Re: JD Model 70 Spark plug fouling

Posted by Steve - IN on May 30, 2002 at 01:37:15 from (12.222.1.20):

In Reply to: JD Model 70 Spark plug fouling posted by Mike weickert on May 29, 2002 at 20:17:58:

Mike,

I agree with Bob's idea. The heat range of the plug does make a big difference. A plug that comes out looking a gunmetal to a light gray color is happiest in the combustion chamber. Black plugs are not able to run hot enough to burn off junk -- your carb is too rich, or there's some oil getting into the cylinder.

If one is black and one is gray, you may need to go to a different heat range on the black plug as a cheap fix -- as the bottom set of rings in that cylinder are probably leaking some oil, or maybe the valve guide are letting in oil -- assuming the carb is giving the same stuff to each cylinder.

As Bob implied, if you change the heat range of the plug to a "cold" plug, you'll want to avoid "stop and go" kinds of running. That is, run it so it gets to good operating temp and stays there. If you're just running it 10 minutes at a time, put in hot plugs that will burn away the crud rapidly. If you use cold plugs and the engine is cold, it can miss until it comes up to a good temp and gets cleaned out.
You can basically judge the heat range of a plug by looking at it. Think of all the metal below the contact point with the head as a heat sink. The cold plugs will generally have more metal, the hot plugs less. Hot plugs run hot because there is not as much metal or surface area to dissipate the heat. Cold plugs like extended high operating temps because they can dissipate the heat.

There are a lot of variables in an engine. Plugs are just one of them. I'm assuming from your post that you don't want to mess with the mixture of one cylinder versus another -- or to dismantle a cylinder to replace oil rings to make a fix, or replace the valve guides in case one is leaking. So plugs are the fastest and cheapest way to attack the problem.

Oh.. to address your stated concerns ad seriatum:
1. Bad ignition. Maybe. But if 1 is good and 2 is bad, then the only path could be from distributor cap, to high tension cable to plug. Replace all those items, fairly cheap.
2. Valves. Check the valve lash. If that's OK, then maybe a little oil is leaking in the thru the guide. You can either pull the head and replace oil leaking guides, or go to a hotter plug in that cylinder.
3. Rings. Yes the bottom oil ring could be passing too much oil on one cylinder. Big job to fix it with new rings -- and a plug might be the cheap fix.
4. Cracked block. See a lot of white smoke out of the exhaust? That's steam and is usually the sign of a cracked block or head. You usually crack into the water jacket and burn coolant. Odds are this isn't the problem.

Hope this helps you. I used to tune engines -- race not tactor -- to a gnats as* -- The engine in my JD 60 only produces ~40 HP out of 321 cubic inches. I used to worry about making 190 HP race legally out of 100 cubic inches naturally aspirated. I think the JD is a lot more forgiving of little problems here and there. The basic design of our 60's and 70's are probably good to live long after you or I will walk this earth. They're big, lazy, and built to last; God bless 'em. So try the plug fixes first. That 70 has lived over 40 years without your help. A few plug experiments probably won't hurt it.

Best of luck....

Steve

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