Engine Noises; Cylinder Leakdown Test; Power Loss; Piston Seizure - Polaris 1999 Sportsman 400 Service Manual

TROUBLES HOOTI NG
chambe r before the proper time o f co mbustio n. Se-
vere engin e da mage ca n resu
lt if
thi s condition per-
sists. Use ofla w-octane gasoline is a common cause
of detonat ion.
Eve n when using a high -octan e gasoline, deto na-
tion can still occur, Other cau ses are ove r-adva nce d
ignition timi ng, lean fue l mixture at or ne ar full
throttl e, inadequate engine cooling, or the exc ess ive
acc um ulation of ca rbon deposits in the co mbustion
chamber.
Power Loss
Several factors can cause a lack of power and
speed. Look for a clogged a ir filter o r a fouled or
dama ged spark plug. Galled piston or cylinder, in-
correct piston clearance or worn or sticky piston
rings may be respons ible for any po wer loss. Loo k
for loo se bolts, defeetive gas kets o r leaking ma -
chine d matin g surfa ces on the cylinde r hea d, cy lin-
der block or cra nkcase.
Pisto n Seizu re
This is caused by incorrect bore clearance, piston
rings
with
an
improper end gap, compress ion leak.
incorrect air/fuel mixture,
a
spark plug of the wrong
heat range
or
incorrect ignition timing. Overheating
from any cause may result in piston seizu re.
Pist o n Sla p
Piston slap is
an
audible slapping or rattling noise
resulting from excess ive piston-to-cy linder clear-
ance. When allowe d to cont inue, piston slap w ill
eve ntua lly cause the piston skirt to shatter.
To prevent piston slap, clea n the air filte r on a
reg ular sc hedule. When pisto n slap can be heard,
disassemble the engine top end and measure the cyl-
inder bore and piston d iameter and check for exces-
sive clearance. Replace parts that exceed wear
limits or show damage.
ENGIN E NOISES
1. Knocking or pinging during acce leration can be
caused by using a lower octane fuel than recom-
mended or a poor grade of fuel. Inco rrect carburetor
jettin g and a too hot spark plug can cause pinging.
47
Refer to Spark Plug Ileal Range in Chapter T hree.
Check a lso for excessive ca rbon build up in the
combustion chamber or a defective ignition system
CD! unit.
II
2. Slapping or rattling noises at low spee d or dur-
ing acceleration can
be
caused by excessive pis-
ton-to-cylinder wall cle ara nce. Check also for a
bent connecting rod or worn piston pin and/or pis-
ton pin hole in the piston.
3. Knocking or rapping while decelerating usually
caused by excess ive rod bearing c
lear ance .
4. Persis tent knocking and vibration or other noise
usua lly ca used by wo rn ma in bearings. If the main
bearings are acce ptable, cons ider the following:
a. Loose engine mounts.
b. Cracked frame.
c. Leaking cylinder head gasket.
d. Exhaust pipe leakage at cylinde r head.
e. Stuck piston ring(s).
f.
Bro ken p iston ring( s).
g.
Pa rtial eng ine seizure.
h.
Excessive connecting rod bearing clearances.
I.
Exce ssive connecting rod
big
end side clear-
ance.
j . Excess ive cra nkshaft
run
out.
k. Wo rn o r dam aged primary drive gear.
5. Rapid on-off squeal indicates a
compression
leak aro und cy linder head gasket or spark plug.
CY LI NDE R L EA KD OW N TEST
A cylinder leakdown test can isolate an engine
malfunction caused by a leaking valve, defective
cy linder head gasket,
worn,
stuck or broken piston
rin g(s), or de fective pisto n. Perform a cylinder
leakdow n test by applying compressed air into the
cylind er, then meas uring the percent of leakage. A
cylind er leakage tester (F igure 17) and an air co m-
pressor are requ ired to perform this test.
Follo w the manufacturer's directions along with
the followi ng informat ion when performi ng a cylin-
der leak do wn test.
I.
Start and run the
engine until it reaches normal
operating temperatu re, then turn
ofT
the engine.
2. Remo ve the air
filter assembly,
Opcn and secure
the thrott le so tha t it is at its wide -open position.
3. Set the piston to TDC on its compress ion stroke.
See Valve Clearance Check and Adjustment in
Cha pter Three.