Spark plugs exploded, why?

[Honclfibr]

Originally posted by elaineo
how bout i just kick him really hard in the 'nads?
<font size=0>.... see you soon darrell... </font>

Hey, I TOLD you I felt partially responsible for the whole thing, and I'd fix your bike for you...

...but if you kick me in the nuts, it's going on E-Bay

[stoinkythepig]

If it's just a cracked valve (or 4), you could fix it really cheaply. Valves are around $12 each and you may be able to get away with just new valves, a bit of lapping and a new head gasket. Don't buy a new engine yet...

[twrayinma]

Originally posted by Honclfibr

...but if you kick me in the nuts, it's going on E-Bay

okay, here's the plan, elaine.
step 1) get keys
step 2) kick 'im in the nads.
step 3) report bike as stolen
step 4) profit!

[zx7rou812]

Originally posted by twrayinma
okay, here's the plan, elaine.
step 1) get keys
step 2) kick 'im in the nads.
step 3) report bike as stolen
step 4) profit!

[Honclfibr]

Is it best to buy engine parts from a dealer or is there a good aftermarket supplier?

Originally posted by stoinkythepig
If it's just a cracked valve (or 4), you could fix it really cheaply. Valves are around $12 each and you may be able to get away with just new valves, a bit of lapping and a new head gasket. Don't buy a new engine yet...

[zx7rou812]

Originally posted by Honclfibr
Is it best to buy engine parts from a dealer or is there a good aftermarket supplier?

I'd say go with the factory parts for the "small" and "important" stuff...usually you'll wind up paying the same, but you know (in THOERY) the OEM stuff will be made better...In thoery

[Honclfibr]

Originally posted by Honclfibr
Is it best to buy engine parts from a dealer or is there a good aftermarket supplier?

Wrong lever elaine

and it's www.ronayers.com they have searchable microfiche for parts for your bike and you won't find cheaper OEM parts...

[stoinkythepig]

Originally posted by Honclfibr
Wrong lever elaine

and it's www.ronayers.com they have searchable microfiche for parts for your bike and you won't find cheaper OEM parts...

That's where I'd be buying 'em. Bought 8 intake valves there in the past. Lemme know if you need the tools to compress the valve springs to get the keepers and retainers off.

Did you get the head off, BTW? If so, be sure to check it for flatness before sinking any cash or effort into it. You'll want to inspect the cylinders bores for scoring too...

[Honclfibr]

Originally posted by stoinkythepig
Did you get the head off, BTW? If so, be sure to check it for flatness before sinking any cash or effort into it. You'll want to inspect the cylinders bores for scoring too...

Haven't touched the bike since the compression test, we're waiting on a service manual which should be coming in shortly.

Check the head for flatness, is that because extreme heat can warp it? How do you go about checking it? The cylinders thing, yeah that makes a lot of sense. Can we check the cylinder walls with just the head off, or is it necessary to pull the crank and check things that way?

Thanks for the advice btw, you know a good bit about DIY engine work, is this a job for the budding shade tree mechanic or not? There's been some speculation that perhaps I should not be allowed to touch this engine

[stoinkythepig]

Originally posted by Honclfibr
Haven't touched the bike since the compression test, we're waiting on a service manual which should be coming in shortly.

Check the head for flatness, is that because extreme heat can warp it? How do you go about checking it? The cylinders thing, yeah that makes a lot of sense. Can we check the cylinder walls with just the head off, or is it necessary to pull the crank and check things that way?

Thanks for the advice btw, you know a good bit about DIY engine work, is this a job for the budding shade tree mechanic or not? There's been some speculation that perhaps I should not be allowed to touch this engine

Use a straightedge, a bright light and some feeler gauges to check the head for flatness (forgot to mention that you should check the deck of the engine too). Lay the straight edge across different flat parts of the head and look for light coming through in the middle if the straight edge does not rock, or on the ends if it rocks. Use the feelers to determine how bad it is. Service manual should tell you what the limit is.

You can check for scoring with the head off and nothing else removed.

This is absolutely a job for a budding shade tree mechanic. Matter of fact, a perfect job. You really can't start from a much worse situation so you have nothing to lose. Worst case scenario is the engine needs to swapped. That's only a 5 hour project by itself. Pulling the head and replacing the valve(s) might be twice the labor. You'll be surprised how easy this work really is. Don't forget all you learned about cam and chain alignment though...

