Technical Distributor Rotor / Cap causing backfire?

I have (had?) a problem over the weekend. I think I might have fixed it, but I can’t explain why.

We went up to Wisconsin for the Hot Rod 100 over the weekend. Drove up Friday afternoon, about 4 1/2 hours north from here to there (Waupaca) taking the non-highway back roads. No problems all the way up. Three gas stops.

Saturday morning, left the hotel and drove 10 minutes to Greg’s Speed Shop. No problems, though the engine was cold and doesn’t idle well until it warms up. That’s normal. Started up, left Greg’s, and headed out for the run, with no problems.

About halfway through the 110 mile trip to Hillsboro, I started getting backfires. It was completely repeatable. Idle is fine. Slow acceleration to cruise is fine. Cruising along at 60-65 MPH, high gear (.7 overdrive) with a 28” tall tire and 2.73:1 rear end, so about 1300-1500 RPM, no problem. On slight acceleration from there, like adding just a bit more gas to maintain speed up a hill, I’d get backfires and stumbling. I could back off back to cruise, or floor it. Once the secondaries open, no more backfire. We finished the trip, and limped home. As long as I stay away from a slow transition from cruise to accelerate, it’s fine. Two more gas stops on the way home.

I cannot reproduce this in the garage. It only does it when the engine is under load. It would do it in 2nd or 3rd, but not as easily as in 4th.

Today, I started working on figuring out what changed.

Engine is a ‘74 SBC. HEI distributor. Holley 1850-3 600 CFM carb. Crane “Fireball II” camshaft. Edlebrock Performer intake.

AFR meter and wideband O2 sensor installed. The bung was already in the exhaust.

At idle, 800-850 RPM, I have 13-15 in Hg of vacuum, vacuum advance disconnected, 28 degrees BTDC, and 15.2 to 17 AFR. That’s where the engine has been for a few years. Verified that nothing changed. And idle is ok.

I ran the engine up to 2500 RPM in a few steps, checking advance, AFR and vacuum. No problems found, but it’s not under load running in the garage. So it won’t do it anyway.

I had trouble reaching the tach connector on the distributor. It’s up against the firewall, with the brake master cylinder in the way, so getting a hand on it is difficult. I removed plug wires 1 and 3 to get access. Put them back on. And got a crank, no start. Not catching at all. Not even a cough.

I put a spark tester in-line with #1, and found no spark. That was unexpected. But, ok. Pulled the cap and rotor for inspection. The rotor tip shows some pitting, but not too bad. The electrode tips in the cap have some grunge, but not a lot, and don’t look worn. They are unknown age, I’ve owned the car 7 or 8 years, and I haven’t replaced them. Lacking any better ideas, I wire brushed the grunge off, blew and wiped out the cap, and put it back together. Starts fine, first crank.

While the cap was off, I checked for timing chain stretch by turning the crankshaft with a wrench. The rotor turns with the crank, with no hesitation, in both directions. So no chain stretch found.

I removed the vacuum gauge and timing light, reconnected the vacuum advance, and went for a test drive. No backfires. The AFR looks good. Cruise is about 15:1. Acceleration goes to 12:1 to 13:1. Idle still leans out at 17:1 (max on the meter).

The solution seems to be cap & rotor. But I can’t explain why. If it’s a misfire caused by junk in the cap, it should get worse at WOT, not better. If it’s leaning out, then having not done anything to the carb, it should still be lean. If it was rich, it should still be rich. If it was bad gas, it should still be bad. I’m back to where I was Friday.

Guesses, explanations, and comments welcome. I hate problems that go away without knowing why. That usually means that they’re going to come back.

 

Change fuel filter, Especially if you stopped at one of those quaint out of the beaten path gas stations or worse yet a mini mart with a gravel parking lot.

 

The only other thing you said you did was unplug/refit plug wires 1 and 3. I assume all of them were then unplugged/refitteded to clean the cap? Maybe a loose plug wire???

 

Run it in the dark to see if spark plug wires are flashing spark around. Moisture in the distributor cap will do funny things too, but with all the miles you put on it should be dry

 

Yes, all 8 removed to remove the cap, along with the coil and battery connector plugs.

 

It seemed dry when I removed it.

 

That may have cured it. Check all the ends to make sure they grab well.
Might be a good idea to carry a new lead set around with you (all nine!).
I hear @Moriarity may have a couple of spare sets.............

 

This is an easy one. I have seen the symptoms you described occur when the power(ignition) wire to the cap has a dirty or loose connection. Also seen it happen with the 3 wire connector to the cap also. Don’t ask me why it only happened under light load.

 

I fought a mis under load and found a crack inside the cap on a points distributor. There was a carbon trail built up on the crack and was trying to fire 2 cylinders at once. Drove me nuts.

 

Cap and rotor are likely, but note that you could not reproduce it in the garage. I wonder if one of the batches of fuel you got had high oxygenates (say by the fuel company accidentally making a shandy in one of the underground tanks). Just enough to make it ping under load, but not enough to ping at cruise and masked once the secondaries richened it up. You drove it home and started eating up whichever load of fuel was bad, getting better fuel quality with each successive topup.

Were you still noticing it between the last fill-up and parking it in the garage?

Cheers,
Harv

 

A backfire implies spark at the wrong time, or perhaps, at the right time but going to the wrong cylinder. If a spark jumps from the rotor to the wrong post in the cap that can happen, and the vacuum advance will change the rotor position relative to the cap posts when you operate the throttle.

