Poly 318 Ignition Timing, Points Adjustment, Distributor Conversion
Ignition timing, distributor points cleaning and adjustment, and electronic ignition conversion can seem confusions and overwhelming, so I wrote this article to discuss the methods I use and recommendations.
Timing is highly dependent on the specific engine build, the air-fuel mixture, distributor specs, and other variables and should be considered. Manual recommendations are baselines for economy mass commuting, not the “correct” timing for all engines. We really need to break timing into the camp of “less ideal” and “ideal.” I highly recommend using the “ideal” method on all engines since it will give the most ideal initial timing for the specific engine, but I provide both methods regardless.
Typical Timing Procedure (good for stock engine street cars):
This is the type of tuning the majority of people come across in repair manuals, which doesn’t make it incorrect, but it doesn’t make it ideal. For the majority of stock and hopped-up street engines, it will do the trick. The following specifications are a good starting point for tuning, and I go into further details to explain what these numbers and terms mean:
- Hot Idle Speed: 500 – 850 rpm as the cam requires in gear for an automatic and neutral for a standard.
- Note: set hot idle with A/C compressor running.
- Initial Timing set at hot idle w/out vacuum attached (see figure A below): 10° BTDC
- Total Advance at 4,000 rpm w/out vacuum attached: 34° – 36° BTDC
- Total Advance at 4,000 rpm with vacuum attached: 42° – 52° BTDC
If someone wanted a basic timing tune-up for a stock street A-block with vacuum advance, this is how I would proceed using a timing gun equipped with integrated tachometer and dwell meter (if running points).
- Note: Performance mechanical-advance distributors—such as a Mallory YL dual-point mechanical—are a different beast and designed for WOT racing, so follow the distributor instructions for adjusting the total mechanical advance and the advance curve.
- With the engine off, remove the air cleaner.
- Loosen the distributor clamp just enough to where I can rotate the distributor with some resistance so it will not vibrate around on its own.
- Unplug the vacuum line from the distributor and plug the hose. Tip: A golf ball tee makes an excellent hose plug (I still use the one my father used in the 1960s).
- Setup the timing gun with 12V power, a confirmed ground, the spark lead to cylinder 1 (see Figure 1 below), and the dwell lead hooked to the coil negative (if running points). Ensure the wires/pickup are clear of exhaust, fans/blades, and belts.
- Start the engine and bring it up to running temperature ensuring the choke is fully open. Resting a hand on the top radiator will tell when the thermostat opens when hot coolant rushes in.
- Set proper idle speed.
- Set the carburetor’s idle air-fuel mixture (See my carburetor tuning article for this adjustment).
- For points, check the dwell and adjust as necessary (see the section on points adjustment below).
- With air-fuel mixture, hot idle speed, and dwell adjusted, set the timing gun to 10° BTDC and strobe the timing mark (see figure 2 below). Slowly retard or advance the timing by rotating the distributor until the timing marks align on 0, meaning timing is set at 10° BTDC. The distributor should rotate with little force necessary but with some drag. Tighten or loosen the clamp bolt as necessary.
- Raise the gun timing to 34° BTDC and switch the gun to tach. Steadily open the throttle by hand until the engine reaches 3,500 rpm. Hold the rpms at 3,500 and switch the gun to the strobing mode. Adjust the timing up or down on the gun until the timing marks align. Advance the engine rpm up to 4,000 to see if the mark moves at all. If it moves, keep increasing engine RPM until the mark stops moving. Holding at this new rpm, adjust the gun until the timing marks align on 0. Record the gun number and bring the engine back to idle. The measurement I just took includes the initial timing of 10° plus the distributor’s total mechanical advance or what we call “total with mechanical advance.” Most V8 engines will run best if this number is between 34° and 36°.
