Prelude Tech Guide

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VALVE LASH ADJUSTMENT

-*-Explanation: As part of every major tune-up or service interval, the valve clearance should be checked and ajusted if necessary. If the valve clearance is too large, part of the lift of the camshaft will be used up in removing the excessive clearance, thus the valves will not be opened far enough. This condition makes the vlave train noisy as they take up the excessive clearance. The engine will perform poorly, since a smaller amoung of air/fuel mixture will be admitted to the cylinders. The exhuast valves will not open far enough to ven t the cylinder completely; retained pressure (back pressure) will restrict the entry of the next air/fuel charge.
If the valve clearance is too small, the intake and exhuast valves will not fully seat on the clinder head when they close. This causes internal cylinder leakage and prevents the hot valve from transferring some heat to the head and cooling off. Therefore, the engine will run poorly (due to less gases escaping from the combustion chamber), and the valves will overheat and burn (since they cannon transfer heat unless they are firmly touching the seat in the cylinder head).

-*-Before Beginning: Valve lash must always be adjusted with the engine cold. The head temperature must be below 100 degrees F. Generally, this means allowing the engine to cool for at least 3 hours without driving. Overnight cold is best. If the valve adjustment is being performed as part of a routine amintenance or mileage service regimen, do it before the engine is warmed up to check timing or idle.
Valve location will vary with the type of engine. As a guide, the intake valves always are aligned with the ports or runners of the intake manifold. The exhaust valves align with the tubes of the exhuast manifold.
Adjusting the valves requires positioning the No. 1 cylinder at TDC, then rotating the engine to certain other precise positions. Rotate the engine with a socket on the crankshaft pulley or just roll your car forward with the shifter in 5th gear. Rotation is made easier by removing the spark plugs first. This elminates compression. Always rotate the engine in a counterclockwise (or pushing forward with it in 5th gear) direction.
If you miss a mark during the rotation keep going in the same direction until it comes around again. Turning the engine backwards may cause the timing belt to jump a tooth or slacken, riisking engine damage when restarted. Remember that cylinder No.1 is the one closest to the pulley end of the engine.
The engine is at TDC No. 1 if ALL of the following are true:
-The TDC mark on the flywheel or flexplate is aligned with the pointer in the timing inspection window at the rear or fthe engine. The TDC mark is generally white. It is never red.
-The distributor rotor is point at cap terminal or plug wire No. 1. Mark the cap with the correspoinding wire numbers, then lift the cap and check the rotor position.
-The alignment marks on the camshaft pulley are positioned as indicated in the individual procedure.
-The rocker arms for at least one of the valves on cylinder No. 1 is loose; the points or lobes of the cam are not putting pressure on the rocker arms.

-*-Procedure:These engines employ Dual Overhead Camshafts and use 4 valves per cylinder. The intake valves are operated by one cam and the exhuast valves by the other. Each camshaft pulley has UP or arrow marks and grooves; when the engine is rotated during the adjustment, the marks must be aligned both with the cylinder head and with each other.

Valve Clearances for 1992 Preludes are:
Intake: 0.004-0.005 in. (.09-.13mm)
Exhaust: 0.006-.0007 in. (.15-.19mm)

Valve Clearance for 1993-1995 Preludes w/o VTEC
Intake: 0.003-0.004 in. (.07-.11mm)
Exhuast 0.006-0.007 in. (.15-.19mm)

Valve Clearance for 1993-1995 Preludes w/ VTEC
Intake: 0.006-0.007 in. (.15-.19mm)
Exhaust: 0.007-0.008 in. (.17-.21mm)

1. With the engine OFF, remove the valve cover.
2. Set the No. 1 piston at top dead center. The UP or arrow marks on the pulley should be at the top, and the TDC grooves on the edge or side of the pulley should align with the cylinder head surface. The distributor rotor should be pointing towards the No. 1 spark plug wire.
3. Check the clearance on all the valves for cylinder No. 1 with a feeler gauge. The gauge should pass the gap with a slight drag.
4. If the guage passes with no drag or cannon be inserted, adjust as follows:
a. Loosen the adjusting screw locknut.
b. Turn the adjusting screw to obtain the proper clearance.
c. Hold the adjusting screw in position and tighten the locknut. Correct locknut torque is 20 ft. lbs on models without VTEC and 14 ft lbs. on models with VTEC. After the locknut is tightened, recheck the clearance and readjust as necessary.
5. Rotate the cranksahft 180 degrees counterclockwise (the cam pulley will turn 90 degrees). The UP or arrow marks should be at the exhuast side. The distributor rotor should point to the number three spark plug wire. At this point the valves on NO. 3 cylinder can be checked. Repeat the adjusting steps previously described as necessary.
6. Rotate the crankshaft 180 degrees counterclockwise to bring the No. 4 piston up to TDC. Both UP or arrow marks should be at the bottom and the distributorrotor should point to the number four spark plug wire. The grooves on the pulley align with the cylinder head surface and with each other. Check the valves for cylinder No. 4. Repeat the adjusting steps previously described as necessary.
7. Rotate the crankshaft 180 degrees counterclockwise to bring the No. 2 cylinder up to TDC. The UP or arrow marks should be at the intake side. The distributor rotor should point at the number two spark plug wire. Check all the valves for cylinder No. 2, repeating the adjustment steps previosuly described as necessary.

