Tune up fail

1994 GMC SUBURBAN 2500 454 4X4 225000 Miles

Plugs wires cap n rotor…NGK plugs. Gapped at about 40…best i could come up with is 35 gap … so i set them at 35…Its missing… Not running right.
The only thing i can come up with is either a chunk of crud on one of the plugs on the way back in or gap is way off…

Set the gap per the specs.


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And triple check the wires are on the correct plugs.


Also be sure you didn’t bend the tang on the rotor. Then check for the 5 7 cross on ignition wires. If you had to make the plugs look odd when changing the gap, it may be too narrow.

also check for a vacuum leak.

A compression check is always a must IMO. The plugs are out anyway so why skip that important step especially with 225k miles? Use of a vacuum gauge would help a lot.

More info would help. Cylinder specific misfire or random? Idles rough but smooths out as the RPMs go up, etc?
The odds of this being caused by the plug gap being off by .05 is zero IMO.

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