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78/83/90 Series Horizontal Shaft Engines
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Prime test
To perform a prime test:
1. Prime the engine through the carburetor throat using a squirt bottle, filled with clean fresh gasoline.
2. Make sure the throttle is in the run position and the safety key if fully inserted.
3. Attempt to start the engine.
4. If the engine starts and runs long enough to burn the prime, the problem is effectively isolated to the fuel sys-
tem. proceed to Chapter 4: The Fuel System and Governor.
5. Check ignition system as described in Chapter 7: Ignition System.
6. If the ignition system is working, check the compression or perform a leak down test.
Leak-down test
A leak-down test is the preferred method to test the engine’s ability to compress the charge. It will also show
where pressure is leaking from.
To perform a leak-down test:
NOTE: A leak down test pressurizes the combustion chamber with an external air source and will allow the
technician to listen for air “leaking” at the valves, piston rings and the head gasket.
NOTE: These are general instructions. Read and follow the instructions that came with the tester before
attempting to perform this test.
• If possible, run the engine for 3-5 minutes to warm up the engine.
• Remove the spark plug and air filter.
• Find top dead center of the compression stroke.
1. Find top dead center by following the steps
described in the valve lash section of Chapter 1:
Introduction
2. Thread the leak down tester adapter into the spark
plug hole.
See Figure 2.1.
3. Connect tester to compressed air.
4. Adjust the regulator knob until the needle on the
gauge is in the yellow or set area of the gauge.
5. Connect the tester to the adapter.
NOTE: If the engine rotates it was not at top dead
center.
6. Check the reading on the gauge.
! CAUTION! CAUTION
If the engine is not centered at top dead center, the engine will rotate when compressed air is
introduce to the combustion chamber.
Figure 2.1
Leak-down
tester adapter
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