Holden HQ–HZ engines are known for their durability, but age, wear, and poor maintenance can lead to noticeable power loss. Whether it’s sluggish acceleration, poor towing performance, or weak throttle response, the following guide outlines the most common causes and practical remedies to restore full power.
Poor Compression
Low compression is one of the most common reasons for lack of power. It reduces combustion efficiency and overall torque output.
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Leaky valves
• Remove cylinder head and grind valves to restore sealing -
Valve stems or lifters sticking
• Free up or replace components to ensure proper valve movement -
Weak or broken valve springs
• Replace springs to maintain correct valve operation -
Incorrect valve timing
• Re‑time valves using correct camshaft and crankshaft alignment -
Leaking cylinder head gasket
• Replace gasket and check head for warping -
Broken piston rings
• Replace rings and inspect cylinder walls for scoring -
Poor fit between pistons, rings, and cylinders
• Overhaul engine to restore compression and efficiency
Ignition System Incorrectly Adjusted
Incorrect ignition timing or weak spark can drastically reduce power and throttle response.
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Ignition not correctly timed
• Set ignition timing according to factory specifications -
Faulty spark plugs
• Clean, adjust, or replace plugs and test spark quality -
Distributor points not set correctly
• Adjust point gap and dwell angle; re‑time engine -
Weak ignition coil or condenser
• Replace with correct‑capacity components -
Vacuum advance not operating
• Check diaphragm and linkage for leaks or sticking
Lack of Fuel
Insufficient fuel delivery leads to lean mixtures and poor acceleration.
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Dirt or water in carburettor
• Clean and overhaul carburettor -
Fuel lines partly plugged
• Clean or replace fuel lines -
Dirt in fuel tank
• Drain and clean tank; fit new filter -
Air leaks in fuel line
• Tighten connections and inspect for cracks -
Fuel pump not functioning correctly
• Test output pressure; repair or replace pump
Carburettor Air Inlet Problems
Restricted airflow can cause poor mixture and reduced power.
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Dirty air cleaner
• Clean or replace filter element -
Choke partly closed
• Adjust or repair choke mechanism for full airflow -
Incorrect air cleaner position
• Ensure proper fitment and sealing to carburettor
Overheating
An overheating engine loses power due to detonation, pre‑ignition, and reduced efficiency.
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Lack of water
• Refill cooling system and check for leaks -
Fan belt loose or worn
• Adjust or replace belt -
Thermostat sticking closed
• Replace thermostat -
Water pump inoperative
• Repair or replace pump -
Cooling system clogged
• Flush and reverse‑flush system -
Incorrect ignition or valve timing
• Re‑time engine -
Brakes dragging
• Check brake adjustment and free up components -
Incorrect oil grade or viscosity
• Use correct oil for engine and climate -
Fuel mixture too lean
• Adjust carburettor for proper mixture -
Restricted air cleaner
• Clean or replace filter -
Defective ignition system
• Inspect coil, leads, and distributor -
Exhaust system partly restricted
• Clean or replace muffler and pipes
Overcooling
Running too cool can also reduce power, as the engine never reaches optimal operating temperature.
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Thermostat holding open
• Replace thermostat with correct heat range
• Check coolant temperature after replacement
Slipping Clutch
A slipping clutch causes power loss between the engine and drivetrain.
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Clutch slips under load
• Adjust clutch linkage or replace worn clutch plate
• Inspect pressure plate and flywheel for glazing or wear
Additional Checks
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Exhaust leaks
• Repair leaks that affect back pressure and torque -
Timing chain wear
• Replace stretched chain to restore valve timing accuracy -
Vacuum leaks
• Inspect all hoses and fittings for cracks or loose connections -
Engine mounts
• Check for excessive movement affecting throttle linkage or vibration
Final Thoughts
Lack of power in Holden HQ–HZ engines can stem from multiple small issues rather than one major fault. Regular maintenance, proper timing, clean fuel delivery, and correct compression are key to keeping these classic six‑cylinder engines performing as they should.
While this guide helps trouble shoot common problems of the Holden HQ-HZ engines the Holden HQ-HZ 1971-1978 Workshop Manuals available from Car Manuals Direct includes all the original specifications, adjustment, torque settings and repair procedures used by Holden mechanics.
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Source
Information compiled from period factory service literature.
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