Carburetor Tune Up Guide
previous page: Instructions for Use and Purpose of Compression Gauge
TESTING THE IGNITION CIRCUIT
First, using a voltmeter, check all connections between battery and distributor (ignition switch "on") for voltage. Voltage drop should never be greater than one-tenth (1/10) of one (1) volt. Voltage reading can only be obtained at the distributor primary connection when contacts are open.
Then connect ammeter in series between coil-primary and distributor-primary connections after having removed regular primary wire. Turn ignition switch "on", and with contacts closed, you should get a reading of between four (4) and five and one-half (5 1/2) amps. Any reading between these two figures is a perfectly normal reading of current flow. A reading of less than four (4) amps indicates an insufficient flow of current, which is caused by a loose or badly corroded connection in the circuit between battery and distributor ground; or contacts that are badly corroded. In this case, all connections must be thoroughly checked and all faulty conditions entirely eliminated. A reading of greater than five and one-half (5 1/2) amps indicates a possibility of bad coil primary winding. In this instance the coil should be removed from engine and tested in suitable tester.
However, if a normal dead-draw current reading is obtained, start up the engine and set throttle for about 15 to 20 miles per hour. At this speed, the ammeter should show a reading of between one and one-half (1 1/2) to two (2) amps if distributor is operating normally. A reading of more than two (2) amps indicates contacts are too close. In this case distributor should be removed from engine and checked in regular distributor tester.
A reading of less than one and one-half (1 1/2) amps indicates that the contacts are spaced too far apart or that the distributor shaft, bearings, or plate are badly worn. The distributor must then be removed for proper servicing in distributor tester.
When the distributor is removed for testing also check the condenser for microfarad capacity, series resistance, insulation breakdown, and ohms resistance. This is all done on one condenser tester.
The ignition coil should also be tested on a suitable coil tester.
When installing contacts in the distributor, they must be set for approximate clearance before being properly aligned. In other words, if the gap clearance should be .020 of an inch, check the contact gap with a .020 feeler gauge before aligning the contact surfaces. In this way you secure a greater degree of accuracy. When they have been properly aligned and set for correct cam angle in degrees, the contact points will function normally for a greater period of time. The cam angle setting will ordinarily be accomplished by the use of a distributor tester or cam angle meter.
When checking the distributor in tester, remember that the contact arm fibre rubbing block will chamfer itself to the cam. Therefore, if the specifications are 36°, set it for 34° and when the rubbing block seats itself, it may automatically set the cam angle to 36°.
Contact arm spring tension is important too. If a spring tension gauge is not available, speed up the distributor tester to three-fourths (3/4) of its maximum speed. If the spring tension is low, the contacts will chatter and leave a break in the neon flash on the center disc of distributor tester. In this case, the tension of the arm spring should be increased until the break disappears at the end of neon flash in disc.
After the distributor has been properly serviced, put it back in the engine. Always reset timing with neon timing light far flywheel mark, mark on crankshaft pulley-balancer in front of timing case, or with vacuum gauge as per instruction on timing with vacuum gauge later in these instructions.