Components That Affect Timing
Distributor – the primary device, which is adjusted to establish minimum timing.
Vacuum Advance – a device that increases or decreases the timing by moving the distributor plate. It is controlled by the amount of vacuum the engine produces. The source of the vacuum is a ported opening in the carburetor. Mechanical Advance- consists of weights attached to the distributor shaft, controlled by coil springs attached to the distributor plate. Centrifugal force created by the speed of the turning distributor shaft is transferred to the weights attached to it.
Total timing is determined by measuring Initial timing (distributor setting) + Vacuum Advance (the setting of the small diaphragm can attached to the distributor) + Mechanical Timing, the advance caused by the weights attached to the distributor plate and controlled by the strength of the springs.
Correct adjustment of timing affects the quickness of the engine starting, gas mileage, amount of horsepower, rod bearing life, valve ping, and idle quality. Timing that is set outside the parameters of the capability of the engine may cause severe damage to the engine.
PROCEDURE
Step One Determine the base line of the settings that affect timing and write them down.
Initial timing
-Start with a COLD non-running engine. Find the timing mark on the vibration damper (the big round wheel about 1 inch thick behind the belt pulleys at the very bottom front of the engine). It is a perpendicular mark that is cut into the damper. Clean the mark with an appropriate shop cleaner such as WD40 so it is clearly visible. If it is not clearly visible after cleaning it use an artists brush with white paint to paint a thin line on the mark.
-Attach Mopar timing tape or measure and mark degree increments on the vibration damper to about 40 BTDC. (Note 1 inch equals 16 and 2 1⁄4 inches equals 36.
-Find the degree gauge usually attached to the timing cover and visible from the top of the engine. Clean it also, so the numbers and marks are clearly visible.
-Find the distributor hold-down clamp and bolt. Loosen bolt then tighten very slightly.
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-Find and disconnect the vacuum line to the distributor. Connect the vacuum gauge to the line.
-START ENGINE, outside of garage, and run until Engine operating temperature is reached and choke is completely off, transmission in Park.
-Determine your vehicle’s ideal idle speed in RPM; usually 600-800 RPM. If you have a high performance cam it may be as much as 1100 RPM.
-Connect tachometer to determine idle rpm (SEE DRAWING No. 1). -Adjust carburetor idle screw to achieve desired rpm.
-TURN ENGINE OFF.
-Connect Timing Light (SEE DRAWING No. 2)
-START ENGINE and point timing light at mark on damper.
Record the timing reading and the vacuum reading.
-Disconnect the vacuum gauge.
-Re-connect the vacuum line to the distributor.
-Run the engine up to 3000 RPM, point timing light as above. - Record the timing reading.
-TURN ENGINE OFF
You have now established the theoretical minimum and maximum timing. Mopar performance recommends a maximum (total) of 35 BTDC for street engines. The following step will identify and quantify the source of timing which exceeds the minimum (initial) timing.
Step Two
-Disconnect the vacuum line to the distributor.
-Connect a vacuum gauge to the end of the vacuum tube you disconnected.
-START ENGINE, RUN UP TO 3000 RPM, repeat with timing light as above. Record the timing reading and the Vacuum gauge reading.
-Subtract the first maximum timing reading from the second maximum timing reading. The difference is the number of degrees of advance timing the vacuum system is adding.
-Subtract the minimum (initial) timing (step 1) from the maximum timing as recorded in step two. The difference is the number of degrees of advance timing the Mechanical advance system is adding.
Now lets look at an example, which will help you decide the next steps. Lets say the minimum timing is set at 8 BTDC, the vacuum advance provides an additional 13, and the mechanical advance provides 21 at 3000 RPM. 8+13+21=42. This is too much advance timing. We have 3 choices:
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- 1) Reduce the minimum timing (initial) to a point that 35 is achieved by the above tests. Set up the minimum timing procedure and retard the distributor to a number less than 8, then re-measure everything again. Keep adjusting and measuring until the number is achieved. OR;
- 2) Reduce the Vacuum advance to a point that 35 is achieved. This done by removing the vacuum line at the canister on the distributor and inserting the 3/32 inch Hex head Allen wrench, through the hole where the line was connected, into the set screw (inside the canister) and turn the screw to the right (in) to reduce the amount of advance (or left to increase). Turn the screw one (1) turn and record the amount “1 turn right”. Re-connect the vacuum line, run the engine back to 3000 RPM and check the timing again. If still too high turn to the right and record the number of turns (or partial turns). Check timing again. Continue to adjust until the 35 is achieved.
-Alternately you may use the hand held vacuum pump and the reading for vacuum from the first and second step instead of running the engine up to 3000 RPM. OR;
- 3) Change the springs on the distributor plate and check timing in the same manner at 3000 RPM. This is a fairly complex process and beyond the scope of this document.
You may have to use some combination of all three to achieve the desirable results which include; easy start, good idle, power throughout the RPM range, and no valve pinging.
Disconnect all equipment and replace hoses. Happy motoring!!!!
The EPA prefers low BTDC settings to reduce emissions. Unfortunately, this is counter to a high performance engine.
If you don’t have the above equipment and/or do not want to go through the involved process, there is a fairly simple procedure to use, which will produce, desirable but not optimum performance.
Steps
-Loosen the distributor hold-down clamp bolt.
-Start the engine and turn the distributor a small amount in the direction that the engine speeds up. This is advancing the timing. -Tighten the bolt without moving the distributor.
-Turn engine off, and then re-start. It should turn over without hesitation and start quickly. If not reduce timing slightly.
-Test drive the vehicle. From cruising speed (about 50 MPH) accelerate slightly and listen for valve noise from under the hood. Accelerate fully and observe valve noise and vehicle performance.
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If no valve noise, repeat the entire process. When valve pinging is heard reduce the advance by turning the distributor slightly in the opposite direction from above. Re-test and adjust if needed. Once this state is achieved your engine is handling all the advance timing it can take.
tvmoparclub.com/files/tech/Mopar%20Timing.pdf
