John deere 40 spark plug problems

hillaagsa

Member
Hi everyone,

I have a John Deere 40 that has been running rough lately and only wants to hit on one cylinder sometimes. So I changed the spark plugs out first and it still did the same thing. But I noticed if you pull the spark plug wire off just a little bit on the number 1 cylinder the tractor runs great, put the spark plug wire back on all the way like it's supposed to be and it drops a cylinder. So the next thing I did was replace the distributor cap and spark plug wires because there was corrosion on both. It still does the same exact thing. You pull the number 1 cylinder spark plug wire off just a little where it's still making spark and it runs great. Any information and help on this would sure be appreciated. Thanks
 
Coolant temp above 180F and a slightly lean mixture will limit plug fouling.
Use auto lite 216 or 437's non resistor plugs and non resistor wiring. Check the coil polarity, they are often backwards.
 
I have a Wheelhorse D180 that does the same thing. I've got a new coil for it but I haven't put it on yet but I'm 95% sure it's the coil.
 
Another approach is to grind about 1/32" off the end of the rotor. Makes the spark a little hotter when it does jump to the plug.
 
Yeah it has a brand new coil on it too. It has good compression and the valves have been adjusted also. I looked and I have 216 autolites in it now. So I"ll try switching to 437 next. Thanks for the info.
 
The most common failure I have seen is dirty contacts internally/externally on the dash ignition switch. Often, I have measured a strong 6-7V at the negative coil lead & still have had problems.

The best quick test, assuming it is original 6V positive ground system, is to run a test jumper lead from large negative battery lug on the starter directly to the negative coil terminal, thus bypassing the ignition switch.

I have disassembled many Dubuque ignition switches & cleaned the contacts as a cure to all electrical system issues assuming the harness is not at fault.
 
Cleaning the points may help. While you are cleaning them, look to see if they are pitted and need to be replaced.
 
I just got the 437 spark plugs in yesterday, installed them last night and it runs runs great now.Amazing the difference changing to a different plug. Thanks for all the advice and help. I really appreciate it.
 

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