VESPA 633844 IT Service Manual page 147

Vespa GTV 250 i.e.
- Check the main bearing seats that limit axial
clearance in the crankshaft show no signs of wear.
The dimension between these seats is measured
by way of the procedure described previously for
measuring the crankshaft axial clearance and di-
mensions.
N.B.
THE JET IS FED THROUGH THE MAIN BUSH-
INGS. PROPER OPERATION OF THIS COMPO-
NENT IMPROVES THE PISTON TOP COOLING.
CLOGGING HAS EFFECTS THAT ARE DIFFI-
CULT TO DETECT (PISTON TEMPERATURE
INCREASE). FAILURE OR LEAK CAN CONSID-
ERABLY DECREASE THE MAIN BUSHING
AND CONNECTING ROD LUBRICATION PRES-
SURE.
N.B.
THE HEAD LUBRICATION CHANNEL IS PRO-
VIDED WITH A SHUTTER JET; THIS GIVES A
"LOW PRESSURE" HEAD LUBRICATION; THIS
CHOICE WAS MADE TO REDUCE THE OIL
TEMPERATURE IN THE SUMP. THE JET
CLOGGING IMPAIRS THE HEAD LUBRICA-
TION AND THE TIMING MECHANISMS. A JET
FAILURE CAUSES A DECREASE OF THE MAIN
BUSHING AND CONNECTING ROD LUBRICA-
TION PRESSURE.
Inspecting the crankshaft plain bearings
- T
o obtain a good bushing lubrication it is necessary
to have both an optimal lubricating pressure (3,2
bar) and a good oil flow rate; the bushings must be
correctly positioned so as not to obstruct the oil
supply channels.
- The main bushings are comprised of two half-
bearings, one with holes and channels for lubrica-
tion whereas the other is solid.
- The solid half-bearing is intended to stand the
thrusts caused by combustion, and for this reason
it is arranged opposite the cylinder.
- To prevent obstructions in the oil feeding chan-
nels, the matching surface of the two half-bearings
Engine
ENG - 53