Guardian 4389 Repair Manual page 109

DC CONTROL
2. Replace any spark plug having burned electrodes or
cracked porcelain.
3. Set gap on new or used spark plugs to 0.030 inch for
single cylinder engines and 0.020 inch for v-twin
engines.
RESULTS:
1. Clean, re-gap or replace spark plugs as necessary.
2. If spark plugs are good, go to Test 62.
SET GAP TO 0.020 INCH FOR V-TWINS
SET GAP TO 0.030 INCH FOR SINGLE CYLINDER
Figure 22. Checking Spark Plug Gap
TEST 57- CHECK ENGINE COMPRESSION
DISCUSSION:
Lost or reduced engine compression can result in (a)
failure of the engine to start, or (b) rough operation.
One or more of the following will usually cause loss of
compression:
• Blown or leaking cylinder head gasket.
• Improperly seated or sticking-valves.
• Worn Piston rings or cylinder. (This will also result
in high oil consumption).
NOTE: For the single cylinder engine, the mini-
mum allowable compression pressure for a cold
engine is 60 psi.
NOTE: It is extremely difficult to obtain an accu-
rate compression reading without special equip-
ment. For that reason, compression values are
not published for the V-Twin engine. Testing has
proven that an accurate compression indication
can be obtained using the following method.
PROCEDURE:
1. Remove both spark plugs.
2. Insert a compression gauge into either cylinder.
3. Crank the engine until there is no further increase in
pressure.
4. Record the highest reading obtained.
5. Repeat the procedure for the remaining cylinder and
record the highest reading.
PART 4
RESULTS:
The difference in pressure between the two cylinders
should not exceed 25 percent. If the difference is
greater than 25 percent, loss of compression in the
lowest reading cylinder is indicated.
Example 1: If the pressure reading of cylinder #1 is
65 psi and of cylinder #2, 60 psi, the difference is 5
psi. Divide "5" by the highest reading (65) to obtain
the percentage of 7.6 percent.
Example 2: No. 1 cylinder reads 75 psi; No. 2 cylin-
der reads 55 psi. The difference is 20 psi. Divide "20"
by "75" to obtain "26.7" percent. Loss of compression
in No. 2 cylinder is indicated.
If compression is poor, look for one or more of the fol-
lowing:
• Loose cylinder head bolts.
• Failed cylinder head gasket.
• Burned valves or valve seats.
• Insufficient valve clearance.
• Warped cylinder head.
• Warped valve stem.
• Worn or broken piston ring(s).
• Worn or damaged cylinder bore.
• Broken connecting rod.
• Worn valve seats or valves.
TEST 58 - CHECK SHUTDOWN WIRE
DISCUSSION:
Circuit board action during shutdown will ground Wire
18. Wire 18 is connected to the Ignition Magneto(s).
The grounded magneto will not be able to produce
spark.
PROCEDURE:
1. On v-twin generators, remove Wire 18 from the stud
located above the oil cooler. On single cylinder genera-
tors, disconnect Wire 18 at the bullet connector.
2. Perform Test 55.
3. If spark now occurs with Wire 18 removed, check for a
short to ground. Set the AUTO-OFF-MANUAL switch to
OFF. Remove the 17 pin connector J1 from the circuit
board.
4. Set a VOM to measure resistance. Connect one test
lead to Wire 18 from the control panel. Connect the
other test lead to a clean frame ground. INFINITY
should be measured.
5. Reconnect the J1 connector to the circuit board.
6. Set a VOM to measure resistance. Connect one test
lead to Wire 18 from the control panel. Connect the
other test lead to a clean frame ground. Set the AUTO-
OFF-MANUAL switch to MANUAL. During cranking the
meter should read INFINITY.
SECTION 4.4
DIAGNOSTIC TESTS
Page 107