Spark Problem

matth121

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May 19, 2011
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Hey Guys, I'm told my spark should jump a 7/16 gap and be bright blue. Well, it's jumping the gap but i cant even see it, i'm going to try again after dark but i'm thinking i've got weak spark. I pulled my plugs which are a month and a half old, they looked like 3 yrs old. I cleaned them up, reinstalled them, ran the motor for 5 mins in a tank, pulled the plugs again, full of oil again. Would this cause my horrible lack of performance? boat does 5 or 6mph max and won't even try to plane. Oh, 82 merc 25Hp on a small 14' fiberglass vhull. the motor starts and runs fine, i marked my prop and hub, not spinning. Can anyone tell me the symptoms of a weak spark? To fix this, i need new coils right? Thanks for any help
Matt
 

Texasmark

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Dec 20, 2005
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14,544
Re: Spark Problem

You can see a good spark in full daylight. Since you have a 25 hp, and assumed later model, one could/would tend to look elsewhere than the high voltage pks that plug directly into the plugs.....don't remember the ignition arrangement back in '82....could have been one coil, a pair of points and a couple of plugs for the 25 hp.

Why don't you make it easy for those of us who are beyond remembering and tell us what your ignition system is/consists of. Then we might can help.

Mark
 

matth121

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Re: Spark Problem

two magnet flywheel, makes it a black stator, wiring from stator and trigger to switchbox, trigger wires from switchbox to each coil, 1 per cylinder. Sorry, i kinda thought they were mostly all the same, but i guess your right. Hope this clears it up. so are you thinking my problem might be a lack of voltage before the coils then? I've seen the spark tester i have in use before and your right, a good spark is easy to see, i see nothing, but it's jumping the gap because the motor dies off when tester comes off the plug. Appreciate any help, thanks
 

matth121

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Re: Spark Problem

come on guys...there has to be some ignition experts out there....opinions, please
 

reddogg

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Re: Spark Problem

Just a thought, but you said you had allot of unburnt oil on your plugs, have you tried thining the mix a bit? Could oil have built up in your fuel tank throwing off your mixing? I had a similar problem once with unburnt oil driping out of my exhaust and really oily plugs. I cleaned the plugs and rigged up a temp fuel tank and lessend the mix a bit and she ran like a raped ape.

Red
 

matth121

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Re: Spark Problem

i haven't tried thinning it, i'm mixing at 50:1 with a new tank- same age as the plugs, id be supprised if Ive run more than 10 gallons thru it since i got it. should i drop it down to 40:1? im ok with this as long as theres no chance of it seizing up but would like to confirm its ok before i do it. Thanks
 

matth121

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Re: Spark Problem

ok, homemade DVA adapter- CDI troubleshooting guide says a DVA plugged into a multimeter set on DC inserted into common household outlet should read a peak voltage of between 160-170V. my outlets say 165- 168V so I think my DVA works. That being said, here's my results.

Blk/W to GND - 0.00 ohms(meter set to 400) DVA- 5V@ idle 3V@2000rpm
Blk/Y to GND - no reading/ Open DVA- 7V@ idle 4V@2000rpm

Brn/Y to GND- DVA- 8V@idle 11V@2000rpm
Brn/W to GND- DVA- 6V@idle 9V@2000rpm
Brn/Y to Brn/W- .78ohms(40K)

Both green wires to coils- DVA- 10-12V@idle falls off as rpms increase.

I know i probably missed a few things, so please let me know if I need to run anymore tests.
This is my current question. With readings that out of whack, should this even be running. I should be getting at least 150DVA on those green wires @ idle and increasing with revs, im getting 10 at idle. Is this why I can't see my spark? is that even enough juice to fire the coils? this thing will run all day long without a problem, just gutless. and fouling plugs. Open to any thoughts or suggestions. Seems like the stator is completely fried but it wouldnt run if that were the case right? Thanks guys
 

Laddies

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Sep 10, 2004
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12,218
Re: Spark Problem

All the voltages are low but the motor runs, the meter may be wrong have you another to try? If you multiply but 19 most of the readings are in the ballpark
 

matth121

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Re: Spark Problem

so I have to multiply by 19? I didnt know that, but what about when the voltages are dropping as revs increase- this should go the other way shouldnt it? and what about the resistance readings? they cant be in the ball park can they? Also, i used a different meter for these tests than my on the water tests, results are the same from my memory of them.
matt
 

Laddies

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Re: Spark Problem

No you should not have to but because all the readings were low I was ondering if the meter might be off in is reading
 

matth121

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Re: Spark Problem

according to the example CDI give on their site -"
ex.jpg
", i think my DVA and meter are good. Maybe I'm wrong, but i'm thinking at the very least, the stator needs replacing. What do you think? And what about the voltage dropping instead of increasing. Thanks for the help.

you'll have to click the small text to read it.
 

matth121

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Re: Spark Problem

what would be the result of removing the blk/w (high speed) stator wire from the switchbox. my motor seems to run the same with or without it with the only exception being my rpms read about 1000 higher, but they arent any higher, just reading higher.
 

Texasmark

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14,544
Re: Spark Problem

Don't want to get involved in your dialogue, but a couple of comments about the DVA (basically a sample and hold circuit reduced to low voltage that a DMM can read).

