Technical Distributor Rotor / Cap causing backfire?

David, would you like me to bring you my spare module?
That's where to start.

 

Your cruise rpm being as low as it is because of your overdrive makes me think its the spark plug wires. That is a good load on an engine...keeping your car moving on the highway at 1500 rpm...then add a hill like you describe? I'd bet on a plug wire or two going bad. That's exactly when they would show symptoms of breaking down internally...not idling..or revving...or accelerating through gears....under that kind of load.

 

THIS!

Ben

 

Another good reason to add louvers to the hood!

 

I was about to say that many HEIs had a problem with running the recommended .060 plug gap and the early rotors at the same time.
HEIs can handle the original .060 gap recommendation as long as everything else is in great shape.
Then came along the problem of the spark wanting to take alternative routes as the plug wires got some miles on them or dirt...
Also they started discovering a path thru the rotor center to the advance weights, then shaft... Rotor burn-thru eventually became visible as the spark decided that an easier path was to jump to the dist shaft. Roughened-up shaft or even welded weights eventually happened if it went undetected for a long time.
Aftermarket thicker rotors cured that part. Reducing the recommended plug gap to .045 was another cure to take the "running on the upper limit" and to make things less stressed, and therefore more durable when neglected.
I have also seen, as others mention, good looking wires from the trigger coil that have an unnoticed bare spot from rubbing the housing or vac advance . The rubbed-bare spot isn't easy to notice.
The first time reading your description sounded like a lean spot at mid thtottle just about the time you want a small accelerator squirt or a slight power valve opening for a moment, but reading it a second time sounded possibly more ignition related if the ign is not in perfect shape.
Checking the accel pump, power valve operation, and float level can't hurt, plus you can cross off a couple more items on the checklist.
Ive had an hei module or two going into failure mode in similar fashion when hot...

 

R45TS gap at .045

 

Thanks. Not sure that would help. If I can’t get it to do it with the parts I have, changing parts won’t show anything. I’m not opposed to loading up the parts cannon, but I don’t want to use up your parts. NAPA will sell me a coil, pickup, cap, rotor, wires, and anything else I want to throw at it.

 

Yeah, it’s basically the highest load on the engine is when it does it. Can’t reproduce it in the garage, that would be too easy. Don’t have a chassis dyno here either.

Wires are a few years old. I replaced them after I bought the car, so less than ten years old, but I don’t remember exactly when.

Thinking, though, that a plug wire would cause a misfire / no fire / maybe a late fire, so I’d get a pop through the exhaust. Not an early fire that pops through the carb.

 

It’s a 37 Chevy, so the hood has louvers (side) from the factory. The inner fenders are missing, so there’s a lot of airflow past the engine. Not quite as open as a hoodless rod, but not a closed in engine compartment either.

 

to run you need spark, compression, and fuel....lot of good answers so far,... but,... what about vapor lock???? it would run like you discribe....fix'n motor problems are hard enough in the garage,... and near impossible on line..... so just adding to the maybes......

 

I think I’m running the .045 gap. I’ll check that though.

Stuck in a doctor waiting room here this morning, waiting for wife. Garage time is on hold for now.

Need to check the cap, rotor, pickup, and wiring this afternoon. Then some more road testing, see what happens. The vacuum advance would just be coming in to play under load and accelerating slightly, which could (theory) then affect the wiring to the pickup coil. While working on it the other day, I could have moved the wires enough to re-establish a connection if the wire is frayed or broken internally.

 

Can’t currently rule it out, but it hasn’t vapor locked in the previous bunch of years, including much more heat stressful events like sitting in traffic idling for hours thanks to whoever planned the single venue entrance for 5000 cars on Power Tour a couple years ago.

Going down the road at 60 on a 70F day, it doesn’t build up heat under the hood.