I have a spare ZG1000 head which is quite similar to the EX 250 if you wish to look at it before you start. It's got at least one broken exhaust valve from ignition "issues". Found the problem with your compression gauge and fixed it with a part Beet gave me along with an engine from a parts bike I paid $250 for (and have sold $900 worth of parts from). Being a shade tree mechanic, knowing great people, and being connected to the Internet has some awesome advantages. If only I knew some great people...

[Honclfibr]

Just a quick update about MY bike (what this thread was about before it got hijacked ), got the carbs synched up and the bike all back together, took it out for a brisk few miles today and it's running excellent! Once again thanks for all the help! Now, time for some warmer weather!

[stoinkythepig]

I was thinking about this problem a bit and I have a theory about what went wrong. You wrote that the OEM plugs are resistor types and the melted plugs are not. It's been a long time since I studied EE stuff so bear with me and be kind.

My theory (which is mine, that is to say, it's my theory, which is mine):

When the magnetic field in the primary winding of the coil is collapsing, it generates a pulse in the secondary winding. It's not an instantaneous event but happens rather slowly (from a speed of light/electricity point of view) The pulse voltage builds until in can overcome the resistance of the air in the gap of the spark plug. If that resistance were increased, the voltage would need to climb higher to overcome it and that would take more time. Assuming that's correct, if the resistance were decreased (say, by not using a resistor plug), the voltage required to bridge the air gap would be lower and the spark would occur earlier. An earlier spark would in essence be a timing advance and that can create more localized heat than the engine design can deal with. The result of that excess heat usually manifests itself in the form of melted spark plugs and other damage.

[elaineo]

my EE sux, but the resistor inside the NGK is only 5 k ohm, which i doubt would make a significant difference when added to the nearly infinite resistance of the spark plug gap. the NGK website says it's only there to suppress ignition noise. adding the resistor would probably result in a weaker spark, but not a delayed spark.

[stoinkythepig]

Originally posted by elaineo
my EE sux, but the resistor inside the NGK is only 5 k ohm, which i doubt would make a significant difference when added to the nearly infinite resistance of the spark plug gap. the NGK website says it's only there to suppress ignition noise. adding the resistor would probably result in a weaker spark, but not a delayed spark.

Thank you for being kind. Did I say it was my theory? I meant some idiot at work came up with the theory and I just posted it, yeah that's it.

[Honclfibr]

But when the spark gap is bridged, the series resistance limits the current through the gap, right? So a lower series resistance would result in higher current across the air gap, therefore a hotter spark...

What I mean is, plasma is a good conductor right? When the gap is actually sparking, I thought the resistance across the gap was pretty low?

[bemused]

Originally posted by Honclfibr
But when the spark gap is bridged, the series resistance limits the current through the gap, right? So a lower series resistance would result in higher current across the air gap, therefore a hotter spark...

What I mean is, plasma is a good conductor right? When the gap is actually sparking, I thought the resistance across the gap was pretty low?

IIRC, you're on the right track: the spark is due to the voltage differential across the gap overcoming the insulating factor of the air or plasma (remember, a plug will fire merrily away even if you've yanked it out into free air).

resistor will drop voltage AND reduce current @ the electrode. lower current will reduce heat, lower voltage will duration of the spark (and ultimately, the time that the current [heat] is flowing).

so-o-o... if any of this is valid, your belief that the fault was caused by use of non-resistor plugs seems sound to me!

[stoinkythepig]

Originally posted by Honclfibr
But when the spark gap is bridged, the series resistance limits the current through the gap, right? So a lower series resistance would result in higher current across the air gap, therefore a hotter spark...

Assuming the current source is unlimited... If the current source was already at it's limit, there would be no more current. There would be more voltage initially but as the current goes to max the voltage has to drop. That said, the resistor has to absorb some of the energy so the spark must be "hotter" without it.

What I mean is, plasma is a good conductor right? When the gap is actually sparking, I thought the resistance across the gap was pretty low?

It has to be lower than air at that point because once an arc is started across air, it can be drawn out substantially, but I couldn't tell you what it's true resistance is.

[stoinkythepig]

Originally posted by bemused
IIRC, you're on the right track: the spark is due to the voltage differential across the gap overcoming the insulating factor of the air or plasma (remember, a plug will fire merrily away even if you've yanked it out into free air).

resistor will drop voltage AND reduce current @ the electrode. lower current will reduce heat, lower voltage will duration of the spark (and ultimately, the time that the current [heat] is flowing).

so-o-o... if any of this is valid, your belief that the fault was caused by use of non-resistor plugs seems sound to me!

