tx750 special interest site IGNITION BREAKER POINTS MAINTENANCE

IGNITION BREAKER POINTS MAINTENANCE

This unit is equipped with two independent sets of ignition points; one for each cylinder. They are both located in a single housing mounted on the middle of the crankcase (L). The points act as circuit breakers for the ignition system. A point cam spins counterclockwise in the center of the ignition unit. A lobe on the cam contgrols the opening and closing of the points.
When the points are closed, current flows to the primary coil (which begins to build a magnetic field). At a precisely calculated point of crankshaft rotation, the cam forces the points apart, which stops current flow to the primary winding in the ignition coil. High voltage is then generated in the coil's secondary winding and causes a spark to jump the plug electrodes.

INSPECTION - WEAR


1. The points gradually become burnt and pitted. This is normal wear. However, metal from one point might transfer to the other. If this metal build-up cannot be cleaned off with a point file, the points should be replaced.
2. Oil may gradually seep past the seal and coat the points or wiring. This will burn onto the points creating an insulating film. It must be cleaned off with ignition cleaning solvent.
3. The fiber cam follower mounted on the pivoting point arm rubs against the cam. Eventually this block wears down which results in a reduction of the point gap and retarded timing of the cylinder. The remedy is to regap the points and check the timing (timing should be checked any time the points are regapped.)
4. If a point return spring becomes weak or broken, the pivoting point will bounce. Timing will become erratic and ignition firing will be uneven. Measure spring tension by attaching a scale (measured in grams) to the pivoting point. It should take 650~850 g to cause the points to separate. (Use a point checker to measure the separation electrically).

REPAIR



1. Point gap on each set of points must be set at 0.3~0.4 mm (0.012"~0.016"). Constant electrical arcs across the points cause some metal to burn away, changing point gap. Clean and regap the points every 2,000 miles. Check timing after regapping.
2. To clean the points, run a point file between the points until the gray deposits and pits have been removed. Spray the points with ignition point cleaner or lacquer thinner, then snap the points shut on a white business card (or paper of hard texture) and repeatedly pull the card through until no more carbon or metal particles come off with the card. (The card may be dipped in lacquer thinner or other cleaner to facilitate this procedure).
3. To gap the points, first rotate the engine until the ignition cam opens the points to their widest position. Slip a 0.4mm (0.016") feeler gauge into the gap. It must be a tight slip fit. If an adjustment is necessary, loosen the point lock screw (1 or 2) as shown in the accompanying drawing, insert a screwdriver into the adjustment slots (3 or 4), and open or close the points until the feeler gauge indicates the correct gap. Retighten the lock screw and recheck the gap.
4. Next, rotate the camshaft until the second set of points opens to its widest point. Then perform the same steps as described in the previous paragraph.
Note:
Add a few drops of light-weight oil into the felt rubbing pad after each points adjustment to lubricate the point cam surface. Do not overoil.

REPLACEMENT


1. Unscrew the point wire securing screw. Completely remove the point lock screw. Lift the entire point assembly up off the point base plate.
2. Locate the new set of points into position by slipping the point assembly locating pin into the appropriate locating hole in the base plate.
3. Insert and tighten the point lock screw. Finish this replacement by attaching the point wire to the stationary point and regapping the new point assembly.

ADVANCE ASSEMBLY



1. The governor is attached to the oil pump shaft. Two centrifugal weights pivot on pins. Each weight has a small extension that fits into the disk notch. As engine rpm increases, both weights begin to swing out on their pivot pins due to centrifugal force acting on the rotating unit. The weights continue to swing outward as rpm's increase until the weights are stopped by fixed stopper pins. As these weights pivot, the extensions in the disk notches cause the disk to rotate. Since the disk is directly attached to the point cam rod, the ignition point cam also rotates, which causes ignition timing to advance.
2. Both weights must pivot smoothly or ignition advance will not occur at the proper rpm, nor will it advance to its fullest extent. On occasion, light-weight grease must be applied to the weight pivot pins.
3. The advance unit mechanically changes ignition firing from 6° BTDC at low rpm to 37° BTDC at high rpm (full advance). The ignition point cam is attached to the camshaft.
4. The point cam is not integral with the assembly. During installation, align the mark on the cam with the pin on the assembly shaft. If reversal, ignition timing will be reversal.