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Spark Plug Will Not Ground


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11-04-08, 01:02 PM   #1  
Spark Plug Will Not Ground

I have an old Ehco trimmer and has always worked fine, except this last time it ran rough, so I rebuilt the carb and a new plug. Now I get spark from the lead with a screwdriver, I get spark from the plug when grounded to the head, but put the plug IN the head and no spark will jump, tried different plugs. WE put a wire under the plug and grounded it to the head and it will try to start on carb cleaner. We have four small engine people looking at this and we are all in the dark. Threads have been cleaned and sanded on the male and female side, so it's not a dirt problem. If you can throw some light on the subject, please feel free to let us know what we are missing.


Thanks, Wayne and friends

 
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11-05-08, 06:20 PM   #2  
If the spark plug is screwed into the cylinder, it's grounded. The more likely issue would be that your ignition module is not grounded good.

How do you know that the spark plug is not firing when it's installed?

 
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11-06-08, 02:17 PM   #3  
No ground

Hi, Thanks for the answer, but we used a spark plug test light in one test and a wire from the plug lead to the tip of the plug in another and still no spark, take the plug out and put the lead on and ground it to the fins and it will ignite gas fumes coming out of the plug hole. Today I put a brass washer around the plug and screwed it in, still no ground and it shows up as no ground on a ohm meter. There is no way to explain this where it would make sense. If I were the only one that came to this conclusion it would be like I'm nuts, but we have four good small engine people getting the same answer. Thanks again, Wayne

 
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11-06-08, 02:49 PM   #4  
You wouldn't have a ground from the tip of the plug (lead end). That is essentially an open circuit the spark bridges in the jump to the ground. However, the casing you screw into the head should show a ground from a continuity tester.

If you are getting a spark from the module/plug with the plug out of the head but grounded, then you install the plug and aren't getting a spark to show in a spark circuit tester, you have a weak spark that isn't able to bridge the spark plug gap under compression = bad ignition module or lead wire.

 
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11-06-08, 04:24 PM   #5  
I can see what this guy is getting at though.

Marbobj correctly explained what happens. But yet this guy says he can get the fumes to ignite by holding the plug against the engine (You really did that?) by the open hole. But obviously it is not firing off for him while screwed INTO the engine. THAT is why I think he thinks he is crazy, as that doesn't readily make sense, as far as the firing off aspect goes.

Spark------->fire---------yet no go? Maybe plug really is firing, but compression or timing issue?

 
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11-06-08, 06:48 PM   #6  
What model Echo trimmer do you have?

Can you Post the Model and Serial numbers??

 
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11-06-08, 08:30 PM   #7  
Plug not grounded? Hardly, we are speaking of 14000 to 16000 volts at the gap. If that voltage can jump a .035 plug gap, the thread body between plug and head would offer no resistance to that much voltage.
The plug firing outside of the cylinder and igniting fuel and not firing when in in the cylinder and under compression makes sense. The slightest spark outside of the cylinder will ignite gas, but that same mixture under 80 to 110 psi compression will not ignite when the spark is not at full potential.
My bet is that the coil base is not grounded or the ignition module is faulty. I think 30 yr tech and Marbobj are correct.
In the fifties, Champion distributed a spark plug testing piece of equipment that had a high tension built in and was connected to an air source of 130psi through a press regulator. The plug was screwed into an enclosed chamber that had a thick glass window behind it and an external mirror. To test a used plug, the plug was screwed into the chamber and the igniter turned on. Any used plug would generally fire pretty and blue. As the air press was turned up , most used plugs would stop firing at 60 to 90 psi while a new one would fire at 130. Tom

 
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11-07-08, 07:09 AM   #8  
no ground

Thanks again all and Tom, I would give the answer you did about the compression were I standing on the outside of this thing. I did a little test today that will blow away the bad ignition and timing theory, I put a fine stranded wire in the treads of the spark plug and ran the other end to a screw in the body, we now have a working trimmer with a funny looking wire on it, not what is needed but tells me there may be a crack in the head somewhere and the spark jump is happening there or somewhere else, but that doesn't make sense because it runs as good as ever. I told you it was screwy. I'm going back to the shop later today and will get the model and serial number. Called Echo yesterday and after a big wait and number punching I left a message and so far no answer, surprise. Wayne

 
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11-07-08, 07:27 AM   #9  
The test you've done would indicate the head isn't grounded (or at least well grounded) to the engine casing, which would be unusual to say the least.

Did you do a continuity test from the spark plug base to the head or to another part of the engine casing? Your jumper wire, I take it is going to a part of the engine casing which would be a common ground to the module. If it is = back to first sentence.

Should you have a crack in the head somewhere, the current would simply follow the path of least resistance = solid material. Should it jump across a gap, the current would be amplified.

Certainly a new one on me.

 
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11-07-08, 07:51 AM   #10  
Posted By: gardener321 To test a used plug, the plug was screwed into the chamber and the igniter turned on. Any used plug would generally fire pretty and blue. As the air press was turned up , most used plugs would stop firing at 60 to 90 psi while a new one would fire at 130. Tom
This is fascinating. So, as the compression increases, it has an affect on the spark itself, even causing a plug that could spark under no compression to stop sparking under compression? That is new on me. Just learned something today. Do you have an explanation of the physics involved here?

