My 1982 Chevy 250 engine has a miss at idle but going down the road it seems to pull ok. I havent had a lot of time to look at it but its missing on number 3 cylinder . I replaced the dist cap,wire and plug. I ran a compresion test it had 140 . It died on the way home this spring . I cant remember every thing that took place but it wasnt firing .I called a buddy and we changed out the dist not the cap and steal no fire at the plugs. Then we replaced the cap it ran. Days later i put the old dist back in with the old replacement cap . I got a new cap and replaced that one. I need to take time to look it over really good.
vacuum/intake---smoke test.
Could also try spraying brakeclean or ether around manifold. Will change revs when spraying where the vacuum leak is.
Really the best way to check most anything tune-up wise on any old or even newer engines is utilizing a vacuum gauge--A miss fire at idle can be attributed to a weak valve spring or bad valve guides, also a cam going flat. A vacuum gauge will also tell if timing is advanced or retarded. * A tool very few use or think about these days because it's old school. Good vacuum at idle on a stock engine should reach 19 to 20 inches. The advantage of roller cams is duration can be increased without effecting vacuum as much as a flat tappet {solid or hydraulic} camshaft. Rollers can can easily run over 262 duration and still pull 18 to 19 inches. * Years ago it was so common to see rebuilt cylinder heads with .060 shims under the valve springs to raise spring pressure. People were real cheap and machine shops would cut the corners instead of insisting on replacing the springs which were not really all that expensive, along with knurling valve guides that were marginal to start with. On top of that the induction hardened valve seats just didn't last with unleaded gas, induction hardened seats are only .010 in depth of seat material at best and some like a Chrysler product were only .007. So it didn't take much to grind past the induction hardening to soft cast, valve job would last about 10,000 miles if that. The other was plunge grinding the stem, many never used a Quicky gauge to check valve stem height, which then would through off the self adjusting hydraulic lifter range. On tear down of old engines you had to check all the push rod lengths because it wasn't uncommon to find lengths ranging from {stock, .030, .060} in one engine. * One area was Ford with early FE {Ford Edsel} was the hydraulic lifter range was .200 then later 1964 up .150. The 289, 302, 351W and 6's with sled self adjusting rockers was a issue. When tightening the center fulcrum bolt it should only take a half turn from zero lash to tight. Many had low vacuum because they were assembled with a full turn from zero to tight. Mr. Gasket made a shim pack that had several different thickness shims to go between fulcrum and cylinder head rocker tower to correct the issue, even from factory many were not correct depending on machining tolerance. * One of the worst common practices in the 80's & 90's was using false bronze guides, those were thin wall & had a tendency of coming loose and traveling with the valve stem. Heads that were equipped with those lasted about 10,000 miles. Ford rebuilt engines in 60's, 70's an into 80's were commonly fit with valves that had .005 to .010 over size valve stems, the factory authorized rebuilders just reamed the old guides and slammed over sized valves in. That was all great till the next time, everything went in the scrap bin. When Quick Way & Rottler came along with valve seat/guide machines and floating tables is when cylinder head rebuilding got better. Mechanics have a tendency of worrying about the bottom end of a engine rebuild and not as much upstairs--upstairs is just as important as the basement. Valve train geometry and correct machine work really attribute to a smooth runner.