[LakeMountLude]

Ignition Timing

-*-Explanation- Ignition timing is the measurement, in degrees of cranksahft rotation, of the instant the spark plugs in the cylinders fire in relation to the location of the piston. Ideally, the air/fuel mixture in the cylinder will beignited and just beginning its rapid expansion as the piston passes TDC. If this hapens, the pistons will be beginning the pwoer stroke just as the compressed and ignited 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. Due to the time it takes for the spark from the plug to completely ignite the mixture in the cylinder, the plug must fire beofre the piston reaches TDC so the mixture is completely ignited as the piston passes TDC. This measurement is given in degrees of crankshaft rotation before the piston reaches BTDC (Before Top Dead Center). If the ignition timing setting is 6 degrees BTDC, this means that the spark plug must fire at a time when the piston for that cylinder is 6 degrees BTDC of its compression stroke. However, this only holds true while the engine is at idle. As you accelerate from idle, the speed of your engine increases. This increase in engine speed 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 system incorporates a means to advance the timing of the spark as the engine speed increases. The mechanical distributor has two means of advancing the ignition timing. One is the centrifugal advance and is actuated by weights in the distributor. The other is a vacuum advance and is controlled in that large circular "can" on the side of the distributor. Fully eletronic systems use the Enginc Control Unit to control the trigger signal to the coil. The computer can provide for much more accurate spark control than mechanical units. In addition, some Honda distributors have a vacuum retard mechansm 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 ignitnion spark under certain engine conditions. This cuases more complete burning of the air/fuel mixture in the cylinder and consequently lowers exhasut emissions. If ignition timing is set too far advanced (BTDC), the ignition and burnign of the air/fuel mixture in the cylinder will try to oppose the motino of the pison in the cylinder while it is still traveling upward. This causes engine ping, a sound similar to marbles in a cofee can. If the ignition timing is too far retarded (ATDC), the piston will have alread started down on the power stroke when the air/fuel mixture ignites. This will cause the piston to be forced down with much less power. This will result in rough engine performance, lack of power and poor gas mileage.

-*-Before Beginning- DO NOT grasp the top of the distributor cap while the engine is running, as you might receive a nasty shock. Instead, grab the distributor housing to rotate (the plastic piece).

-*-Procedure- 1. Remove the rubber cap from the timing window on the engine.
2. Start the engine and allow it to warm up. The engine is fully warmed when the cooling fans come on at least once. Switch the ignition off.
3. Locate the service check connector and, if applicable, remove the cap. On 92-95 Preludes, the service connector is located under the middle of the dash. Use a jumper wire to connect the two terminals of the service connector. The two wires that are connected to this connector are Brown/White and Blue/White.
4. Following the manufacturers instructions, connect 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 wire.
5. Make sure that all wires are clear of the cooling fan and hot exhuast manifold.
6. Set the parking brake. Start the engine. Check that the idle is set to specifications.
7. Point the timing light at the timing marks. The timing mark is located on the flywheel and can be viewed by removing the small rubber pice of the block.When the engine is idling, the red mark must align with the pointer. Some timing marks are indicated by 3 lines; the center one must be aligned with the pointer.
8. If necessary to adjust the timing, loosen the distributor hold-down bolts and/or nut and slowly rotate the distributor in requred direction while observing marks. To advance rotate distributor toward the firewall and rotate it towards the front of the car to retard it.
9. To complete the adjustment operation, tighten the hold-down bolt, taking care not to disturb the adjustment.
10. Switch the engine OFF. Reinstall the rubber plug in the timing window and remove the timing light. Remove the jumper wire.