Camshaft; Compression/Mechanical Compression Release; Compression Release Assy Diagram; Lobe - Cam/Remp Type Compresseion Release Diagrams - Tecumseh SPECTRA 37 Service And Repair Manual

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CAMSHAFT

Check gear and cams for damage or wear. Check that
shaft is not bent and that bearing dimensions conform to
sizes given in table. On vertical shaft engines check that
oil passages are clear, replace shaft if necessary.
Check dimension of cams (see tables).
Compression release
In order to assist in easy starting the 4-stroke engines
may be fitted with a compression release, the function of
which is to hold a valve off its seat at cranking speed thus
lowering the compression ratio.
Mechanical compression release
The mechanical compression release is activated by a
pin working through the crankshaft directly on the tappet
(Fig 28).
When the engine reaches running speed, a centrifugal
weight on the camshaft withdraws the pin thus allowing
the valve to seat fully.
NOTE - Mechanical compression release usually
operates on the inlet valve.
Fig. 28 - Compression release assy
a. pin
b. rocker arm
c. centrifugal weight
d. return spring
Alternatively, the centrifugal weight has a ramp on it which lifts
the valve at cranking speed. This ramp retracts at normal running
speed. (Fig 28-1).
Lobe - cam (Fig 29)
Tecumseh has a compression release system called 'Lobe
- Cam' which is not centrifugally operated as previously.
The drawing illustrates the reshaped cam lobe which
actuates the exhaust valve. It is important that the clearance
of this valve is set at 0,10-0,20 mm (.006"-.010").
NOTE - Check valve clearance with piston at TDC
compression stroke.
Ramp type compression release (Fig 29-1)
Some engines now use a ramp type compression release
system on the inlet valve.
Fig 29-1 Illustrates the new lobe profile
Decompressing on the inlet valve prevents unwanted
emissions being expelled from the engine.
NOTE - Do not interchange Mechanical, Lobe or Ramp
style camshafts, or poor starting and erratic engine
running may occur.
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Fig. 28
Fig. 28-1
Fig. 29
Fig. 29-1