Using household voltage as a means to calibrate your DVA is better than nothing, but not bullet proof. Reason being is that the sine waves are occurring at 60 hz (60 revs per second) and are nice and fat if you will. At idle, your outboard is running at roughly 600 rpm's or 10 revs per second.....1/6 the speed of your line voltage. Additionally, the trigger pulses are high frequency (sharp leading/trailing edges) and narrow.....the DVA has to be able to grab it while it's there, not start to grab it and it disappear before it finishes charging up to the peak value and hold onto it until it reoccurs.

The frequency response of those two signals are miles apart and you need high frequency components to accurately detect the triggers. The triggers come and go fast and they are far apart. So, you need a high frequency storage unit (capacitor) to catch the fast ones and a large storage unit (low frequency) to hold the voltage up until the next one arrives.

I have the schematic for the DVA and have built one. I don't remember the values, but you need a high speed capacitor shunting a low speed capacitor and a good fast diode (switching diode) detector....a plain old rectifier may be too slow. The high speed storage part would be a ceramic disc of roughly 0.1 uf whereas the low speed would be an electrolytic of the size mentioned in the schematic (10 uf swag). The size of the bleed resistor (across the capacitors) plays into the holding part of the measurement also...larger the bleeder, the smaller the electrolytic can be and produce the same results.

In short, your readings may appear low (per Laddies' comment) but in reality the signal may be where it is supposed to be; you just can't read it correctly.

Just something to think about.

HTH,

Mark
 

matth121

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Re: Spark Problem

Thanks for your thoughts Mark, I Guess I may need to get a proper DVA or have it tested. But what do you think about those #'s dropping instead of increasing with rpm's? Also, an online mercury manual i've been looking at under ignition troubleshooting, they list weak spark as a sympton of a defective stator. What do you think of that? Please don't hesitate to get in on this conversation. I need all the help i can get :D

edit- what about using an oscilloscope for the measurements? I could have that done way cheaper than a DVA test by my local shop.
 

Texasmark

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Re: Spark Problem

I thought about the drop with an increase in rpm's and that is confusing to me also. The stator is basically an iron core coil, like a transformer, but only one coil. The magnet(s) embedded in the flywheel pass by these coils (stator/alternator) and as it passes it magnetically couples energy into the coils which either generates electricity or a trigger pulse.

If you are talking about the DVA voltage dropping, if you just took the DVA into account, the faster the pulses, the higher the voltage until it had charged up as far as possible and there was no more "droop" to the stored voltage therein. I don't see it an issue.

So, that translates to me as a stator output problem, or as far as triggers are concerned, an overloaded power supply possibly in the switch box.

I just went to the shop and got my manual. It is for later model higher hp engines so the wiring color coding is probably/appears to be different, but the theory is the same.

I noticed that the Stator output, includes outputs to the voltage regulator and other things. Of all the wires from the Stator, the only wires that are of the same color, (as your green wires are, and my yellow ones are) are the AC outputs to the rectifier, the output of which goes to the voltage regulator.

So.....before we go any farther since you said that the voltage from the green wires is 10-12v at idle and falls as rpm's increase I want to know:

Where are the other ends of the green wires connected (to what).....a rectifier bridge whereas the output goes to your voltage regulator....you may only have one module containing the rectifiers and regulator? If that is the case, the green wires will attach and red and black wires will exit with the black going to ground and the red going probably the starting solenoid hot, where the wire to the battery attaches.

If this is the problem, and "if" these suppositions are correct, your stator very well could be the culprit and it could be sucking down the battery voltage and reducing all the engine's voltages and hence the high voltage pulses to the spark plugs also, the higher you go in rpm's and would account for your poor performance and gooey plugs.

Stators, being magnetic devices usually fail whereupon moisture enters the wiring and shorts out a turn or more. Once that happens it's time for a new one.

Buttttt it could also be a problem with the rectifier or voltage regulator (if you have one).

Have you had problems with a discharged battery?

Measuring the voltage right at the battery terminals, what does it do when you increase rpm's? Does it drop? If yes, we are on the right track.

If yes, remove the green wires from whatever they are attached to and with them hanging freely in the air, attach your DMM's test leads and put the scale on AC volts, range 20V or so. Do the rev test and tell me what happens to the voltage. If it goes up, you probably have a module problem. If it still goes down then I would say that the stator needs replacing.

Get back with me.

Mark
 

matth121

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Re: Spark Problem

Great info Mark, but i think i should point out this is a manual start with no battery. I Have 2 stator wires(BLK/W, BLK/Y) and 2 Trigger wires (BRN/Y, BRN/W) that connect directly to my switch box on one side- the other side of the switch box has the 2 green wires that connect directly to each coil. Since they fire the engine and I have no start motor to wind it over, im not sure if I could test with them off. Would cranking with the pull rope be enough? Dont think I have a rectifier or regulator unless its part of the switch box. The Green wires again, they're the wires that carry voltage to the coils before the coils discharge to the plugs....my test info says they should read 150V at idle and go up to around 400V at WOT with DVA. This is why the dropping concerned me, should definitely be going up, right? Stronger spark for higher revs? Appreciate any thoughts you have, Thanks
Matt
 

Texasmark

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Re: Spark Problem

I'm going to have to back off Matt. Without a battery, you have to leave your wires connected for the engine to run and therefore the tests I mentioned won't work. Ping Silvertip or Chris1956 and pick their brains. They can probably help you.

Sorry,

Mark
 

matth121

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Re: Spark Problem

no worries Mark, thanks for the thoughts anyway. Cheers
Matt
 
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