- Adjust initial timing to where total mechanical advance will be 34°, reset the hot idle speed, and recheck the total with mechanical following the steps above to confirm 34°. As an example, let’s say my timing gun read 32°. I would raise my initial timing from 10° to 12° to bring my total with mechanical to 34°
- Once total with mechanical is confirmed at 34° BTDC and hot idle set, plug the vacuum hose into the vacuum advance can on the distributor.
- Check initial timing since the vacuum advance may have advanced initial. Record this new number for notes.
- Proceed to repeat the steps I used to check total with mechanical, only this time set the timing gun at 44° before revving up the engine. Record the number when the timing mark stops moving at high rpm and is zeroed on the damper/tab. This number is total all-in advance and, ideally, will fall between 42° and 52°. Subtract total with mechanical from this number to get the amount of advance coming from vacuum. For example, if total with vacuum is 44° minus 34° total with mechanical means the vacuum advance is giving me 10°. If the total with vacuum falls within the 42° – 52° range, I am ready to take a test drive and see how the engine reacts. If it falls below 42°, I either need to raise the initial timing or modify the slots in the mechanical and/or vacuum mechanisms to increase the limits. Since my initial in this example is at 12° and total mechanical is 34°, I could raise initial by 2° to keep me at 36° total with vacuum. If my total with vacuum is higher than 52°, I risk detonation at higher rpms while under load, especially on a hot day or with cargo. I can either retard the initial or modify the mechanical and/or vacuum mechanisms to lock out advance. With my initial at 12° in this example and total mechanical at 34°, I would not want to retard initial much more than 2°. Keep in mind that initial above 16° is getting up there for an iron-head V8, and anything below 6° likely won’t run well.
- Note: Remember to disconnect vacuum advance when adjusting initial timing.
- Once my total with mechanical and my total all-in are set, I quickly rev up the engine by hand using a smooth but relatively fast motion to about 4,000 rpm. I am looking to hear how the engine responds without load. If it backfires, falls on its face, or acts otherwise problematically, I know I need to diagnose the issue since the engine will not run well on a test drive under load.
- If I am comfortable with how the engine responds to throttle, I use a marker to mark a line across the distributor base and the intake manifold to note my initial timing location.
- Snug down (do not overtighten) the distributor clamp to where the distributor will not turn with reasonable one-handed force, and confirm the marker lines align. Replace the air cleaner.
- I then take the vehicle out to test drive in normal around-town driving with traffic and stoplights/signs paying attention to low-rpm and mid-rpm throttle response and any signs of bogging down or detonation. I take my tools with me for ease of tuning. If I hear/feel detonation at low rpms, either initial timing is too advanced, or the vacuum advance curve is coming in too early. For vacuum cans with adjustment, I use an Allen key through the vacuum port to either slow down or speed up the curve (counterclockwise delays vacuum, clockwise advances it). If throttle response lags or the engine stumbles, initial timing might be too retarded, vacuum might be coming in too late, or I might need to adjust the accelerator pump.
- Once things are working well around town at low- and mid-rpm, I find my preferred boondocks road or freeway on-ramp where I can test with wide-open throttle passes from a rolling start and during hard kick-down shifting, again paying attention to throttle response, how the engine revs up, and sounds of detonation. If the throttle response is slow or the engine stumbles above 2,000 rpm, I might need more vacuum advance or to adjust the vacuum pod’s spring if equipped (use an Allen wrench through the vacuum port) to adjust the vacuum curve to come in earlier. Fuel and spark quality may also be culprits to consider if I have difficulty resolving the issue with timing. If I hear/feel detonation, I have too much all-in advance and need to either lock out vacuum by modifying the mechanism’s slot or bring in the vacuum later by adjusting the vacuum curve with the Allen wrench.
