plug in the front allowed it to fire, it did not cause it to fire, the ignition working properly with a good plug is what caused it to fire, whilst that junk plug in the rear cylinder still did not fire.

Brian :

the issue has been isolated to the spark plug.

Customer:

What I don't know is that the rear plug that I put in the front was working for how long before it would fail?

Brian :

Sadly neither do I & the testing for that is just time.

Brian :

If you want to spend time further looking into the ignition system you could check the ignition coil resistance & the module itself for signal to the ignition coil.

Customer:

Have you heard of this problem before?

Brian :

Based on the age of the bike, or the year, it would not be out of the question to be looking at a new hall sensor at least or a new ignition module. I do not think however that the issue went that far.

Brian :

Yes, & we just use NGK plugs.

Brian :

You did nothing different than a shop would do you just looked at it from the wrong angle, & you yourself immediately isolated it to the front plug.

Customer:

So I dont' know if it is the coil or ignition

Brian :

it is the plug

Customer:

Why always the front cylinder? And why over and over

Customer:

Something in the conditions must be different in the front so that plug goes before the other.

Brian :

same kind of plugs is all that I can think, without further testing with a timing light to determine actual loss of spark vs. ohm testing of a plug. Only when you know you have no spark can you then test the ignition coil & then the module for issues. Approximately 95% of single cylinder miss issues are going to be found between the ignition coil & the plug.

Brian :

I'm sorry that you do not want to spend $4 on a set of NGK plugs to try to solve the issue first, but that is my advice. If after you install the plugs & the issues resumes only then can we test the remainder of the system.

Customer:

Got you. I agree that is the case but it seems crazy when I have changed the leads and plugs so often. Also the plugs that have run for years in the bike are Harley plugs and the problem only started when I had a short.

Brian :

And, I don't doubt that there could be further issues, but until issue is replicated & testing done at the time of lost spark you just can't know. You sure can guess, but until you can test the situation further you just can't know. As it stands now though the issue is isolated to the plug itself.

Customer:

I'll give it a go and see. I can't believe that it is plugs failing because of nothing else.

Customer:

I'll let you know.

Brian :

please do

Customer:

thanks

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