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What causes a 1987 Blazer with 350 to idle good then in gear rough and under load sputter backfire is not fuel injected?

1. Spark plugs

Check the spark plugs and make sure they are properly gapped and in good condition.

2. Spark plug wires

Check spark plug wires and make sure they are not cracked or damaged and that they are properly seated.

3. Distributor cap

Check the distributor cap for cracks or damage.

4. Ignition coil

Check for a weak ignition coil.

5. Carburetor

Check for a dirty or clogged carburetor.

6. Fuel Filter

Check that fuel filter is not dirty or clogged.

7. EGR Valve

Test the EGR Valve.

8. Oxygen Sensor

Check the oxygen sensor.