Are there specs for distributor timing accuracy - and ways to measure it?

[MoPar~Man]

Regarding a distributor with points, are there specs for what is within tolerance as far as opening / closing timing and variability under constant RPM conditions?

I've been making some measurements with a scope when my engine is idling (rpm = 530) and looking at a string of ignition pulses (looking at the voltage signal across the balast resistor) and I see a lot of variability in a single sequence of 8 ignition pulses I've analyzed. On average, the dwell time for this sequence is 29 degrees (duty cycle of 65%) but individually this ranges from 25 to 33 degrees. The resulting RPM, if calculated on a per-ignition basis, ranges from 498 to 553 RPM. The engine does not idle smoothly at this RPM given this variability.

So I'm thinking either there is variability in the valve train / timing chain, or fuel mixture, or distributor shaft / points (spring bounce?) that is the underlying cause for the rough idle. I would assume that the mechanical vagaries of the distributor with points would be much improved by replacing it with electronic version.

This is a 318 with 2-bbl carb, if that matters. I have had a good look at the timing chain and in my opinion there is very little slack in it. The condition of the valves / cam shaft is unknown, but the last compression test I did 2 years ago showed adequate numbers (150-155) and low variability. The points / condenser are new, their pedigree is unknown. Spark plugs are white (after the 850 mile round trip to Carlisle). System voltage while cruising is 14.4 but it can / will drop to slightly under 12 while idling at a stop light. Electronic voltage regulator. Ballast resistor is 0.5 ohms, coil is stock (ie vintage, not new). Idle vacuum is 17 - 18. No idea if idle mixture is correct or too rich or lean.

Maybe the system voltage drop while idling, perhaps the alternator is putting an irregular load on the engine, is causing the uneven idle?

But I'd like to know if the distributor / points could be the cause of uneven / rough idle, or is the carb more likely to be the reason, or system voltage drop (14.4 to 12 or high 11) is - problematic for spark intensity.

 

[CBODY67]

IF you want to really get geekier, pull the distributor and check the points gap (how far the points open) with a dial indicator. When I did that on a used 440+6 used dual point distributor, I was amazed at the variance in the how much difference there was lobe-to-lobe.

The reason I did this was that I was not getting the dwell total out of the situation it should have had, individually (single point method) or collectively (both points working).

Now, when the cars were newer, we just chose ONE lobe and set the point gap on that one lobe. Nobody worried about variations between the lobes! They were presumed to be "all the same", but nobody bothered to check them. Each new point set had a little vial of lube to put on the rubbing block to prevent wear on the lobes, although the rubbing block was supposed to be the "sacrificial" surface. Somewhere in the 1980s, those little vials of lube were not coming in the new point sets, by observation, which tended to coincide with people buying an older car with points ignition and complaining that the points did not last past about 8kmiles, when we could run them to about 15K miles back when. The issue was not dwell change, but the transfer of metal between the point contact surfaces, usually.

Dwell and ignition timing are directly proportional. I forget which way it is, but I do know there is a direct relationship. Which is why the point gap is set first, then the base timing.

To me, I woild be more concerned about the height of the spark plug firing trace. That can vary with compression in the cylinder. A higher spike (more voltage) means the compression in that cyl is higher than the others. Kind of like a "light" compression check? For each firing event, the coil will only produce enough spark energy to fire the spark plug that one time. Even if the coil CAN produce a 50KV spark.

As for "timing accuracy", that can be easy to see in the scope trace line. If using a Sunn distributor machine, there were marks for EACH firing sequence on the machine. Considering the other variables in the plugs firing, I'm not sure than a 1* plus or minus would really matter in a normal, non-race max output, engine.

The duration of the spark is important in ensuring the air/fuel mixture gets fired-off most completely, BUT the gap style of the spark plug can be more dependent upon this area of combustion dynamics. There are several YouTube videos on lawn mower engines indicating that the "fine wire Iridium" plugs are best for this. Both in ZAP and run time on a controlled amount of gas.