The key to dialing in timing this way is patient, experimenting making single and small changes at a time, and recording everything I do/change. Using marker lines of different colors on the distributor/intake as I make adjustments to initial timing lets me track changes and quickly move back to earlier settings on the fly without using the timing gun. If I am having issues diagnosing a problem, unplugging the vacuum advance, plugging the vacuum hose, and driving without vacuum advance is a basic step in troubleshooting to remove that variable. A healthy engine should run just fine with no vacuum advance and 34° total with mechanical for the purpose of diagnostics. It will not be optimal for street driving long term, but it will tell me if the vacuum advance is the issue if the problems go away.
Optimal Timing Procedure (for performance engines):
With performance comes a more complex method of timing, and even a stock engine will benefit from this method. The first thing we need to do is “baseline” the distributor. I must give credit to an excellent magazine article I read many years ago written by an engine builder whose name I wish I wrote down. Having a note-taker in this procedure really speeds up the following process.
- With vacuum advance plugged in and the timing gun hooked up as described above, start the engine.
- Adjust the gun until the timing marks are zeroed. Record this initial timing degree. For example, 10° BTDC.
- Slowly and steadily, increase the throttle by hand until the timing mark just begins to move. Adjust the timing light until the marks align and record this timing degree and the RPM. Continue this procedure in 500 rpm increments until the timing mark stops advancing, recording the numbers at every 500 rpm. The last recording will be total all-in advance.
- Back at idle, unplug the vacuum advance and plug the hose.
- Slowly and steadily, increase the throttle by hand until the timing mark just begins to move. Adjust the timing light until the marks align and record this timing degree and the rpm. Continue this procedure in 500 rpm increments the same as above until the timing mark stops advancing. This is total with mechanical advance. Subtract this number from total all-in and I have advance from vacuum. Subtract initial timing from the total with mechanical and I have my advance from mechanical. For example, initial is 10°, total all-in is 46°, and total with mechanical is 36°. Subtract 10 initial from 36, and I know mechanical is giving me 26°. Subtract 36 from 46 and I know vacuum is giving me 10°.
- Take graph paper with RPM on the Y axis and degrees advance on the X and plot all the 500 rpm numbers to form a graph. Mark the different rpms that advance comes in and mechanical advance stops. There’s the advance curve for this exact distributor.
Now that I have the distributor baselined, I can focus on timing. Good idle quality is key to performance engines, especially those with a lot of cam overlap.
- Disconnect vacuum advance and plug the hose.
- Set initial timing to 6°.
- Adjust hot idle speed to where the engine will idle well in gear. Record this rpm in park/neutral.
- Adjust the idle air-fuel mixture (see my carburetor tuning article for this procedure).
- For automatics, place the car in gear and record the idle rpm and vacuum.
- With the engine in park, bump initial by 2° to 8° and record the rpm and vacuum, which should both be higher.
- Reset idle speed and record the new vacuum reading both in park and in gear, which should be higher.
- Continue this process until vacuum just begins to suffer.
- I shut off the engine and sit down to analyze all the data I have recorded. Out of all these recorded results, I choose the highest vacuum at the lowest initial timing when the engine was in gear. This initial will likely be between 8° and 16°. Err on the side of lower timing than eking out every last bit of vacuum. For example, if the best vacuum is 12 inHg at 14° advance, but going to 12° advance only decreases vacuum to 11.5 inHg, go with the lower initial and sacrifice the vacuum. This will be the ideal initial timing for the engine, so all timing adjustments should take place in the mechanical and vacuum advance mechanisms thereafter.
- If I luck out, the distributor will give the perfect amount of mechanical to put total with mechanical between 34° and 36° and total all-in with vacuum at 42° – 52°.
- However, if mechanical is too much, I have to remove the mechanical plate and weld the stop to limit movement.
- To adjust vacuum advance limits, they make different vacuum cans. Double the can number to give you engine advance, so a #7 can will give 14° of total vacuum advance. To lock out vacuum, I use epoxy and a piece of rebar tie wire as a stop since welding like I do on the mechanical plate will melt the rubber diaphragm. If the engine detonates at low rpms, try turning the Allen screw through the vacuum port counterclockwise to delay vacuum. Clockwise will bring in vacuum advance at a lower vacuum if the engine is sluggish at low rpm throttle.
