Hi I've got 67 olds w/ 425 engine. I switched it to a HEI distributor. whenever I tune it it runs really smooth for a couple of weeks. Then I start getting a miss, a slight one, really notice it at idle. No vacuum leaks, changed plug wires. Still using the plugs for that year at a .45 gap. Would it help if I used plugs for a '75 455 with a .60 gap ?

 

You might have a plug wire not seated all the way. Did you bump your timing up a bit and have you checked your carb adjustments?

 

i'd use the plugs that are spec'd for an hei engine. that should take care of your problem...

 

Is it a popping at idle? Along with the other suggestions IMO I would customize the advance curve(s) in the dist. Did you pull the HEI from a smog era engine or is it new? If it’s from a smog era engine the curve is not beneficial to the 425. You probably have too much advance at idle. Get an adjustable vac advance can and spring and weight kit and play around with it. You'll need setback timing light. Start with a base timing of 12*. Have the vac advance bring in another 10-12* at idle. Then have the centrifugal bring the rest in so you have a total advance of 36-42* at 2800-3000 RPMs. Each engine is different so this is just a general starting point. The gas and your driving habit as well as your altitude where you live all will have an effect. If it’s out of your realm of comfort take it somewhere or send it out and have the curve customized.
How do your spark plugs read?

 

Sounds like the HEI distributor has issues, was it new or used?

Put some NGK R5 plugs in it and gap them @ 40-45 or what they say to.

Just because you put a HEI distributor on an earlier model engine doesn't mean you have to use a wider gap plug. If you are using Champion plugs, toss them, they are junk.