The only way to get rid of any variations in valve timing is to use a Jessel timing belt system. They always caution to check valve-to-piston clearance, but this is more important on an engine with a long duration, high lift cam. Using the Jessel belt system means that any variation in valve timing will be due to manufacturing tolerances when the camshaft was made, leaving things like pushrod deflection (as little as it might be), working against valve springs, to be where the actual timing variation might be. Using overhead cams gets rid of this factor.

THEN, more into the fuel supply system. Namely the intake manifold and its cyl-to-cyl air/fuel mixture variations. The reason the single-plane intakes (i.e., Edelbrock Tarantula, circa 1969) existed was to get a more-equal charge to each intake valve. With that principle later having variations not tuned to 6000rpm engine speeds. The old Chrysler Direct Connection Race Manual addressed how to lessen the variations of particular, popular intake manifolds with epoxying popsicle sticks, on their side, at the mouth of some ports in the plenum area.

Can't forget the intake manifold flow variations due to casting differences in the cyl head intake ports. One advantage of the plastic intakes of modern vehicles.

Comparing the 1960s engines' design to 2000s engine designs, it is obvious,in the newer engines, having more-zero differences in cyl-to-cyl filling, cyl-to-cyl air/fuel charge amount differences being "zero", and all of the ports (intake and exhaust) being much more consistent and separated from each other. With timing belts or double-roller timing chain sets and crank-trigger electronic ignition systems.

Even if you invest in a timing belt system and a crank-trigger electronic ignition system, you'll still have the built-in variabilities of the fuel supply and induction systems for "variances".

The OTHER thing is that even if you get everything completely optimized and equalized, the "betterment" might only be 10 horsepower at the top of the power curve, in real life. Emissions can be reduced (a main driving force of the modern engine design orientations) as highway mileage can be increased a little bit (a reflection of better combustion). But is it worth it for the thousands of $$$$ spent?

Suggestions? If you want to keep the points, get a new points distributor from Rock Auto and swap the breaker lobe item with the one in your current distributor. Then set the points to specs to achieve a 30* dwell reading.

A set of NGK V-Power plugs gapped to .035-.040" gaps. These plugs have a gap design which maximizes the exposure of the spark kernel to the air/fuel mix. Only the fine wire Iridium plugs are better (but cost more and last longer). Making sure the rest of the ignition system is operating optimally, too.

Then tune everything to (insert your own favorite unit of measure, as "rch", "ka", or similar) and enjoy what you have done.

Just some thoughts,
CBODY67

 

[halifaxhops]

Also it sounds like a bent upper shaft can be the issue.

 

[garyh]

Somewhere in the 1980s, those little vials of lube were not coming in the new point sets

I wonder this myself. Was the lube no longer Necessary ? or was it a cost saving measure?
I have a tube of Standard brand (lubricam) lobe lube since I run points in four of my Mopars.

 

[halifaxhops]

You still need it they just cheaped out.

 

[CBODY67]

When I realized that new points were NOT coming with the little vial of lube, I started to look on the closeout tables for NOS point sets. It was UNCOMMON to find any with vials in their unopened boxes. This was the earlier 1980s!

I then started to look further, in other auto supplies. I'd go in and ask for "point grease" and the high-school age guys looked confused and brought out (in one case) a cartridge of chassis lube. I smiled and kindly told him that might work, but was not what I was seeking.

At another auto supply chain store, I had seen some hanging on a shelf end cap. The sales guy on duty was an older guy (about my age at the time) and he remembered seeing it hanging there, but it was not to be found. He at least understood what I was looking for!

Then, I walked into the local SuperShops location and there was a BIG tube of Mallory Ignition Point Lube laying on a shelf on their J-hook wall. I bought it! Later giving it away as I did not need that much of it.

This was not an "all-consuming" search, but it was consistent.

Yes, rubbing blocks still wear so lube is needed to decrease this and lengthen the working life of the point set. As a mental exercise, I wonder if the Ford-spec wheel bearing/chassis lube grease (which has moly in it!) in the synthetic form (that Valvoline sells in a caulking gun tube) might work pretty well? "A little dab will do ya"?

Enjoy!
CBODY67