But the kind of damage seen could not have been caused by a "hot" spark. It looks more like detonation to me which goes back to timing...

[Honclfibr]

One other thing also, it seems to me that before the gap is bridged, the spark gap looks electrically like a capacitor. So changing the series resistance is going to affect the RC time constant of the charging circuit, affecting the voltage rise time, is this what you were saying Dave?

Though I mentioned this to my father and he seemed to think that in the context of a charging circuit the change would be fairly minimal. Something else he mentioned that I didn't know was that the plug wires themselves are often 10-20k ohms, making the series resistance of the plug pretty nominal.

Seems ignition systems are not as simplistic as I had imagined, I may need to do some homework here

[stoinkythepig]

Originally posted by Honclfibr
One other thing also, it seems to me that before the gap is bridged, the spark gap looks electrically like a capacitor. So changing the series resistance is going to affect the RC time constant of the charging circuit, affecting the voltage rise time, is this what you were saying Dave?

Though I mentioned this to my father and he seemed to think that in the context of a charging circuit the change would be fairly minimal. Something else he mentioned that I didn't know was that the plug wires themselves are often 10-20k ohms, making the series resistance of the plug pretty nominal.

Seems ignition systems are not as simplistic as I had imagined, I may need to do some homework here

That's exactly what I was saying.

Pretty sure the wires on that bike are solid core and offer negligible resistance. They are solid on my ZG1000 and it is of similar design vintage. The resistors on the ZG are in the caps though, not the plugs (and they're 4.7 kohms).

[Honclfibr]

Originally posted by stoinkythepig
But the kind of damage seen could not have been caused by a "hot" spark. It looks more like detonation to me which goes back to timing...

My thinking went along the lines of a hotter spark would mean more heat into the plug. Which combined with a plug that's designed to dissipate less heat (hotter heat range) might cause enough heat to build up in the plug to cause preigntion?

[stoinkythepig]

Another thought...

What if the ignition module on the bike is susceptible to RFI? The resistor plugs are used solely to reduce RFI.

The ignition module relies on weak pulses coming from the pickup coils, that travel through unsheilded wire and into the moudule itself at pre-determined intervals of crank rotation. If the module were getting extra pulses from RFI, it may fire at a completely wrong time.

It's a long shot but not entirely without merit. I used to do a lot of RFI testing (emmisions and susceptibility) when I worked in Bell Labs and it's shocking what a lack of simple sheilding will allow to happen to a circuit.


EDIT: Found this after posting (maybe not a long shot after all):

http://www.ngksparkplugs.com/techinf...200&country=US

[elaineo]

Originally posted by stoinkythepig
But the kind of damage seen could not have been caused by a "hot" spark. It looks more like detonation to me which goes back to timing...

Can't we just chalk it all up to the fact that the spark plugs I used were several ranges hotter than the spark plugs intended for my bike? (Champion recommends Z6-something for the EX250, i used Z9Y)

The rate at which the voltage rises is going to be the same regardless of whether or not there is a resistor in there. The owner's manual for that bike doesn't even recommend the resistor spark plug, it recommends C8HSA. that probably means the wires have enough resistance already to suppress RFI.

[stoinkythepig]

Originally posted by elaineo
Can't we just chalk it all up to the fact that the spark plugs I used were several ranges hotter than the spark plugs intended for my bike? (Champion recommends Z6-something for the EX250, i used Z9Y)

The rate at which the voltage rises is going to be the same regardless of whether or not there is a resistor in there. The owner's manual for that bike doesn't even recommend the resistor spark plug, it recommends C8HSA. that probably means the wires have enough resistance already to suppress RFI.

Oh sure, take the easy, obvious and logical route.

[elaineo]

Originally posted by bemused
IIRC, you're on the right track: the spark is due to the voltage differential across the gap overcoming the insulating factor of the air or plasma (remember, a plug will fire merrily away even if you've yanked it out into free air).

resistor will drop voltage AND reduce current @ the electrode. lower current will reduce heat, lower voltage will duration of the spark (and ultimately, the time that the current [heat] is flowing).

well... the plug will fire at the same voltage, resistor or not. The resistor will reduce the current flow but the total energy/heat dissipated has to be the same, it'll just result in a longer spark (and weaker spark, because of the energy dissipated in the resistor).