IHC H plug fires only with wire removed...

With new spark plugs installed in both engines everything seems fine for say about an hour or so but then several plugs start to misfire and will only fire when the spark plug wires are either pulled out of the distributor socket say one quarter of an inch or off the plug by probably the same distance. The plugs look normal to the eye... but I suppose there could possibly be a coating that isn't discernable. New plugs and everything is fine for another hour or so. It's been too long ago for me to remember if cleaning the plugs helped for a while.

Does anyone have any ideas they would be willing to share?

What happens is as the tip insulator becomes fouled it becomes partially conductive. Voltage from the distributor then "bleeds" along the insulator to the plug shell rather than appearing as a spark at the gap. However by pulling the plug wire off a bit, voltage must build up sufficiently to bridge the new additional gap, and when it does there's enough energy to cause a spark at both the new gap AND at the plug. So the misfire disappears when you pull the wire off a bit.

Causes can be many - oil burning, plug heat range too cold, excessive idling or running too lightly loaded, carb set too rich, conductive additives in the fuel (lead, mystery oil, etc.) Sometimes a small coolant leak (eg. head gasket) will cause this too.

First thing I'd suggest is replacing the plugs with a set 1 or 2 heat ranges higher and see what happens.

In my case, I've suspected this carburetor all along. Darned thing has a "plug" where the shutoff solenoid should be and there is no high side adjustment. Further, it gets more than it's fair share of fuel at anything above an idle.

Thanks Bob,

Allan

This whole phenomenon has turned my "wrenching world" on it's ear.

Normally, I would have just said that the engine is pumping oil and needs overhauled.

However, I just went thru a similar episode with my Hydro 70 gasser. Darned thing would "carbon foul" with light use.

Guys here told me to install the electronic ignition, so I did and also changed plug brands.

My mind just can't wrap around this "cure", but that old tractor has not missed a single beat since. Runs like a brand new engine.

Allan

Earlier on, I had tried setting the gap on the plugs to roughly .035" and that didn't help one darned bit with the points and the only way I could clean up that flutter was by working the tractor hard.

Put in the electronic ignition using the same old coil, but pushed the gap out to around .045" or so.

Whatever the reason, I am so darned impressed with this "fix". It really, really works! :>)

Allan

Ray