 
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11-07-08, 09:28 AM   #11  
Think of it in terms of air density and the spark has to penetrate it to get to the ground. Air itself isn't a conductor of electricity and offers only resistance. The denser it is (higher compression) the more resistance. The weaker the spark, the less likely it can bridge the gap to the ground in a compressed air environment.

 
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11-07-08, 09:44 AM   #12  
Ecman, With due respect sirI have read many of your replies and know that you are very knowledgeable. I do not have a scientific explanation why a weak plug will not fire under compression. If you read the second paragraph of my post, I had several years experience using the Champion plug test machine in the lift truck shop. A weak plug more noticeable in a multi cylinder engine, because a plug failing will sometimes skip occasionally and not stop the engine.
CJD77 I still think there is more at issue here than the plug being ungrounded while screwed into the engine.
To each his own, but I hold to either a poorly grounded ignition coil to block or the possibility of the new plug being faulty. I agree with you that this does not make sense but there is some reason other than the plug and head not making ground.
As 30yr tech and marbob have stated, check the coil for ground.
It is rare, but while helping a friend out who is in the small engine repair business, on several occasions where there was no fire, we removed the coil and cleaned the coil iron to block surfaces and solved the problem. I would also check the coil to mag clearance and try another plug after rechecking the gap of course.
I might be tempted to run the trimmer if it were my own, but I would bust or attempt to solve the problem although I too may walk off defeated. Tom PS I would go so far as to try another coil known to be good. This is very interesting and I am anxious to hear the outcome. Pls keep us informed

 
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11-07-08, 01:53 PM   #13  
Posted By: ecman51` This is fascinating. So, as the compression increases, it has an affect on the spark itself, even causing a plug that could spark under no compression to stop sparking under compression? That is new on me. Just learned something today. Do you have an explanation of the physics involved here?
It is true that a spark plug requires more voltage to fire under compression, and it can require as much as double the voltage. This is why I inquired as to how it was known that the plug was not firing when installed. The best test would be an inline tester where the spark has to "jump" a gap and not the type with a neon type bulb.

marbobj, gave an excellent explanation of why a plug requires additional voltage. I would like to know the model and SN of this particular unit, so I can look at which type of ignition set up is utilized, as this may shed some light on this issue (but then again may just further confuse as well)

 
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11-07-08, 03:19 PM   #14  
Posted By: marbobj Think of it in terms of air density and the spark has to penetrate it to get to the ground. Air itself isn't a conductor of electricity and offers only resistance. The denser it is (higher compression) the more resistance. The weaker the spark, the less likely it can bridge the gap to the ground in a compressed air environment.
But it is not just air. It is mixed with fuel. It would seem to actually make MORE sense to me to believe that the air-fuel mixure is MORE conductive, the more it compresses (as opposed to not conductive) under pressure , because it would become more solid-like - and then because the conductance increases, then as the high voltage traveling through the tip to the electrode reachces the highly conductive compressed, more solid-like air-gas mix, that instead of sparking, the current would simply travel out of the electrode into the mixture, to cylinder metal ground. Not it?

 
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11-07-08, 03:25 PM   #15  
No Ground

Hi all, this has turned out to be a good information forum if nothing else. The serial # 00105354 and the Model SRM 140DA. It's late now so will leave it for now and do all of your suggestions, ONE AT A TIME, and check for continuity after each different try and let you know when we got spark. I'm leaning toward the bad ground on the coil, but I have been wrong, can't remember the last time I was right on this job.

 
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11-07-08, 03:28 PM   #16  
Posted By: 30yearTech The best test would be an inline tester where the spark has to "jump" a gap and not the type with a neon type bulb.
Suppose I am on to something in my post just before this reply. If what I said makes sense, then couldn't the in-line test be a false indicator if say at the plug itself, that everytime a coil pulse were released, that the spark potential voltage simply bled through the fuel mixture rather than causing a spark to jump at the plug?

See what I mean. If the tester were inline with the plug wire, and caused the tester to flash, that would show coil pulses, but would that necessarily prove a spark actually JUMPED at the plug electrode, as opposed to just leaking the voltage out into the mixture under higher compression?

 
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11-07-08, 04:04 PM   #17  
Posted By: cjd777 Hi all, this has turned out to be a good information forum if nothing else. The serial # 00105354 and the Model SRM 140DA. It's late now so will leave it for now and do all of your suggestions, ONE AT A TIME, and check for continuity after each different try and let you know when we got spark. I'm leaning toward the bad ground on the coil, but I have been wrong, can't remember the last time I was right on this job.
Boy this is a really old weed trimmer!!! It has points and condenser. I would check the point gap. There is a good possibility that the points are just barely opening, this would will affect the timing and spark intensity. Points should be located underneath the starter assembly.

 
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11-07-08, 04:11 PM   #18  
Posted By: ecman51` Suppose I am on to something in my post just before this reply. If what I said makes sense, then couldn't the in-line test be a false indicator if say at the plug itself, that everytime a coil pulse were released, that the spark potential voltage simply bled through the fuel mixture rather than causing a spark to jump at the plug?

