Ignition timing is the measurement, in degrees of crankshaft rotation, at the instant the spark plugs in the cylinders fire, in relation to the location of the piston, while the piston is on its compression stroke.
Ideally, the air/fuel mixture in the cylinder will be ignited (by the spark plug) and just beginning its rapid expansion as the piston passes top dead center (TDC) of the compression stroke. If this happens, the piston will be beginning the power stroke just as the compressed (by the movement of the piston) and ignited (by the spark plug) air/fuel mixture starts to expand. The expansion of the air/fuel mixture will force the piston down on the power stroke and turn the crankshaft.
It takes a fraction of a second for the spark from the plug to completely ignite the mixture in the cylinder. Because of this, the spark plug must fire before the piston reaches TDC, if the mixture is to be completely ignited as the piston passes TDC. This measurement is given in degrees (of crankshaft rotation) before the piston reaches top dead center (BTDC). If the ignition timing setting is 6° BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 6° BTDC of its compression stroke. However, this only holds true while your engine is at idle speed.
As you accelerate from idle, the speed of your engine (rpm) increases. The increase in rpm means that the pistons are now traveling up and down much faster. Because of this, the spark plugs will have to fire even sooner if the mixture is to be completely ignited as the piston passes TDC. To accomplish this, the distributor incorporates means to advance the timing of the spark as engine speed increases.
The distributor has two means of advancing the ignition timing. One is called centrifugal advance and is actuated by weights in the distributor. The other is called vacuum advance and is controlled in that large circular housing on the side of the distributor.
In addition, some Honda distributors have a vacuum retard mechanism which is contained in the same housing on the side of the distributor as the vacuum advance. Models having two hoses going to the distributor vacuum housing have both vacuum advance and retard. The function of this mechanism is to regulate the timing of the ignition spark under certain engine conditions. This causes more complete burning of the air/fuel mixture in the cylinder and consequently lowers exhaust emissions.
If ignition timing is set too far advanced (BTDC), the ignition and burning of the air/fuel mixture in the cylinder will try to oppose the motion of the piston in the cylinder while it is still traveling upward. This causes engine ping . If the ignition timing is too far retarded (after, or ATDC), the piston will have already started down on the power stroke when the air/fuel mixture ignites and expands. This will cause the piston to be forced down with much less potency. This will result in rough engine performance and lack of power and gas mileage.
CHECKING AND ADJUSTING
See Figure 1
Honda recommends that the ignition timing be checked at 12,000 mile intervals (1973-74), 15,000 mile intervals (1975-78) or when problems are suspected (1979-82). On 1983 models, check the timing and adjust (if necessary) every 60,000 miles.
If the vehicle is equipped with a conventional ignition system, the timing should always be adjusted after installing new points or adjusting the dwell angle.
On all non-CVCC engines, the timing marks are located on the crankshaft pulley, with a pointer on the timing belt cover. All are visible from the driver's side of the engine compartment. On all CVCC engines, the timing marks are located on the flywheel (manual transaxle) or torque converter drive plate (automatic transaxle), with a pointer on the rear of the cylinder block. All are visible from the front right side of the engine compartment after removing a special rubber access plug in the timing mark window. In all cases, the timing is checked with the engine warmed to operating temperature (176°F [80°C]), idling in Neutral (manual transaxle) or Drive (automatic transaxle), and with all vacuum hoses connected.
- Stop the engine and install a tachometer to the engine. The positive lead connects to the distributor side terminal of the ignition coil and the negative lead to a good ground, such as an engine bolt.
On some models you will have to pull back the rubber ignition coil cover to reveal the terminals.
- Following the manufacturers instructions, install a timing light to the engine. The positive and negative leads connect to their corresponding battery terminals and the spark plug lead to No. 1 spark plug. The No. 1 spark plug is the one at the driver's side of the engine compartment.
- Make sure that all wires are clear of the cooling fan and hot exhaust manifolds.
- Set the parking brake and block the front wheels. Start the engine. Check that the idle speed is set to specifications with the transaxle in Neutral (manual transaxle) or Drive (automatic transaxle).
- If the distributor is equipped with a vacuum advance mechanism, disconnect the hose(s), plug it (them) and reinstall on the vacuum advance.
Any engine speed other than specified, the distributor advance or retard mechanisms will actuate, leading to an erroneous timing adjustment.
- Point the timing light at the timing marks.
F or red notch on the crankshaft pulley. On CVCC engines, align the pointer with the red notch on the flywheel or torque converter drive plate (except on vehicles where the timing specification is TDC in which case the T or white notch is used).
- If necessary to adjust the timing, loosen the distributor holddown (clamp) bolt(s) and/or nut, then, slowly rotate the distributor in the required direction while observing the timing marks.
- To complete the adjustment operation, tighten the holddown bolt, taking care not to disturb the adjustment. If equipped with a vacuum advance mechanism, unplug and reinstall the hose(s).
Some models are equipped with two bolts, others are equipped with a bolt and a nut, which may be loosened to adjust ignition timing. If there is a smaller bolt on the underside of the distributor swivel mounting plate, it should not be loosened, unless you cannot obtain a satisfactory adjustment using the upper bolt. Its purpose is to provide an extra range of adjustment, such as in cases where the distributor was removed and then installed one tooth off.