Spark plug mystery

I went ahead and replaced the 6 year old plugs ( champion truck plugs ) on my 1993 F150 ( 4.9 eng , e4od trans ) with Bosch platinums. The truck was running great before the change but with the change I was getting an
occasional stumble and misfire at idle ( close to stalling ) . Otherwise the truck appears to run just fine. I then installed a NEW set of champion truck plugs and again, I still get that stumbling at idle. I then re-installed the OLD champion truck plugs and everything is fine, stumble is gone. The old plugs are pretty badly worn but not too much in the way of deposits. Can somebody explain why new plugs would cause the truck to run so crappy?
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Did you gap them??

the
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Yes, I set the gaps on both sets of plugs according to specs. The old plugs are way overgapped due to electrode wear. The old plugs are in now (still overgapped) and truck is running great... what gives?

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gap them wider. may be hotter ignition system. john

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"flicks" wrote

You could have twisted a plug wire too much when removing/installing, and broken the internal conductor, and are just lucky that it's making good contact now.
The wire's plug-end terminal might also be loose, and making bad contact with the plug, and again, you "are just lucky that it's making good contact now".
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the
truck
the
How about using new Champions? I could be that the Bosch plugs aren't as good as you think.
Jeff
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flicks opined in

HAh... you find the worst two plugs you could put in a ford.
Champions are just bad, and Bosch Plats dont work in my fords, for some reason... tried in 351, 302 and 2.3... work ok for a day or two, then trhe same problem you are having
Put in a new set of Motorcraft or Autolite...
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Hiya all,
Always use NGK in my Granada. The dealer for the Chrysler changed the plugs under warranty, threw out the Champions and fitted NGK to cure a misfire.
Try a slightly bigger gap anyway, possibly up to 50 thou depending on how good your ignition system is, a bigger spark gives a better bang.
pottsy.
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Widening the gap away from factory specs fixes the stumble . It looks like some other issue is rearing it ugly head with the smaller gap, something like a lean mixture, bad injectors or something.

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