See what I mean. If the tester were inline with the plug wire, and caused the tester to flash, that would show coil pulses, but would that necessarily prove a spark actually JUMPED at the plug electrode, as opposed to just leaking the voltage out into the mixture under higher compression?
If the spark jumps at the tester it would have to jump at the plug, otherwise it could not complete the circuit and jump at the tester. The tester will not provide an accurate indication of the condition of the plug, but if it's new we can assume it's good. The current will take the shortest route to ground, the .025 - .030" gap at the electrode of the spark plug will be much shorter then anything else in the proximity, so I don't see the spark jumping through the gas air mixture to the cylinder wall.

Here is some basic ignition theory you can look at. While it's written for Briggs & Stratton the basics apply to all ignitions.

How does the Magnetron Ignition System function? How can it be tested?

 
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11-07-08, 04:49 PM   #19  
Posted By: marbobj Think of it in terms of air density and the spark has to penetrate it to get to the ground. Air itself isn't a conductor of electricity and offers only resistance. The denser it is (higher compression) the more resistance. The weaker the spark, the less likely it can bridge the gap to the ground in a compressed air environment.
That is an excellent explanation.
I thought of another after you said that.
In a diode vacuum tube. electrons will flow from the negative cathode to the positive plate, but only if the tube is in a deep vacuum. Tom

 
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11-07-08, 04:56 PM   #20  
For ecman51. No the fuel in the mixture at the proper ratio would not bring the air up to what you call a conductive state.

However, if you get far enough above the proper ratio (used to be 16:1, but now I believe it's changed to 18:1) and into a flooded condition, the conductive characteristics of the compressed fuel/air mix changes and a no spark condition cames about. That is why you have to purge the cylinders of the fuel air mix to get the engine to start in a flooded condition.

That can be done with the plug removed or with full throttle, no choke - or a number of other combinations. FI systems are a little harder to get out of a flooded condition.

 
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11-07-08, 05:19 PM   #21  
Posted By: marbobj That is why you have to purge the cylinders of the fuel air mix to get the engine to start in a flooded condition.
I think I will just give up and read. The reason I say that is because I could also see that happen when the engine floods, being a result of the (wrong?)theory I gave.

Here is how it also would apply, with a flooded conditiion, according to my (twisted logic?) theory: That too much gas would make it not 'spark' because the current would just connect the electrode to the gas, and go to cylinder ground, without jumping the gap.

 
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11-07-08, 05:24 PM   #22  
Posted By: 30yearTech If the spark jumps at the tester it would have to jump at the plug, otherwise it could not complete the circuit ...
Why not?

What if the current went down the tip of the plug, full volts accordign to coil pulse, and instead of jumping the gap, it grounded to the block, as how plugs can do when carboned up for instance. How can the tester know the difference, as long as the current is completing the path to ground when the coil pulses?

 
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11-07-08, 05:37 PM   #23  
Posted By: ecman51` Why not?

What if the current went down the tip of the plug, full volts accordign to coil pulse, and instead of jumping the gap, it grounded to the block, as how plugs can do when carboned up for instance. How can the tester know the difference, as long as the current is completing the path to ground when the coil pulses?
The spark tester cannot tell if the plug is fouled or not, I guess I was not clear in my post.

Posted By: 30yearTech The tester will not provide an accurate indication of the condition of the plug, but if it's new we can assume it's good.

 
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11-07-08, 06:36 PM   #24  
Posted By: ecman51` I think I will just give up and read. The reason I say that is because I could also see that happen when the engine floods, being a result of the (wrong?)theory I gave.

Here is how it also would apply, with a flooded conditiion, according to my (twisted logic?) theory: That too much gas would make it not 'spark' because the current would just connect the electrode to the gas, and go to cylinder ground, without jumping the gap.

No, it's not twisted logic - it's accurate. You simply cross a threshold of fuel/air mix, before it applies. Once you have sufficient fuel in the fuel/air mix, with the compression factored in, the electricity of the intended spark to ground electrode, would be grounded into the compressed fuel/air mix/cylinder and head = no spark to ignite the mix.

 
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11-10-08, 05:08 PM   #25  
No ground

Hi all, Yes this was a old trimmer, but worked fine until just lately.
Things I did:
Took the coil out, cleaned and gapped, checked ground .001 ohms, great fire outside, would not fire ether in chamber.
Cleaned points, (were a little dirty, gap OK) gapped and diamond filed, good spark, put all back and no fire in chamber on ether.
Checked compression 80 psi.
Was going to change to an electronic ignition, but it hit me wrong about spending more money on a dead horse.
You should be able to buy some form of this object in a year of so in a foreign car.
I don't give up very easy, but enough is enough.

Thanks for the help, and it was a learning process for all.

Wayne

 
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11-11-08, 09:22 AM   #26  
You had me scratching my head all the way thru this one. I still don't know why your jumper wire works and removing it kills the spark because my intuition says the head HAS to be grounded by the bolts that hold it on but your jumper wire proves otherwise. Interesting problem!

 
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