350 5.7
Pertronix HEI
ACCEL wires
AC Delco r43ts @.035 gap
Firing order is 100% correct.

Dumb question it seems but after changing my plugs from r45ts gapped @.045 to r43ts @.035 and doing some work to the starter (shimming and putting some lock tight on the new bolts, olds kept backing out), my car has a HARD *** shutter, its firing in order but has an intermittent, uneven miss. It's so hard that it jerks the engine as if it's a dead miss. It somewhat clears up if I pick up the rpms but once they fall it begins to spit and backfire as if its loading up on fuel. Timing hasn't changed, nothing internal has been adjusted or changed. Just a plug change and threw some shims and new bolts in the starter. Any ideas?

 

43 is a colder plug, why? But it should still fire. Recheck #s 5 and 7, it is really easy to get them interchanged.
How hard was it to pull the spark plug wires off of the old plugs? If some were really hard, you might have damaged a plug wire. Dielectric grease helps.

 

Want paying attention when I bought the plugs but figured it wouldn't be that much of a difference, 5&7 are in the correct places and the wires are new with the plugs. It's got me stumped because it's a hard shudder. Imagine a bouncy house with 15 kids in it but it only jerks once over 2 seconds=my 350. I dont think it's a valve or spring or anything because although you can still feel and hear the miss with your foot on the gas, it clears up a lot.

 

I'm wondering if it is actually NOT a problem with the ignition, but if the ignition IS correct now, and that just revealed your next weak link in the chain. Either that, or your engine just doesn't like those 43 plugs.

 

I'm not sure, it did have a hiccup before but you really had to pay attention to notice it. I'm going to take the plugs out tomorrow and gap them to .045 and see if that helps. If not, then I'll throw the 45s back in and see, hopefully it's not the gear drive alor valve train

 

43 is too cool for a stock motor. Likely to foul during extended idling; and once that happens, even if it clears up when you get going, it will return at unpredictable times.

Put R45TS back in, like should be there in the first place.

Gap doesn't matter much, within reason. Anything from .035" to .050" is fine. Changing from .035" to .045" isn't going to fix it, I wouldn't bother. .040" to .045" is more or less ideal. .060" is too much... it'll run fine but will stress out the cap rotor & wires more than necessary... too much maze, not enough cheeeeeze.

 

It was the plugs, changed back to 45s and gapped @ .060, runs like a clock, go figure lol

 

Gap them to .045, .050 max. That .060" thing never worked out well and puts an un-needed extra load on your ignition system.
You might find the rotor button melted into the distributor cap if you leave it at .060".

 

Yup, set them to .045" max.

All you accomplish with the wide gap is, increasing the voltages developed in the high-voltage (secondary) side of the coil. The rotor, cap, & plug wires are subjected to 2 - 3 times the overall stress that they should be. The rotor in particular breaks down and burns up right at the center, through the plastic, from the contact to the top of the shaft. In exchange for, ABSOLUTLEY NOTHING in terms of any positive benefit, in a stock motor.

Go back and gap them properly, instead of what you did. Or, gap them properly next time you have to work on it, which is guaranteed to be quite soon.

 

Check the new/removed plugs for a cracked one. It could have been dropped.

RBob.