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Restoration & Repair Tips Board

john deere 40 won't start bad coil

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Author  [Modern View]

05-31-2012 20:55:28

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hi i have a john deere 40 and it won"t start now. it cranks but no firing. i have timed it. there is spark. gas on. i was wondering if it could be the coil?. i was having problems before where it was running on 1 cylinder and it would bog down and have no power and the throttle would go back up again.

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El Toro

06-02-2012 08:01:03

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 Re: john deere 40 won't start bad coil in reply to nickkalisz, 05-31-2012 20:55:28  
Check your sparkplugs to see if they're wet. You may not be getting fuel into the engine if they're dry. Hal

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06-01-2012 05:45:21

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 Re: john deere 40 won't start bad coil in reply to nickkalisz, 05-31-2012 20:55:28  
Sometimes you can have a when taking
a plug wire off, and holding it near the block
while turning the engine over...BUT...the plug
won't fire under compression. You've got some
spark, but not hot enough to fire! Condenser or
coil are weak!

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John T

06-01-2012 09:17:16

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 Re: john deere 40 won't start bad coil in reply to greygoat, 06-01-2012 05:45:21  
Cant guarantee its accuracy, but I read somewhere an arc of 3/16 inch in free air is approximate to an arc of 0.025 inside the cylinders at 100 PSI or so. Therefore, if the spark at 3/16 appears good and blue outside the tractor, it ought to be sufficient inside under compression.

Trouble is we have no idea what or where or the quality of the spark or gap he has when he says he "has spark"

John T

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John T

06-01-2012 04:51:33

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 If its sparking the coil is likely okay in reply to nickkalisz, 05-31-2012 20:55:28  
You say it has spark!!! And if its producing a spark the coil isnt the problem.. Where is that "spark" you say its producing???

If you get spark say from out of the HV coil wire but none to the plugs, its a cap or rotor or plug wire problem.

If you work through my Troubleshooting Procedure linked below it will help diagnose the casue of non firing, but if its producing a good spark, the coil is NOT the problem, you just have to get it routed from the coil out to the plug wires and thats the job of the distributor cap and rotor etc and then via the plug wires to the plugs.

John T

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