MTC Engine Performance & Tuning 2 — Questions and Answers
Question 1: What does a high vacuum reading on a manifold vacuum gauge (18-22 in. Hg at idle) typically indicate?
- Rich air/fuel mixture
- Normal engine operation (Correct answer)
- Retarded ignition timing
- Leaking intake manifold gasket
Correct answer: Normal engine operation
A steady 18-22 in. Hg vacuum at idle indicates the engine is running normally with good ring seal and proper valve operation.
Question 2: When tuning a carbureted marine engine, which jet size change would richen the mixture at wide-open throttle?
- Smaller main jet
- Larger main jet (Correct answer)
- Smaller pilot jet
- Larger needle jet
Correct answer: Larger main jet
A larger main jet allows more fuel flow at WOT, enriching the air/fuel mixture for increased power or to address lean conditions.
Question 3: A marine engine exhibits detonation (knock) under load. Which of the following is the MOST likely cause?
- Fuel octane too high
- Ignition timing too retarded
- Ignition timing too advanced (Correct answer)
- Rich air/fuel mixture
Correct answer: Ignition timing too advanced
Over-advanced ignition timing causes the air/fuel mixture to ignite too early, leading to detonation under load.
Question 4: Which tool is used to measure crankshaft end play during engine assembly?
- Micrometer
- Dial indicator (Correct answer)
- Feeler gauge
- Plastigage
Correct answer: Dial indicator
A dial indicator measures crankshaft end play by measuring axial movement when force is applied to the crankshaft.
Question 5: On a four-stroke marine engine, what stroke follows the compression stroke?
- Intake stroke
- Exhaust stroke
- Power stroke (Correct answer)
- Overlap stroke
Correct answer: Power stroke
After the compression stroke compresses the air/fuel mixture, the spark plug fires initiating the power (combustion) stroke.
Question 6: What is the purpose of an intercooler on a turbocharged marine engine?
- Cool engine coolant before the thermostat
- Cool compressed intake air before it enters the engine (Correct answer)
- Reduce exhaust gas temperature
- Cool engine oil after the oil cooler
Correct answer: Cool compressed intake air before it enters the engine
An intercooler cools the compressed air from the turbocharger, increasing air density and allowing more oxygen into the cylinders for better performance.
Question 7: A marine technician notices black smoke from the exhaust at full throttle. What does this typically indicate?
- Too lean a mixture
- Water in the fuel
- Too rich an air/fuel mixture (Correct answer)
- Worn piston rings
Correct answer: Too rich an air/fuel mixture
Black exhaust smoke at full throttle indicates an excessively rich mixture where unburned fuel exits through the exhaust.
MTC Engine Performance & Tuning 2
What does a high vacuum reading on a manifold vacuum gauge (18-22 in.
Hg at idle) typically indicate?