Briggs & Stratton Twin Cylinder L-Head Repair Manual page 18

If plugs are dry, look for –
1. Leaking carburetor mounting gaskets
2. Gummy or dirty carburetor
3. Float needle valve stuck shut
4. Inoperative fuel pump
5. Defective anti-afterfire solenoid or wiring.
A simple check to determine if the fuel is getting to the
combustion chamber through the carburetor is to
remove either spark plug and pour a small quantity of
gasoline through the spark plug hole. Replace the
plug. If the engine fires a few times and then stops, look
for the same conditions as for a dry plug.
Check Compression
Briggs & Stratton does not publish any compression
pressures, as it is extremely difficult to obtain an
accurate reading without special equipment.
It has been determined through testing, a simple and
accurate indication of compression can be made as
follows:
Remove both spark plugs and insert a compression
gauge into either cylinder (one cylinder at a time). Turn
engine over with engine starter until there is no further
increase in pressure. Record this reading. Repeat
procedure on other cylinder and record that reading.
The difference between both cylinders should not
exceed 25%. More than 25% indicates loss of compres-
sion in the cylinder with lower pressure. See example.
Example:
Cyl. #1 Cyl. #2
Eng. #1 65 PSI 60 PSI
Eng. #2 75 PSI 55 PSI
If compression is poor, look for –
1. Loose cylinder head bolts
2. Blown head gasket
3. Burned valves, valve seats and/or loose valve
seats
4. Insufficient tappet clearance
5. Warped cylinder head
6. Warped valve stems
7. Worn bore and/or rings
8. Broken connecting rods
Cylinder Leakdown Test
The cylinder leakdown tester, Tool # 19413, may be
used to test the sealing capability of the compression
components of each cylinder and quickly identify the
problem component.
Cylinder Balance Test
If the engine is hard starting, runs rough, misses or
lacks power, perform a cylinder balance test to
determine whether both cylinders are operating to their
full potential.
1
Tools Required
1. Tachometer, Tool #19200
2. Two #19368 Ignition Testers
3. Screwdriver with insulated handle
Attach ignition tester, Tool #19368, between the spark
plug lead and each spark plug, Fig. 11.
Start and run engine running at top no load speed and
note spark at ignition testers. If the spark is equal at
both ignition testers, the problem is not ignition related.
A spark miss will be readily apparent. Now note RPM of
engine. Ground out one cylinder with screwdriver by
contacting alligator clip on ignition tester and a good
ground on engine, Fig. 12. Note RPM loss. Then
ground out the other spark plug and note the RPM loss.
If the difference between the two cylinders does not
exceed 75 RPM, the amount of work the two cylinders
are doing should be considered equal.
Diff. % Diff.
If the RPM loss is less than 75 RPM between cylinders
5 PSI 7.6%
and the engine runs poorly, the problem is common to
20 PSI 26.7%
both cylinders.
Things Which Affect Both Cylinders
1. Carburetion
2. Crankcase vacuum
3. Ignition timing – Magnetron Ignition
IGNITION
TESTER
#19368
ATTACH
TESTER
TO EACH PLUG
Fig. 11
IGNITION
TESTER
#19368
GROUND WITH
SCREWDRIVER
Fig. 12 – Cylinder Balance Test
a. A partially sheared flywheel key will effect
ignition timing and engine performance.
9