- At this point, I am ready to test drive the car as described above.
- For even more precise tuning of the advance curve, I either cough up the money to pay a dyno tuner or go to the drag strip. Ideally, the advance curve will be set by a dyno tuner (kill two birds with one dyno tuner by having carburetor jetting tuned too), but I can get in the ballpark at the track. The general practice for performance street cars is to have the mechanical advance all-in by about 2,000 rpm, and I use different gauge springs. In comparison, most stock mechanical advance does not come all-in until 4,000 rpm, way too high for performance builds. The rules to follow when tuning the curve are (1) speed up the advance curve as much as possible without causing running issues until reaching the best trap speed and (2) if denotation is an issue at full throttle, delay the advance curve.
While points ignition can be perfectly reliable and last a very long time on our classic vehicles that are not driven daily like they used to be, LA-design electronic ignition is a superior design and easy to convert onto an A-block. If you plan on driving your vehicle more than 3,000 miles annually or if you are looking for the best performance, I recommend upgrading to electronic ignition; if you have a period-correct build with a stock distributor or Mallory dual-points or if you drive less than 3,000 miles per year, run the points and have fun.
Since the distributor is a crucial, delicate component of the ignition system, I recommend staying away from cheaply made imports, including Pertronix and Proform. Pertronix is all over the internet largely due to their marketing strategies to the point one might confuse them for a high-quality brand. Do your own research online to see if their parts are actually made in the USA or China regardless of what they advertise. For my personal experience, I have purchased two Pertronix distributors in the last ten years a few years apart for two different engines. With the first distributor, the timing mark jumped all over the place. I removed the distributor and setup a dial indicator on the distributor cam and found it was out of round creating the fluctuating timing. About five years passed where I thought maybe the company had improved, but the next distributor I purchased had a similar issue with the timing fluctuating, only I found the shaft bent. I’ll pass on the third try.
Some people also recommend the 2000 – 2013 Mopar Performance distributor with the miniaturized Accel GM HEI internals, but these units are known to have electronic issues including erratic timing that is the root cause for a host of issues.
For these reasons, many Mopar owners and racers, myself included, have had excellent results with Richard Ehrenberg’s trademarked HiRev 7500 distributor (P3690430), HiRev 7500 ignition control unit, coil, 0.5 ohms ballast resistor, and wiring harness (see Figure 3 below). His kits can be purchased through him on ebay, user name rehrenberg. They are very high-quality components with a reputation in performance circles who want the stock look since they look nearly identical to a factory electronic setup and are based off the 440 A-134 hi-performance units. If desired, the control unit can be hidden in the engine compartment to keep the factory points look. Ehrenberg sells the components direct, but some of them can be purchased through other vendors.
Important Note: When adjusting the gap between the distributor cam and pickup, use a brass feeler gauge since a steel gauge will create drag and can also disrupt the magnetism.
Points Adjustment and Distributor Inspection/Cleaning
What do contact points even do? Essentially, they function as a type of trigger that helps direct coil current. The coil ground is connected to one side of the points. When the points are closed, the coil primary circuit is completed and lets the primary current flow for the coil to build up its charge; when the points open, the circuit is broken, primary current stops flowing through the coil, and the magnetic field in the coil collapses. Voltage builds high enough to move through the coil wire, transfer from the distributor cap to the rotor, jump from the rotor to the cap terminal, and flow down the spark plug wire where it eventually arcs at the spark plug. For this process to function properly, the points need to be clean and properly adjusted.
There are two adjustments for points: gap and its related dwell angle. Referring to Figure 4 below, points gap refers to the gap between the two contact points when the rubbing block is up on the distributor cam lobe at full lift. Gap in measured while the engine is off. The dwell angle is measured with the engine running and measures how long points stay closed in degrees out of a 360° rotation of the distributor shaft. Dwell is the important measurement of these two, and gap simply puts the points in the ballpark so the engine will run in order to adjust dwell precisely.
