I have a 72 cutlass convertible with a 350 automatic that has been giving me a hard time!
The car will run but really misses bad under acceleration, compression is good on all cylinders.
I have replaced points, plugs, condenser, cap, rotor, wires, coil, removed distributer and checked it on an old sun machine it works correctly, and the carb has been rebuilt also.
There seems to be an issue with sending spark to the plugs.
I can take a timing light and put it on the wires and one or more do not fire light they should, the problem is the wires aren't always the same wire!
I removed the Petronix thinking that was the problem, changed coils, points, condenser, cap , rotor thinking maybe one of the new ones was bad.
Something is causing it to misfire and it is definitely electrical.
Any help would be greatly appreciated, tired of thinking about it!
Thanks

 

Have you checked the coil?

 

He says he replaced the coil but we all know that doesn't necessarily mean anything; electrical parts are often questionable out of the box.

Check your engine grounding straps and then check your plug wire routing to make sure nothing's cross-firing. The routing pictured in the chassis service manual is almost always the best way to install them.

Wouldn't hurt to check resistance on the plug wires either.

 

I've put three coils on it so far, two condensers, two sets of points, two rotors, two caps, distributer was checked with points and condenser installed on the Sun machine and checked wiring routing / firing order.
I'm stumped, thought for sure it was the Petronix at first.
Something is causing it to fire erratically, have not checked grounding straps but I doubt that could cause this but I've been wrong so far!

 

Was the misfire there before the new parts? Did it run well for you at some point and then started acting up?

Could be low primary voltage to the coil. Try a jumper to the coil through an external ballast resistor.

Good luck!!!

 

Just a thought, check the distributor shaft sloppiness. If the bushings are worn excessively, this could change the spark not going to different plug wires each time. How does the distributor gear feel ? A worn gear could be part of the problem.

 

I have told this story a few times before, some of you may get tired of hearing it. I once had an old BowTie pickup that had a miss when I traded for it. I figured a tune-up would fix it. Did the tune-up including points, condenser, cap, rotor, plugs, and wires. Tuned carb and set timing. Still had a miss. Replaced the dist. with an HEI and new wires again. Still had a miss. Found a freshly rebuilt set of heads and replaced those along with new intake and carb. Still had a miss. Replaced cam and lifters, still missing. Ended up sitting on the livingroom floor hugging my knees into my chest and mumbling arcane curses, wondering what to do and my loving wife asks if I replaced the plugs? "Of course!", I replied and then she says "Replace them again, they're cheap aren't they?" I replaced them again with AC Rapid Fire plugs and that engine ran GREAT!

Try a new set of good plugs?

 

That is exactly what I was planning to do next.
Thanks

 

I am still fighting this problem.
I have changed carburetors twice, cap and rotor again, plugs, plug wires etc..
Tired of screwing with it, getting ready to buy an HEI distributer and wires.
Almost positive it's an electrical issue.
I can check my wires with a timing light and sometimes they are firing normal and sometimes very erratic! Not always the same wires either!
Any suggestions before I cover it back up and try to pretend it's not mine anymore!!!
Thanks

 

Yes, the distributer feels fine but....IDK

 

Have you closely examined your engine harness? even if looking good on the outside, the wiring age itself maybe causing poor continuity at this point.

Are you running an external voltage regulator? if so, maybe swap it for new to see if any difference?

Also, have you checked your ignition switch wiring? Any intermittent (non-hot related) start issues?

 

Check the wire that grounds the advance plate in the distributor and the wire that runs from the points to the coil- terminal.

 

Does the misfire/blinking of the timing light change if you leave it on the same plug wire but turn the distributor to change the ignition timing?

 

Did that already, thanks

 

Actually, did put a new ground wire in the distributer a while ago. Haven't checked to see if turning distributer will affect firing. Yes it does an external voltage regulator, might be able to swap the one from my 72 Jimmy in there, have to look and see if they are the same, it also has a condenser on the outside of it, may swap that also?
Thanks for the suggestions, hate to buy an HEI setup because I'm still not sure that will solve anything, but I'm about ready to!
I will try these next week and let you know if anything worked.

 

What is the dwell when this occurs?

 

Also, have you tried using a different timing light? It could be a lean misfire and your timing light is wonky.

 

Can you find a shop that has a ignition oscilloscope? A 10 minute hook up will tell you if the problem is with the ignition system and pin point the problem if it is in fact the ignition.
You have changed every component in the ignition system at least twice and changed the carburetor twice. Unless you are repeating the same installation mistakes it is time to look else where.
Have you tested fuel pump pressure?
Have sampled the quality of the gasoline? Is it cloudy or does it contain water?
Have you connected a vacuum gauge? I’d be looking for weak or broken valve spring(s)

 

Have not checked fuel pressure. Have checked vacuum and fuel is definitely good.
Fuel pressure would have nothing to do with spark though?

 

A worn out timing chain could be causing it.

 

If there is a tach installed, try disconnecting it. What is your dwell?

 

I haven't messed with it lately, but I am inclined to think it may be the timing chain also.
I'll get to it at some point.
Thanks

 

Timing chain wear is easy to verify. Remove the distributor cap, rotate the engine BACKWARDS until the timing mark!is at zero. Slowly rotate the engine in the normal direction while watching the rotor. As soon as you see it move, stop and look at the timing mark. I don’t know if there is a textbook spec on timing chain slack but if it’s more than 10* I’d say that’s excessive.

Unfortunately, if there is excessive slack that most likely means the nylon has started to flake off the cam gear. If so, then the oil pan needs to come off. All that nylon is stuck in the oil pump pickup.