Engine Tune Up
 69-3 Purpose of Tune-up
The purpose of an engine tune-up is to restore power and performance that may have been lost through, loss of adjustment, wear, corrosion or deterioration of one or more parts or units. In the normal operation of an engine, these changes take place gradually at quite a number of points so that it is seldom advisable to attempt an improvement in performance by correcting one or two items only. Time will be saved and more lasting results will be assured by following a definite and thorough procedure of analysis and correction of all items effecting power and performance. Because of Federal laws limiting exhaust emissions, it is even more important that the engines tune-up is done accurately, using the specifications listed and the tune-up sticker found in each engine compartment.
Economical, trouble free operation can better be assured if a complete tune-up is performed each 6,000 miles.
The parts or units which affect poser and performance may be divided for analysis, into three groups in accordance with their function in producing (1) compression, (2) ignition and (3) carburetion. The tune-up procedure should cover these groups in the order given. While the items affecting compression and ignition may be handled according to individual preference, correction of items in the carburetion group should not be attempted until all items in compression and ignition have been satisfactorily corrected.
Division III
Service Procedures
69-4 Engine Tune-up Operations
a. Compression
NOTE: To make sure hydrocarbon and carbon monoxide emissions will be within limits, it is very important that the adjustments be followed exactly.
The suggested procedure for engine tune-up is as follows:
1. Remove all spark plugs.
2. Position throttle and choke valve in full open position.
3. Connect jumper wire between distributor terminal of coil and ground on engine to avoid high tension sparking while cranking engine.
4. Hook up starter remote control cable and turn ignition switch to "on" position.
5. Firmly insert compression gage in spark plug port. Crank engine to obtain highest possible reading.
6. Check compression of each cylinder. Repeat compression check and record highest reading obtained on each cylinder during the two pressure checks.
The recorded compression pressures are to be considered normal if the lowest reading cylinder is more than 75 percent of the highest reading cylinder. See the following example and the "Compression Pressure Limit Chart." See Figure 69-2.
Figure 69-1 Example
|
Cylinder No.
|
Pressure (PSI)
|
1
|
129
|
2
|
135
|
3
|
140
|
4
|
100
|
Seventy-five percent of 140 (highest) is 105. Thus, cylinder No, 4 is less than 75 percent of No. 3. This condition accompanied by low speed missing, indicates an improperly seated valve or worn or broken piston ring.
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7. If one or more cylinders read low, injects about a tablespoon of engine oil on top of pistons in the low reading cylinders through spark plug port. Repeat compression check on these cylinders.
a. If compression improves considerably, rings are worn.
b. If compression does not improve, valves are sticking or seating poorly.
c. If two adjacent cylinders indicate low compression and injecting oil does not increase compression, the cause may be the head gasket leaking between the cylinders. Engine coolant and/or oil in cylinders could result from this defect.
b. Compression Pressure Limit Chart
This chart may be used when checking cylinder compression pressures. It has been calculated so that the lowest reading number is 75 percent of the highest reading number.
Figure 69-2 Compression Pressure limit Chart
|
Maximum Pressure Pounds/Sq. Inch
|
Minimum Pressure Pounds/Sq. Inch
|
Maximum Pressure Pounds/Sq. Inch
|
Minimum Pressure Pounds/Sq. Inch
|
134
|
101
|
188
|
141
|
136
|
102
|
190
|
142
|
138
|
104
|
192
|
144
|
140
|
105
|
194
|
145
|
142
|
107
|
196
|
147
|
146
|
110
|
198
|
148
|
148
|
111
|
200
|
150
|
150
|
113
|
202
|
151
|
152
|
114
|
204
|
153
|
154
|
115
|
206
|
154
|
156
|
117
|
208
|
156
|
158
|
118
|
210
|
157
|
160
|
120
|
212
|
158
|
162
|
121
|
214
|
160
|
164
|
123
|
216
|
162
|
166
|
124
|
218
|
163
|
168
|
126
|
220
|
165
|
170
|
127
|
222
|
166
|
172
|
129
|
224
|
168
|
174
|
131
|
226
|
169
|
176
|
132
|
228
|
171
|
178
|
133
|
230
|
172
|
180
|
135
|
232
|
174
|
182
|
136
|
234
|
175
|
184
|
138
|
236
|
177
|
186
|
140
|
238
|
178
|
EXAMPLE: After checking the compression pressures in all cylinders, it was found that the highest pressure obtained was 182 PSI. The lowest pressure is 145 PSI. By locating 182 in the maximum column, it is seen that the minimum allowable pressure is 136 PSI. Since the lowest reading obtained was 145 PSI, the car is within limits and the compression is considered satisfactory.
c. Spark Plugs
1. Inspect, clean and regap or replace spark plugs as required. Correct gap is .030.
2. Install spark plugs. Tighten to 30 lb. ft.
d. Secondary Ignition System
1. Inspect ignition cables for broken, swollen or deteriorated insulation.
2. Check terminal ends and condition of rubber boots. Replace as required.
3. Inspect the condition of the distributor cap and rotor.
4. Clean the ignition coil and inspect for cracks or carbon paths which could cause high voltage leakage.
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