Intermittent Misfire

First off, I SUCK at diagnostics. I've known since I was a kid that "parts-replacement" is the worst way to solve a mechanical problem, and this situation reinforces that. Even though I know better, I've been replacing parts in an effort to solve an intermittent misfire at idle/low RPM of at least cylinder one on my 1967 Dart with a 273 (stock aside from a 500 cfm Edelbrock AVS2 on an SP-2P intake through cast iron manifolds and duals).

Here's the issue: At idle with a timing light, I can clearly see the timing light skip a flash every so many rotations of the engine. I've verified this with two different timing lights. Once the car is warmed up and the choke is off, there's also a very faint stumble when you roll into the gas off idle or light cruise.

When this began, I checked the distributor cap and found two opposing spark plug wire ports (and the corresponding plug wires) blue-green with corrosion. This was cylinders 1 and 6. The other ports looked shiny-new (the cap wasn't old). The two corresponding spark plugs also looked black and sooty, while the others looks good.

Since then, I've replaced (in order):

• Distributor cap, plug wires, and spark plugs – no change
• Timing chain (was stretched) – but no change
• Rebuilt distributor with new rotor (provided by Halifax Hops, converted to Pertronix, with a spectacular ignition curve) – car runs WAY better, but no change in the misfire
• Coil (MSD Blaster 2) and another set of plugs – no change

I figure if the misfire was compression-related, it would be more constant/consistent, so I haven't pulled the valve cover or performed a compression check yet. Even with the misfire, the car runs SO well. I just imagine how much better it will be if I can solve this. What obvious possibility am I missing?

Double check the dist. carefully around the trigger device, the special device that is on the shaft for Pertronix as well as cleanliness, rust, debri around the module/ pickup and for distributor shaft wear or wobble

See if you can round up an oscilloscope, either automotive or even electronics bench type.
You can monitor the coil NEG and look for dropped triggers. It's possible it's a bad coil, something in the trigger device, or the module, or even a power issue. That last is easy, jumper battery power direct to coil+

Check plug wires for continuity and carefully look for any grounding points along wires or boots. I would try a different plug in the offending cylinder(s) even if you have to move/ exchange plugs to see if the problem goes to a different cylinder

An "open" or a "short" (such as fouled) condition can cause similar problems at times

Another thing you might condsider, is that it does not hurt to have a spare with Pertronix. If it fails "on the road" where will you buy parts? Therefore consider buying a spare one and swap it in to see if it fixes problem

67Dart273 said:

Another thing you might condsider, is that it does not hurt to have a spare with Pertronix. If it fails "on the road" where will you buy parts? Therefore consider buying a spare one and swap it in to see if it fixes problem

Click to expand...

Those are all valid points, and I do intend to purchase and carry a back-up Pertronix module for emergencies anyway. I'd also considered running full power via a relay directly from the battery, as I've read that HEI ignition needs full 12v power, and I thought it could apply to the Pertronix as well.

However, the problem seems to be identical between the previous (and completely different) points distributor that's on my work bench right now. I've also replaced ALL of the plugs. Twice. No change.

I'm having trouble finding a common culprit considering all the ignition parts that I've swapped out.

As a test with the engine running at idle, squeeze the PVC-hose shut for a moment.
See if the miss disappears.
Idle will usually improve while doing this nonetheless.

Could be a lean mixture, or one or two cylinders getting a lean-mixture. Perhaps the PCV-system host is connected wrong, or brake-booster hose is connected to somewhere on the intake manifold, instead of to the appropriate port on the carb. Usually a rear port).

Have you ever adjusted the valves? Are they still adjustable?

and what about a vacuum leak at the intake. And the SP2P is a crappy intake. Good thing you have the smallest V8 made.

If you see the miss with the timing light it has to be ignition ie coil, distributor, plug wires or plugs and or the wiring for the ignition.

bcschief said:

If you see the miss with the timing light it has to be ignition ie coil, distributor, plug wires or plugs and or the wiring for the ignition.

Click to expand...

No, it does not always mean so. The light, especially inductive, depends on some current in the wire. Position on the wire and direction of the transformer can affect the light trigger. Other things that can affect it are surrounding conditions, such as a broken or internally faulty plug or wire, or fouled, or as mentioned, lean. Something like a stuck valve might do this also

This may not be such a problem on older lights with a direct electrical hookup

I have never particularly favored a timing light as something like a miss indicator.

Run a jumper from the ign system [+] terminal direct to bat [+] terminal. If the problem is fixed, you have low voltage or intermittent supply problem.

Msd give a lot of people the bad signal

413 said:

Have you ever adjusted the valves? Are they still adjustable? and what about a vacuum leak at the intake. And the SP2P is a crappy intake. Good thing you have the smallest V8 made.

Click to expand...

Yes, I had a slant six, so I have practice adjusting valves, and they're still there on my 273. Adjusted them a few months ago, and it ran better, and got rid of the faint popping from the exhaust at idle.

Agreed – the SP-2P is crappy for performance, but perfect for max fuel economy that I'm after, and it seems to work great with the little AVS2. This car is strictly for cruising and long highway trips with my son. I never even touch the secondaries.

Bewy said:

Run a jumper from the ign system [+] terminal direct to bat [+] terminal. If the problem is fixed, you have low voltage or intermittent supply problem.

Click to expand...

I'll try that next. Thx!

If the cylinder missfires, whether rich or lean, the ignition system can be perfect, yet the light can show a skip.
Lay the plug wire on any grounded surface, and start the engine. Now go observe and listen to, the snapping going on at the end of the wire. My guess is that it will have a nice steady rhythm to it.

Since you earlier mentioned that a valve adjustment temporarily made the engine run better (stopped the popping) My guess is that you have a problem with the valves in that cylinder.
My guess would be it is with an exhaust valve. And
My guess would be that it has insufficient closed valve spring pressure. When this happens,; on the intake stroke, the piston is able to pull a little bit of exhaust back into the cylinder. This reduces the pull on the plenum. So that cylinder at times, may not have the correct AFR at idle to support combustion. The flame goes out. The partially unburned mixture enters the exhaust. It may be fired up by another cylinder coming up on the exhaust stroke and ... POP, there it is.
So, pull the valve cover and look; there is no other good test to prove this. Put the piston of the cylinder in question, near TDC compression. Check for a broken spring, or a spring that is easy to spin (compared to several others). Or lean on it a little, to make sure it doesn't want to fall into the cylinder; just don't drive it into the piston-top.

vynn3 said:

Those are all valid points, and I do intend to purchase and carry a back-up Pertronix module for emergencies anyway. I'd also considered running full power via a relay directly from the battery, as I've read that HEI ignition needs full 12v power, and I thought it could apply to the Pertronix as well.

However, the problem seems to be identical between the previous (and completely different) points distributor that's on my work bench right now. I've also replaced ALL of the plugs. Twice. No change.

I'm having trouble finding a common culprit considering all the ignition parts that I've swapped out.

Click to expand...

Yes look at the Petronixinstructions I sent you it wants 12V. I would do a copression check first.