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JD420 fouling spark plugs

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Tom Helms

05-28-2002 21:01:28

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I have a jd 420s,jd40s, and a jd m. The 420 and 40 are especially bad about fouling the spark plugs. I usually have to rplace the plugs at least twice a growing season. Any ideas on why the foul so much? Also, what is the correct spark plugs to use, just in case I don't have the right?

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Tom Helms

05-29-2002 18:08:04

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Re: JD420 fouling spark plugs in reply to Tom Helms, 05-28-2002 21:01:28

Thanks for all your help. I'll try some of these suggestions and see if they help. I might be back with more questions later. Thank again and keep poppin with those johnny poppers!!!!! !!!!!

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mike

05-29-2002 17:29:43

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Re: JD420 fouling spark plugs in reply to Tom Helms, 05-28-2002 21:01:28

I never heard these tractors have a bad reputation until I read this board. They ran fine when new and should not have any problems when rebuilt to specs. and given a proper tune up and adjustment. I even threw a set of bearings and rings in a MT
with a worn out bore,the block wasn't worth spending money on as it was broken,rebuilt the carb and tuned it up and the owner has used it with no problems mowing. I'm sorry I just can't see the problem as I have had good luck with these tractors.

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CPeter

05-29-2002 17:28:49

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Re: JD420 fouling spark plugs in reply to Tom Helms, 05-28-2002 21:01:28

I have a 40C and a 420U and both have had some problems with plug fouling. All of what has been said is true, but I have a theory as to why one plug fouls before the other and it has to do with the 180 & 540 degrees between firing. This changes the air speed going through the carb and affects the mixture. My only experience is with 2 cylinder verticals so I don't know why this doesn't happen to the horizontal ones. I use antifoulers under the plugs and it makes a huge difference. One thing that I have found with the M-420 carbs is that the throttle shaft wears and makes an vacuum leak in the throttle bore that causes them to run rough which we adjust out by enrichening the fuel mixture. This may contribute to the problem. Today I got in the mail a kit to replace the shaft in my 420 for this very reason. The antfoulers under the plugs have worked miracles for me and I suggest you try them. I get mine at the local non-chain auto and truck parts store. Cost is about $2.50 each. Worth $5.00 to try.

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mike

05-29-2002 17:36:23

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Re: Re: JD420 fouling spark plugs in reply to CPeter, 05-29-2002 17:28:49

Be careful with anti-foulers as the package warns not to run in all cylinders and you are allready limited. I put some in all 6cyl. of a scout engine and it would not even run. hence the warning on the package so instead of a rebuild I clean the plugs as needed. because this application is not worth the price of a rebuild to me.

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Clooney

05-29-2002 03:44:25

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Re: JD420 fouling spark plugs in reply to Tom Helms, 05-28-2002 21:01:28

Tom, you are correct, the M - 420 seem to foul spark plugs easily.
~I don't have any problems with my own personal 40-420'S & don't own an M, but they are all in good condition with good compression & clean air cleaners, [I use Champion H-12 plugs]. ~It seems that once a plug misfires a few times it fouls pretty fast after that. ~One thing that seems to help is using H-12 spark plugs instead of H-10 & if not working it hard, use 2 spark plug gaskets under the plug as that raises the heat range slightly.
~There is a little bronze drain in the bottom of the carb [intake side] that tends to plug up & if it is plugged it will allow fuel to puddle in the bottom of the carb intake [I'm not sure it that has an effect on the plug fouling or not but all mine are cleaned & allow fuel to drain through].
~All the plug wires must be solid metal core [not automotive TVRS "resistor" type & the distributor bushings must be in good condition.
~One thing I have been trying to confirm on the M - 420 with plug fouling problems is what type of condenser they are using. I have worked on a few that have fouled plugs & they all seem to have new condensers in them, on the other hand all mine have original condensers [I never replace a condenser if the point wear looks even].
~Other things to check are: `that the air cleaner inner mesh isn't partially plugged [very common on old tractors], `that the coil is getting good voltage [the ignition switches tend to have high resistance in the M - 420's as they get older],
` that the coil is wired to the same polarity as the battery [the same terminal of the coil should go to the points as the battery has ground],
`that the fuel is fresh & not premium,
`that the idle is kept high enough [don�t try to idle it as low as possible because you like the slow popping] + lower idle speeds usually require a richer carb mixture setting.
~On mine, I always run the carb out of fuel to stop the engine if I am going to store the tractor for more than a week or so. If old fuel is allowed to stand in the carb all the [light ends] tend to evaporate & leave only the heavy non-easily ignited parts that tend to foul plugs & start hard.
~Try the H-12 plugs first, maybe that is all it will take��.

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Henry J

05-29-2002 05:24:46

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Re: Re: JD420 fouling spark plugs in reply to Clooney, 05-29-2002 03:44:25

Clooney

Interesting that you should admit that you use champion. Bet a million guys with green hats are saying tisk tisk right now lol. I don't have any problem with champions either, matter of fact just stuck a couple in the Kawasaki powered lawn mower today insteads of the famous Bosch. I have been having great spark plug results (havn't fouled one yet this summer) and it seems that I have stumbled on a couple of things that you do and has helped. I never idle the 420 low. I usually either let it run about a thousand or turn it off. Also, I try to run the main jet as lean as possible with the engine still running at peak performance. At this setting, the tractor will not pull hard at wide open until the temp gets well up in the green, then it runs like a champ - no pun inteded. I am really a believer in getting the main mixture down, they will run well at a very rich setting but as I found out last year it isn't long before it is time to pull the rear plug out.

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G Taylor

05-29-2002 09:38:40

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Re: Re: Re: JD420 fouling spark plugs in reply to Henry J, 05-29-2002 05:24:46

In addition to the vital point of running lean & clean as previously stated and running temps in the green. Check with a thermometer & make certain the coolant temp is at least 180 & 195 is better. It going to foul & soot up when the combustion chamber/port walls are cold enough for fuel to condense onto.

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