 

here's a test for ya...






back it out of the garage.... start it up.... run the idle up to 2000 rpms.... try to get it hot.... check the temp of the carb and all the lines.... you could be boiling the fuel in the bowl...or like I said vapor locking,... but,,, running at 2000 rpms and letting the engine get hot is as close as you can get to running up hill on the road... my 37 refuses to get hot, and I was having what sounds like the same problem,,,, and always on the freeway...so I put a towel blocking half the radiator.... sure enough,.... she started sputtering... engine heat boiled the fuel....

 

I have had my HEI module checked at AutoZone. Their problem is whether they have lost the wire connector for their test equipment to this now ancient 4 pin module.

 

I believe a lot of things are happening here. First you’re cruising with a 1.91 rear end ratio at an rpm meant for a diesel. From the 50’s I learned that lugging any engine caused the spark plug gap to widen so if you’re going to keep what you have I would follow everyone’s advice to reduce the plug gap. Lugging an engine the ignition works harder and will find the weakest link.
On the trip did you ever try leaving the trans in hi gear 1 to 1 for a few miles to see if it backfired then? I also have a 2.73 rear and 27” tires and the engine runs in the sweet spot of 2500 at 70. I put in the 2.73’s so I would not need an OD transmission.
HEI’s need to breathe. Does the cap have a way to let out the ozone from the sparking. You cleaned the caps points I’ll bet cleaning the tracking you probably can’t see will also make a difference.
I have 3 engines running electronic ignitions and all of them have a plug gap of .035”. 2 of them are HEI styles. Hope you find it.

 

Ohm out the plug wires.

 

Ohm out = check the resistance per foot and see if it’s correct for a HEI.

 

So it’s “medical stuff for wife” week here, but between appointments and tests and stuff, I got some garage time in yesterday. I don’t have a definite smoking gun to report. Because pictures help illustrate…

Plugs are R45TS at .045 gap. This is #1, and it looks good to me other than having picked up some gunk because I dropped it getting it out.



Cap inside, terminals show some wear, but seem ok.



Rotor doesn’t show arcing to ground through the distributor body. Back side of the centre contact is clean.



No signs of arcing on the advance weights.



I removed the module and the three wire harness piece to get access to the pickup coil connector. Meter on connector, got 997 ohms. Specs I found online say that 500 to 1500 ohms is correct, so being right in the middle seems good. Pulling on the wires (gently), they do not feel stretchy or broken. Moving the connector around to the limits of where it will go, the meter never moved off of 997 ohms. I think it’s safe to say that I don’t have a broken wire at the pickup coil.



Removing the module, this B/C connection to the harness seemed not tight. I used a pick to increase the tension on the female connectors. These are low voltage, but I don’t like “not tight” in connectors. Cause? Probably not, but it’s a thing I didn’t like.



Comparing the old rotor to a new one, I found something else I don’t like. The tip of the old rotor is about 1/32” shorter than the tip on the new one.

Not easy to get this in pictures, but the old one, the tip is basically in line with the plastic body of the rotor. Here, with a ruler:



On a new rotor from NAPA, the tip is proud of the rotor body by about 1/32”:



I didn’t get an exact measurement, and it probably doesn’t matter if it’s exactly 1/32” anyway. Assuming that the rotor to cap terminal spec is X fractions of an inch, X + 1/32” is probably not helpful. Unknown age on the rotor and cap, so “worn parts” here.

The cap is a crossfire style, so I’m going to order a replacement cap and rotor. Might parts canon the coil, module, and pickup coil while I’m at it.

One other thing I found, which may also point to “worn parts” is this:

https://www.championautoparts.com/Technical/Tech-Tips/Defective-Spark-Plug-Wires.html

A good spark will be blue-white and will be plainly visible in daylight. If a good spark is present, the problem is probably not in the ignition system. Check the fuel system and/or stark timing.

Weak sparks are orange or red and may be hard to see in daylight.​

I put my spark tester in-line with #1, and it’s not blue-white, it’s more of an orange.

I’m now thinking “weak spark” at the plug, from overly large gap at the rotor to cap terminals. I have temporarily crutched this by wire brushing clean the rotor and cap terminals, and it’ll probably come back as they arc. With Power Tour coming up soon, and limited garage time, I’m loading up the parts canon here and getting ready to fire it.

