Spark Plug goof!

[67resto-coupe]

So, I'm wondering how much of a difference a spark plug error could have on my engine's performance. It seems to run alright, but *something* is not right.
Anyway, I think I figured out where I goofed. My roller 5.0 is a "normal" 5.0 bottom end from the '86-'93 family. I thing the last 2 sets of plugs have been
Autolite 764...which I believe is the correct part for that ere of Mustang 5.0. However, my heads are '98 GT-40P heads....and would actually require the correct
plugs for the '98 Ford Explorer 5.0. Duh!

The car runs decent....has a nice lumpy idle from the E303 cam, but I've always felt as if there was just something not right.
I'll get a new set of '98 Explorer plugs this weekend and fit them on Sunday. Until then, do you guys think that the wrong plug can
make that much of a difference in performance - all else being equal?

Thanks.

 

[SELLERSRODSHOP]

it may cause an issue depending on the differences in the plugs depth. the gt-40p heads changed the placement of the plug in the chamber compared to the older heads. not sure if one plug was longer than the other or not. if the early plugs are shorter & not seated deep enough, it may cause a shrouding issue & not burn the charge properly. we ran into that on an old drag car years ago.

you running a carb or efi on that thing?

 

[67resto-coupe]

running a carb. I feel like it's been running rich, but I've already adjusted the jets and timing. Oddly, this change in performance came after I rejetted and reinstalled NEW 5.0 plugs. I think the plug change will be an improvement. At the least, it'll give me a better baseline from which to tune from......