Could be that you had one wire semi-loose from the cap, the spark had to jump to the wire (or past a gap inside a bad wire), and that extra gap caused an alternate path to the neighbouring cap post to be a shorter, simpler path to ground - and electricity is lazy. Most of the time the spark went to the right cylinder, but at the right throttle opening, vacuum advance etc. the alternative path was the easier option.

I'm guessing an ignition oscilloscope would have shown something interesting, but not everyone has one.

 

OT car (still a Chevy Small Block) with similar issues under a a load, although I don’t recall backfires. Anyways, cap and rotor did cure it, but a week or so later it came back.
Spoke with a GM mechanic I ran into , told him about it. Told me sounds like the pickup coil.
He was right.

 

Moriarity, I have used and driven cars and trucks for many years with HEI. However, I tend to be a thinking mechanic, but, the movement of the pickup coil with the vacuum advance and the pickup coil wire breakage never crossed my mind until you just mentioned it. Combine that with the crappy parts that are so common today, I believe our upcoming 3100 project will have points. Also, this will give me an excellent opportunity to teach Little Truckdoctor about the basics of ignition theory.
Back to the original post, I had an O/T Chevy van at work about 6 months ago that we did a tuneup on and it would only fire on 2 cylinders when we finished it. The new cap was bad out of the box. That being said, I would also use a DVOM on the pickup coil and perform a wiggle test on the pickup coil wires looking for an intermittent open. I would also replace the cap and rotor for good measure.

 

Hairline cracks in the cap causing the backfire, dollar to a doughnut. The no start, you will notice, came AFTER you worked on it. Has nothing to do with the original problem.
Above based on experience.

Yeah, GM HEI distributors are ugly. The reason my Buick sports the original, modified with GM HEI internals, and the module mounted elsewhere.

Ben

 

All 8 wires seemed well connected to the cap. A firm tug removed them. A decent “click” of the terminal going on the connection when reinstalled.

 

Hmm. So, in trying to get to the tach connector, perhaps I disturbed the battery connector next to it. Wasn’t loose, firmly clipped in place with the retaining clip engaged. Could be dirty, or some corrosion, though. Remove and reinstall it in taking off the cap could then have scraped a new path on the connector.

It’s the only under load part that bugs me.

I hate intermittent problems.

 

I suggest replacing the four wire HEI module. I recommend the two choices available at NAPA. They were under $40 last I checked.

 

Fuel seems like it was ok. Major brands (Shell, BP, Mobil), nothing from off brand stations. Having run 40-50 gallons through it, seems like it’s not fuel or filter. It did it all the way home (Saturday). Parked Sunday, didn’t touch it. Worked on it Monday, and now it’s not doing it. There was a bit over half a tank left Saturday, now it’s a bit less than half a tank. 10 gallon tank.

 

Yeah, a ‘scope might have been helpful, but I don’t have one of those. Don’t know anyone who does, either. Just have the basic home garage tools here.

I’m thinking some tracking in the cap, I guess.

 

Pickup coil wires is interesting, I’ll check that.

Would an intermittent open wire at the pickup cause a backfire at the carb? I’d expect it to cause a no-fire or maybe a late fire with the still burning gasses getting in to the exhaust. I’m hunting a backfire through the carb, which seems like the pickup coil wouldn’t cause. Unless maybe it’s firing late enough to be early on the next cylinder?

Possible, maybe.

 

An after fire through the exhaust is usually no or late ignition
back fire through the carb tends to be premature or wrong cylinder ignition

 

There was a post here a few years about HEI troubleshooting.

https://www.jalopyjournal.com/forum/threads/gm-hei.1163229/

It also made mention of the ground wiring and ground strap used on early HEI coils. The ground strap in particular seems to get lost or just not reinstalled occasionally. In some instances at least it seems the vehicle would still run, until it didn't.

This post in particular had some good pictures of the wiring and ground strap.

https://www.jalopyjournal.com/forum/threads/gm-hei.1163229/#post-13230232

Replacement straps seem to be readily available.

 

Also, on the more mechanical side of things, poor valve sealing or sticky valves. Or insufficient valve lash. Particularly with exhaust valves and solid lifter cams. This tends to show up more under heavier engine load and at higher engine temperatures.

 

I would take a good look at the rotor. I would also take an ohmmeter to each plug wire. If you have a partially open plug wire, the HEI will jump the spark to the place of least resistance. Many times that is through the rotor to the distributor shaft. I have seen a lot of HEI rotors with a burn spot on the under side of the rotor.

Actually the best way to check the system is with an ignition scope. You can see bad plug wires in an instant.

 

Exactly.

 

Thanks. I have the ground wire style, no strap. Grounds are funny things, electricity will find one. The trick is making sure that it finds the one you want it to use.

 

I also thought that the ground strap wasn't required for all HEI distributors. But one list of applications I looked at showed what seems like nearly universal GM usage for 1974 thru 1986 models. This was a Standard Ignition Parts listing at O'Reilly Auto Parts, not GM or ACDelco Parts.

 

In all this convo about it I think this is something that a long ride (lotta heat soak) conjures up. That said electronics tend to head south when they heat up. Signs of eventually left with nothing on the side of the road with no spark at all. That says pickup coil or module to me, and sadly they probably check out within range when cool/normal. Just a theory, and ugly as they may be they're often good for lots and lots of miles.