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Spark From Engine To Gas Tank Strap


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#1 Bloodyknuckles OFFLINE  

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Posted June 11, 2014 - 07:22 PM

1972 MTD 990 with 15 hp cast iron B&S. I recently picked up this tractor and am trying to get it to run smoothly. I had taken the front engine shroud and the points cover off just to have a look at things. When I tried to start the engine it produced a large yellowish/white spark from the rear shroud bolt to the metal gas strap every time it turned over. See photo. The engine never started. Also, when I removed the spark plug wire it stopped producing the spark. I don't know what I did but it started when I removed the points cover.

 

I replaced the points and condenser but did not reinstall the cover. When I tried to start it again the large spark was gone and it started and ran for 4 or 5 minutes. I would love to know what could have caused this large spark. If anyone has any ideas I would like to hear them. Thanks.

 

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#2 New.Canadian.DB.Owner OFFLINE  

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Posted June 11, 2014 - 07:32 PM

Magic? 



#3 DougT ONLINE  

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Posted June 11, 2014 - 07:34 PM

Something was looking for a ground. Where was the plug wire in relation to the strap? What about the battery ground cable? was it hooked up? Does it go to the block?


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#4 LilysDad OFFLINE  

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Posted June 11, 2014 - 07:40 PM

The only way I could see that spark is with binoculars!



#5 motobreeder OFFLINE  

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Posted June 11, 2014 - 07:54 PM

I can't see the spark, but sounds like a ground issue.


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#6 gopher OFFLINE  

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Posted June 11, 2014 - 08:20 PM

The only way have a spark jump is from high voltage from secondary winding of coil. Bad plug wire or cracked coil probably the wire when you disconnected it moved from where shorted out. Thing is your tank strap must be grounded better then the motor.  


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#7 Bloodyknuckles OFFLINE  

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Posted June 11, 2014 - 09:14 PM

I didn't get a pic of the spark itself, just where it was happening. The neg battery cable is attached to the frame. The spark plug wire is probably 7inches away at it closest point. I took a set of jumper cables and attached one side to the muffler pipe and the other end to the neg side of the battery and it still did it. I filed a clean spot on the pipe just to make sure I had a good connection. I wouldn't consider the tank strap to be a good ground at all. It fits around the tank and it's saddle but isn't bolted to it. I believe I will replace the coil as well. 

 

It was a real odd situation. The metal tank saddle is bolted to the engine. The problem went away after I changed the points and condensor. Later in the day the generator wouldn't turn over. I sanded the connections and that fixed it. 



#8 dodge trucker OFFLINE  

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Posted June 11, 2014 - 09:28 PM

had that happen on an old Slant 6 I used to have.... started fine at work  ran fine... went home and had a shower and was headed out to the (then) girlfriend's house.... no start.... crank and crank and crank  dead...  When it  was almost dark (I was younger and dumber I had ground on it enough to kill the starter)  I saw a spark from the coil wire to the uppr radiator hose.... yes it jumped to that rubber hose!   Moved the coil wire  over 2 inches, started right up....  well after a starter change (those were quite easy to do)  and like I said it was getting dark by the time I figured out my no start  it was getting a little late to go out that nite...


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#9 JDBrian OFFLINE  

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Posted June 12, 2014 - 06:06 AM

I'd say you have a bad ground connection somewhere. The parts of the chassis of the engine and the tractor should be at the same potential and shouldn't ever have any voltage between them to cause an arc. Clean and tighten the mechanical connections between the arcing parts and between the engine / tractor chassis and the battery - terminal.


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#10 Bloodyknuckles OFFLINE  

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Posted June 14, 2014 - 09:27 PM

I installed a flat braided ground strap from the cast iron part of the engine to the frame. I did have the engine off the frame to install a new gasket between the engine and engine base prior to it arcing. I guess the gasket could be acting like an insulator.


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#11 dodge trucker OFFLINE  

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Posted June 14, 2014 - 10:50 PM

I installed a flat braided ground strap from the cast iron part of the engine to the frame. I did have the engine off the frame to install a new gasket between the engine and engine base prior to it arcing. I guess the gasket could be acting like an insulator.

how did it "not" act as an insulator for the years that have passed since new then?



#12 motobreeder OFFLINE  

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Posted June 15, 2014 - 08:12 AM

Interesting curiosity.

 

I'd guess the two parts would have been electrically connected by the bolts.

Did you coat the bolts with a loctite or something non-conductive?



#13 Bloodyknuckles OFFLINE  

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Posted June 15, 2014 - 10:02 AM

how did it "not" act as an insulator for the years that have passed since new then?

I don't know, good question though. 



#14 Bloodyknuckles OFFLINE  

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Posted June 15, 2014 - 10:12 AM

Interesting curiosity.

 

I'd guess the two parts would have been electrically connected by the bolts.

Did you coat the bolts with a loctite or something non-conductive?

I used anti-seize on the bolts. It must ground through the bolts. As dodge trucker mentioned it worked before with a gasket installed.



#15 dodge trucker OFFLINE  

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Posted June 15, 2014 - 11:53 AM

antiseize should not have hurt  conductivity  but curiously what kind did you use?  I like the copper based stuff myself better than the silver nickel based stuff..... when I was working industrial maintenance in the steel mill they actually had a conductivity "paste" that was very much like the copper based antiseize that they used on just about  anything electrical.   and with all the juice flowing thru an electric arc furnace  that was very important.


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