Brief theory (what must be true)
1. Four‑stroke timing relationship — The crankshaft position determines piston position; the camshaft position determines valve events. The cam must turn at half crank speed so valve opening/closing happens at the correct crank angles for intake/compression/power/exhaust. If cam timing moves relative to the crank (chain stretch, tooth skip, loose guides or failed tensioner) valve timing is wrong → poor idle, loss of power, backfire, noise, and risk of valve/piston interference.
2. Chain and tensioner function — The chain links crank and cam sprockets and keeps them phased. Over time the chain elongates and guides/tensioner wear or collapse, letting slack develop. The tensioner (spring, hydraulic, or ratchet type depending on engine) takes up slack and damps chain motion; if it fails, slack and slap occur.
3. Setting timing — You must put cylinder #1 at TDC on the compression stroke (both valves closed and rod under compression), align the crank and cam timing marks, fit chain so that marks stay aligned with no slack on the driven side, preload the tensioner, then verify by rotating the engine and rechecking marks and valve motion.

Ordered procedure (theory + what you do)
1. Prep and safety (short)
- Battery negative disconnected, engine cold, remove accessories and timing cover so you can see marks and access chain/tensioner. Remove #1 spark plug to confirm piston/valve movement and reduce compression when turning.

2. Establish #1 TDC (compression stroke)
- Turn the crank by hand (socket on crank bolt) until piston #1 is at TDC of compression: you must be on the compression stroke, not exhaust. Confirm by watching rocker arms/valves: both intake and exhaust valves for #1 will be fully closed (or nearly so) and you’ll feel pressure when turning against compression with the plug removed. This is the reference position for the crank.

3. Align factory timing marks
- Locate the timing marks on the crank sprocket and cam sprocket(s) and on the timing cover/chain. With #1 at TDC compression, align the crank sprocket mark to its TDC reference and the cam sprocket mark(s) to their reference(s). On a 4‑stroke cam will only align every 720° crank; align using the marks provided by Toyota.

4. Observe and note chain slack orientation
- Identify the slack side of the chain (normally the side opposite the tensioner). Proper installation must place the slack on the side opposite the loaded/guide side so the chain remains tight between the sprockets that determine phase. The chain should have minimal play on the drive side when marks are aligned.

5. Remove tensioner and chain (if replacing)
- Release or remove the tensioner per the service method and remove the chain and sprockets if replacing. Inspect sprockets, guides, chain, tensioner, and oil passages. Replace any worn guides, sprockets with damaged teeth, the chain, and the tensioner. A worn chain can have uneven link stretch that won’t hold timing even if the marks are matched.

6. Install new chain/sprockets with marks aligned
- Put the new chain on with the sprocket marks aligned to the cover marks while crank remains at TDC compression. Ensure the chain is seated correctly on the teeth and that the marked links line up to the sprocket marks per the service diagram. Keep the slack on the correct side (usually the tensioner side has no slack; the opposite side is the loaded run).

7. Preload/activate tensioner
- Install the tensioner and set it to its service preload position (this differs by tensioner type: compress and lock a ratchet, pull out the catch pin on a hydraulic plunger after installation, etc.). The tensioner must take up remaining slack and apply steady pressure to the guide so the chain cannot skip.

8. Rotate engine and verify
- With everything installed but covers off, rotate the crank by hand clockwise at least two full turns (720°) and re‑stop at #1 TDC compression. Recheck that the crank and cam marks realign. If marks do not come back to alignment there is an installation error (chain skip, wrong orientation, or wrong sprocket fit). Watch valve movement to ensure there is no interference.

9. Check valve lash and clearances
- On a pushrod OHV (like the 2F), recheck and set valve lash/clearance to spec after reassembly because cam timing or wear replacement can change lash. Incorrect lash changes valve event timing and can cause noise and wear.

10. Final assembly and test
- Torque fasteners to spec, reinstall timing cover, accessories, reconnect battery, start engine and listen: chain noise should be gone or reduced and idle should be smooth. Observe operation and recheck for oil leaks. After initial run, recheck tensioner and clearances per service manual if required.

How the repair fixes the fault (concise)
- Replacing a stretched chain and worn guides restores the correct distance and geometry between crank and cam sprockets so the camshaft turns at the correct phase relative to the crank. That restores correct valve opening/closing timing, eliminating misfires, poor power, and backfires caused by mistiming.
- Replacing or properly preloading the tensioner removes slack and prevents the chain from skipping teeth or slapping the guides; that eliminates start‑up rattle, irregular timing changes under load, and sudden timing jumps that can bend valves.
- Replacing damaged sprockets removes uneven tooth engagement that causes noise and irregular timing; replacing both chain and sprockets ensures even wear and stable meshing.
- Verifying timing marks and rotating the engine confirms no teeth were skipped and that the valve events match piston position — this verifies the fault is fixed and prevents catastrophic valve/piston contact.

Key practical checks (short)
- Always confirm #1 is on compression stroke before final alignment.
- Rotate engine two full turns and recheck marks.
- Replace tensioner and guides with the chain, not just the chain alone.
- Recheck valve lash after reassembly.

That’s the ordered theory + procedure and why each repair element cures the symptoms.
rteeqp73

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