Here is the process I use for inspecting and cleaning the distributor and setting gap and dwell:
- With the ignition switch off, unplug the coil wire and remove the distributor cap. Mark and unplug the distributor wires as necessary, making sure to mark the #1 plug on the cap for reassembly.
- Remove the rotor. Inspect the rotor tip for corrosion. If lightly corroded, use fine-grit sandpaper to lightly clean up the tip. If heavily corroded/pitted/melted, replace.
- Inspect the cap terminals for corrosion. Clean or replace similar to the rotor.
- Inspect the points rubbing block for excessive wear/damage.
- Inspect the cam lobe for debris. Wipe off the cam if necessary and wipe on a very, very small dab of distributor cam lube. Excessive or incorrect lube can fling off onto the points and cause issues.
- Inspect the ground and condenser wires for damage. Address if necessary.
- Push/pull the distributor shaft side to side and up/down looking for excessive play. Address if necessary.
- Using a small, clean screwdriver, gently push the points arm away from the stationary point far enough to visually inspect the points (about 1/4″ is enough). Do not tweak the arm, use excessive force, or pry the arm beyond its pivot range. Use a flashlight to inspect both sides of the points for burning/corrosion/pitting.
- If the points are corroded to where they require heavy filing, remove the points from the distributor. Place a clean points file in a vice and sandwich the points around the file. Holding the points so they do not flex/contort, move them back and forth across the file until the points are clean and trued up. The maximum amount of pitted surface area is 30% after filing. Any more pitting, the points need replacing. Blow off the points with compressed air and reinstall.
- Rotate the engine using a ratchet on the crankshaft bolt until the points arm is at full lift on top of the cam lobe. Once close, gripping the mechanical advance mechanism at the top of the shaft and rotating it will usually give enough slop to precisely open/close the points.
- Factory A318 points gap is 0.017”. Take a 0.017” feeler gauge (I use a wire sparkplug gauge since it is easier to get into tight spots) and gently try to insert it between the points. The goal is to be able to insert the gauge with slight drag/resistance but not enough to push open the points farther.
- If the gap needs adjusting, locate the locking screw attached to the stationary point. Loosen the screw keeping some light tension on it. Locate the prying point on the baseplate and the point bracket, which are often a small nub or slot. Using a clean screwdriver, gently pry the point bracket away from the other point to increase gap or toward the other point to decrease gap. Never pry on the point itself.
- Once gap is set, tighten the lock screw and confirm one last time the gap is properly set.
- Reinstall the rotor and wipe off any grease/fingerprints with lacquer thinner or brake clean, for grease can inhibit proper electricity transfer. Reinstall the cap and wires.
- Hook up a dwell meter. If using a two-lead meter, the black goes to chassis/engine ground; red goes to the coil negative post. If using a timing gun equipped with a dwell meter, ground the gun ground lead and attach the dwell lead to coil negative. Ensure the wires are clear of any exhaust, fans/pulleys, and belts.
- Fire the engine and bring it up to running temperature, holding your hand on top of the radiator to feel when the thermostat opens and hot coolant rushes into the radiator. Note the meter reading. An 8-cylinder engine should have 26° – 28° dwell ideally. If necessary, shut off the ignition switch, follow the above procedure for removing parts, and adjust the points. If I increase the points gap, the dwell angle goes down; if I decrease the gap, the dwell angle goes up. For example, if dwell is 30°, I would increase the gap very slightly (as in by .002”) and recheck dwell. If the dwell is 24°, I would decrease the gap very slightly and recheck dwell. Yes, this process can be a pain, which is why Chevy invented points that could be adjusted with the engine running using a long hex T-wrench through a door in the distributor cap.