And now it’s time to go to the next doctor appointment. And then the appointment after that.

 

Pull the plug wires one at a time , pull the boots back ( both ends) carefully inspect the ends especially where the core comes out & wraps around under the crimp on end . I've seen this a few times ,symptoms exactly as you describe .
Module wires also a likely candidate !

 

Make sure wires #5 and #7 are not laying next to each other.

 

Looks like a combination of things acting together to me. Maybe its time for a good old fashioned tune up. Plugs, cap, rotor, and wires. Change the fuel filter too, just because.

You can pull really hard on those wires to the module to test them. If the copper wire is broken, cracked, or corroded inside, a hard pull will completely break them. If the wire is good, you won't be able to pull it hard enough to pull it apart. Those wires are 18-20 gauge, you can not pull that size wire apart if its good. If it passes the hard pull test, you are good to go. I'd rather break it at home then to have it break along the side of the road. Most of the GM module failure is because of those wires broke, if the module failed, its dead. True GM module failure is usually permeant, not hit or miss.

At all those gas stations, you could have arrived at one where the tanker truck just left and the junk from the bottom of the station's tank was still stirred up, and you got a load of that crap. If that happened, the filter caught that junk. It probably wasn't enough to cause your problem, but it may be in the future. Add a batch of gas that maybe wasn't great, could compound the ignition problem. Now you have run enough gas through the motor its OK, but the junk is still trapped in the filter, and it keeps accumulating that junk every time the crud comes into the filter. After it accumulates enough, gas won't pass through.

 

Do you think it might be possible that over time you may have built up some carbon on your pistons due to the low rpm operation? The spark plug looks rather carbony (?) on the base even thogh the electrode loks fine. I'm wondering if maybe blowing the carbon out of it might correct the problem if nothing is really wrong with the ignition set up.

 

Updates…

I loaded up the parts cannon and fired it. New cap, rotor, coil, pickup coil, module, vacuum advance, and wiring harness. The mechanical advance weights and springs are new-ish from last year, so I left those alone. The plug wires are a few years old, I left those too.

A long test drive today, it took about an hour, then I was able to get it to do it again. It was about 90F out, sunny. So, it’s not the distributor itself. And I’m on another new tank of gas, so pretty sure gas quality can be ruled out.

I have my a/f meter and wideband o2 sensor installed.

There is a particular throttle spot where I get a surge. From cruise, it’s just enough throttle movement that I can feel the change in my ankle. The a/f shows the engine going rich (11:1) here. This is also where I backfires. I think it’s going rich, and when really good and hot, it’s popping the unburnt vapor in the exhaust.

So now I’m looking at the carburetor. I’ve read that an off cruise surge can be too much or too little fuel from the accelerator pump. Since the a/f says it’s going rich, I tried moving down one discharge nozzle size, from 31 to 29. I only had time for a short test drive, but this seemed to help. Maybe. I also need to have a look at the accelerator pump cam.

I’ve also read that this surge can indicate an idle circuit / transition adjustment is needed. I think that’s more for a lean condition, though.

A friend suggested that the power valve may be contributing. I don’t think I’m anywhere near the vacuum level to engage the power valve, but I don’t know. I made a magnetic mount for my vacuum gauge and installed it under the dash. It’s not illuminated, so I’ll test this tomorrow. I can’t yet rule out a damaged or failed power valve, haven’t yet pulled it out to examine it. But most of the time, the a/f readings seem reasonable, so I don’t think it’s likely to be the power valve.

 

I don’t think it’s carbon.

 

It’s a crossfire cap. 5 and 7 are next to each other. Nothing I can do about that.

 

Yea, but keep the wires separated just so they are laying next to or on top of each other when they are routed.

 

You realize it only takes 10-20 h.p. to maintain highway speed ?

 

Yes, exactly.

Also, if you’re not from the midwest, “hill” is any change in elevation. There is a hill ahead here:



It’s not like a “hill” in Pittsburgh.

 

Thats a mountain in western Ks.