Looking in detail at my ignition (318) regarding rough idle

MoPar~Man

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Bit of a mistake describing the spark plug situation.

The plugs that were in the engine when it was put into storage 25 years ago were RN12YC. And N12YC. Um, ya. Three were non-resistor. Not sure of the story behind that. They are black. Electrodes are not in bad condition, the non-resistor plugs gap a little more than 35 but not 40, the resistor plugs gap over 40 but not 45.

It now has RN14YC. Every time I've had the engine running, the temp guage has barely got to the cold line on the gauge. I know it's mostly idling, it should be driven I know.

Then I decided to go hi-tech:

113.jpg


I'm using a scope to look at cylinder 1 plug wire, and I'm also looking at the signal across the primary leads of the coil.

Looking at the traces and going the math, I was coming up with the engine running at 250 rpm. But then I realized - the crank turns twice for every 8 sparks. So the engine was idling at 500 rpm here.

106.jpg


I had 10 turns of wire wrapped around spark-wire 1, but I was seeing a signals for each spark on this wire. Obviously a lot of cross talk, but here's what a weak (cross-talk) signal looks like, vs probably the real spark:

109.jpg

(weak signal, likely cross-talk)


108.jpg

(strong signal, probably the real spark)

In the future, I'll use fewer turns of wire to pick up this signal without the cross-talk.

Here's what the coil signal looks like:


112.jpg


Those are 3 consecutive coil charge/discharge cycles. I brought these pictures into a graphics editing program and took some measurements from them, put those into excel and did some math. Those 3 consecutive coil sparks, when converted into instantaneous RPM, work out to 509, 524 and 528 RPM. So there's a lot of timing variability here, where exactly it comes from I don't know.

The ratio of how long the points are open vs closed I can get from that trace, and that does change from 37% to 32%. If 100% = 45 degrees (360 / 8 = 45) then 32% is 14 degrees and 37% is 17 degrees. So my dwell is between 14 and 17 degrees? Am I doing this math right?
 
Is that dwell reading at distributor rpm or engine rpm?

Reading those traces is nice, but you still need a dwell tach to know what you're getting, to me.

Is there a cylinder number selector on that scope? Have you downloaded the instruction manual for it?

Just curious,
CBODY67
 
The scope I'm using is a general purpose lab or bench scope. Hard to believe it's a Tektronix model that is pushing 25 years old. Its one of the scopes we have at work.

From what I understand, dwell angle is referenced to distributor rotation. There is a fixed relationship between dwell angle and duty cycle. Duty cycle (of anything) is the percentage of time something is on or off, open or closed.

There are some hand-held digital meters that can measure duty cycle, and some that can measure dwell angle (but you must input the number of cylinders to get a dwell measurement).

But there are formulas and tables to convert duty cycle to dwell angle.

The spec for the '67 318 for dwell angle is 28 to 32 degrees. The spec for the points gap is 14 to 19 thou.

I see I made an error when stating that the duty cycle I was seeing was 32 to 37%. That was the percentage of time that the points were open. The inverse of that (ie 100 - 32 = 68%) is the real duty cycle (the percentage of time the points are closed).

So by that, going to the tables, the dwell I'm seeing is 28 to 30 degrees, which is what the book says it should be.

I might putz with a reflectance opto-isolator to get a signal from the flywheel to tell me when it's at TDC, and I can use that and the #1 spark to get my distributor advance setting.

But I am very surprised that I was working with an idle of 500 rpm. BTW, the manual says to back the idle screws 1 full turn for approx. adjustment. I always thought it was 2. The manual is showing 318 with Stromberg (which I've never had, it's always been Carter or is that Ball and Ball?). 318 idle spec *IS* 500 according to the manual, and for some 383 applications it's also 500, for others its 550. For the 440 it always seems to be 650.
 
Last edited:
The scope I'm using is a general purpose lab or bench scope. Hard to believe it's a Tektronix model that is pushing 25 years old. Its one of the scopes we have at work.

From what I understand, dwell angle is referenced to distributor rotation. There is a fixed relationship between dwell angle and duty cycle. Duty cycle (of anything) is the percentage of time something is on or off, open or closed.

There are some hand-held digital meters that can measure duty cycle, and some that can measure dwell angle (but you must input the number of cylinders to get a dwell measurement).

But there are formulas and tables to convert duty cycle to dwell angle.

The spec for the '67 318 for dwell angle is 28 to 32 degrees. The spec for the points gap is 14 to 19 thou.

I see I made an error when stating that the duty cycle I was seeing was 32 to 37%. That was the percentage of time that the points were open. The inverse of that (ie 100 - 32 = 68%) is the real duty cycle (the percentage of time the points are closed).

So by that, going to the tables, the dwell I'm seeing is 28 to 30 degrees, which is what the book says it should be.

I might putz with a reflectance opto-isolator to get a signal from the flywheel to tell me when it's at TDC, and I can use that and the #1 spark to get my distributor advance setting.

But I am very surprised that I was working with an idle of 500 rpm. BTW, the manual says to back the idle screws 1 full turn for approx. adjustment. I always thought it was 2. The manual is showing 318 with Stromberg (which I've never had, it's always been Carter or is that Ball and Ball?). 318 idle spec *IS* 500 according to the manual, and for some 383 applications it's also 500, for others its 550. For the 440 it always seems to be 650.
Here's a shadetree method I developed to find an idle speed that works well, that I developed back in the later 1970s, when some lead was still in the gas. It works best for a single exhaust. NO meters involved! Just a screw driver, a good sense of what hydrocarbons smell like, and some hand soap (unscented).

Start the car and get it up to operating temperature. Set the parking brake fully by first firmly applying the foot brake and then the parking brake. With the foot brake applied, shift the transmission into "D". Carefully release the foot brake to ensure the car does not move. This is important! Then quickly step to the rear of the car and carefully place the palm of your hand in the exhaust stream. IF the idle speed you now have results in you feeling each cylinder's pulse, the idle speed is too slow. Step to the front of the car and increase the idle speed by 1/4 turn. Recheck for smoothness of the exhaust flow. When the flow just smooths out, that will be the lowest idle speed you want, in gear, with the a/c working and the headlights on bright.

Once this has been accomplished, then place the other hand's palm into the exhaust flow, keep it there for a second, then quickly bring it near your nose. IF you smell hydrocarbons for sure, tweak the idle mixture adjustment to verify the speed neither increases or decreases from their current positions. If you might desire, lean the mixture screws 1/8 turn inward to lean the mixture slightly. Recheck the smoothness of the flow and the hydrocarbon smells. You will need to wash your hands after being in the exhaust flow for best results. If no real change, that is your final setting of idle speed and mixture. It will also put the speed into the "smoothness" range of idle speed, too.

In the mean time, search online for a schematic to build you own dwell tach instrument. You can also decrease the tolerances of the various resistors and such with the end result of having a smaller tolerance than any store-bought similar item. Usually, most dash-mounted tachs had an accuracy tolerance (unspoken of, usually) of about 2% of full range. Surely, you might be able to be better than that? Radio Shack and others used to sell dwell tach kits, with necessary schematics, back when your car was new.

As always, perform the above procedures, at your own risk.

Enjoy!
CBODY67
 
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