CRANKCASE VACUUM TEST
T h e r e s u l t s of a c r a n k c a s e vacuum t e s t c a n be v e r y u s e f u l i n
the g e n e r a l
of a n engine. A n e g a t i v e p r e s s u r e of s p e c i f i c value ( o r v a c u u m ) should b e p r e s e n t within the c r a n k c a s e
when a n engine i s r u n n i n g a t n o r m a l o p e r a t i n g s p e e d and t e m p e r a t u r e . C r a n k c a s e vacuum r e a d i n g
s h o u l d b e t a k e n w i t h t h e engine o p e r a t i n g a t 3200 R P M ( o r 3600 R P M o n m o d e l s o p e r a t e d a t t h i s s p e e d ) .
V a c u u m c a n b e c h e c k e d with a
t u b e w a t e r m a n o m e t e r
w i t h a v a c u u m g a u g e which c a n h e i n s e r t e d
tightly i n t h e oil f i l l bole. An engine i n good condition w i l l h a v e a c r a n k c a s e v a c u u m r e a d i n g no l o w e r
t h a n t h e v a l u e s s t a t e d in t h e a c c o m p a n y i n g c h a r t .
T Y P E BREATHER
R e e d Type B r e a t h e r
C l o s e d B r e a t h e r
12"
NOTE: Values shown i n i n c h e s of w a t e r
c o l u m n a s r e a d o n w a t e r m a n o m e t e r with engine
ing a t 3200 o r 3600 R P M .
If vacuum i s l o w e r t h a n t h e m i n i m u m v a l u e s s t a t e d , t h i s could b e a t t r i b u t e d t o s e v e r a l d i f f e r e n t
f a c t o r s . The condition e a s i e s t t o r e m e d y should be c h e c k e d f i r s t .
A .
Clogged C r a n k c a s e B r e a t h e r : C a n c a u s e low v a c u u m o r p o s i t i v e p r e s s u r e s t o build up. An
i m p r o p e r l y a s s e m b l e d a n d a d j u s t e d r e e d valve c a n a l s o c a u s e t h i s . D i s a s s e m b l e b r e a t h e r ,
t h o r o u e h l v c l e a n t h e n r e c h e c k c r a n k c a s e
a f t e r s e r v i c e d b r e a t h e r i s r e i n s t a l l e d .
B.
W o r n
Oil S e a l s : Oil
u s u a l l y e v i d e n t a r o u n d w o r n o r d a m a g e d oil
s e a l s . R e p l a c e o i l s e a l s a n d r e c h e c k vacuum.
C.
L e a k y Valves: C a n b e the c a u s e of p o s i t i v e c r a n k c a s e p r e s s u r e s . T h e s e condi-
t i o n s c a n h e c o n f i r m e d b y taking c o m p r e s s i o n r e a d i n g s on e n g i n e .
C o n s t r u c t i o n D e t a i l s
-
Tube M a n o m e t e r : To c o n s t r u c t w a t e r m a n o m e t e r , p r o c e e d a s follows:
( A )
P r o c u r e l e n g t h of c l e a r p l a s t i c tubing. Bend t u b e t o f o r m
a n d mount on b o a r d a s
shown i n a c c o m p a n y i n g i l l u s t r a t i o n . Make g r a d u a l , r a t h e r than s h a r p bend i n tube.
( B )
M e a s u r e i n s i d e , s t r a i g h t s e c t i o n of tube and m a r k i n i n c r e m e n t s f r o m
t o
24
i n c h e s .
( C )
P r o c u r e c o r k having o u t s i d e d i a m e t e r which will be a snug f i t in the o i l f i l l hole. D r i l l
hole in c e n t e r of c o r k t o r e c e i v e one end of tube.
( D )
P o u r w a t e r ( c o l o r e d f o r e a s i e r r e a d i n g ) i n t o
u n t i l l e v e l r e a c h e s the a p p r o x i m a t e
halfway m a r k on s c a l e .
When u s i n g m a n o m e t e r , p l a c e c o r k e n d into oil f i l l hole ( o t h e r end open t o a t m o s p h e r e ) and
m e a s u r e d i f f e r e n c e b e t w e e n c o l u m n s .
If
w a t e r c o l u m n i s h i g h e r i n tube connected to engine, v a c u u m
o r negative p r e s s u r e i s i n d i c a t e d . If the h i g h e r c o l u m n i s o n the a t m o s p h e r i c s i d e of m a n o m e t e r ,
p o s i t i v e p r e s s u r e i s p r e s e n t .
