Inspecting Impulse Passage - Shindaiwa T2510 Service Manual

A. Impulse Function Critical
Checking
Impulse
Like a two-stroke, the T2500 C4 engine relies
on crankcase impulses (positive/negative
Passage
pressure) to operate the carburetor's fuel pump
diaphragm. Impulses are delivered to the
carburetor through a passage in the insulator
block/reed valve component.
NOTE:
An engine can run with a plugged impulse
passage!
Impulse blockages are often overlooked
because the engine will start and run.
However, poor high-RPM performance can
result from a blocked impulse passage.
B. Impulse Blockage Test
STEP 1. Remove carburetor.
STEP 2. Add a drop of oil to the impulse
opening on the insulator block.
STEP 3. Crank the engine over slowly and
watch impulse passage. (Remove spark plug if
easier cranking desired.) If impulse passage is
working properly, the drop of oil should suck in
and blow out as the engine revolves.
STEP 4. If test shows no, or low impulse,
inspect the following:
check gasket for telltale impressions made by
mismatched alignment with port. Enlarge as
necessary.
soot/carbon deposits can affect pump
performance. These deposits from crankcase
gasses are more likely in well-used engines
(worn rings).
Edition 2 • 09/2007

Inspecting Impulse Passage

Cracked insulator block (venturi/impulse).
Mismatched or homemade gaskets. Always
Carburetor diaphragm. Dry-side build-up of
To test impulse passage, place drop of oil in
opening (arrow) and crank engine. Oil will suck
in and blow out if passage is clear.
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