Guardian 4389 Diagnostic Repair Manual page 109

SECTION 4.4
DIAGNOSTIC TESTS
3. Attach the spark plug lead to the spark tester terminal.
4. Crank the engine while observing the spark tester. If
spark jumps the tester gap, you may assume the engine
ignition system is operating satisfactorily.
NOTE: The engine flywheel must rotate at 350 rpm
(or higher) to obtain a good test of the solid state
ignition system.
To determine if an engine miss is ignition related,
connect the spark tester in series with the spark plug
wire and the spark plug (Figure 21). Then, crank and
start the engine. A spark miss will be readily
apparent. If spark jumps the spark tester gap
regularly but the engine miss continues, the problem
is in the spark plug or in the fuel system.
NOTE: A sheared flywheel key may change ignition
timing but sparking will still occur across the spark
tester gap.
Figure 21. Checking Engine Miss
RESULTS:
1. If no spark or very weak spark occurs, go to Test 58.
2. If sparking occurs but engine still won't start, go to
Test 56.
3. When checking for engine miss, if sparking occurs at
regular intervals but engine miss continues, go to Test 16.
4. When checking for engine miss, if a spark miss is
readily apparent, go to Test 59.
TEST 56 - CHECK SPARK PLUGS
DISCUSSION:
If the engine will not start and Test 55 indicated good
ignition spark, perhaps the spark plug(s) are fouled or
otherwise damaged. Engine miss may also be caused
Page 4.4-12
PART 4
by defective spark plug(s).
PROCEDURE:
1. Remove spark plugs and clean with a penknife or use a
wire brush and solvent.
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.
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 minimum
allowable compression pressure for a cold engine is
60 psi.
NOTE: It is extremely difficult to obtain an accurate
compression reading without special equipment. For
that reason, compression values are not published for
the V-T T win engine. Testing has proven that an
accurate compression indication can be obtained using
the following method.
PROCEDURE:
DC CONTROL