Short caveat: most Nissan D22 Frontier/Navara/Terrano Hardbody models use fuel injection; only some older/local-market D22s had carburetors. Below is a practical, ordered theory-first guide to diagnosing and repairing a carburetor on those carbureted D22s. No questions.

1) Carburetor theory — circuits and parts (high level)
- Float bowl & needle valve: maintain a reservoir of fuel at a set level. If level wrong, mixture across all ranges is wrong or it floods.
- Idle (pilot) circuit: supplies fuel at closed/throttle and low RPM. Controlled by idle jet and mixture screw.
- Main circuit (venturi + main jet + air bleed): supplies fuel at mid-to-high throttle. Pressure drop in venturi draws fuel through jets.
- Accelerator pump: gives an immediate shot of fuel on throttle opening to prevent flat spot.
- Choke (manual/automatic/thermostatic): enriches mixture for cold starting by restricting air and increasing fuel.
- Air/fuel metering diaphragms, piston or vacuum systems (depending on carb type): control transition and mixture.
- Vacuum ports/lines, intake manifold gasket: leaks introduce extra air → lean condition.
- Linkages, return springs: ensure smooth throttle response.

2) Symptom → likely root cause (short mapping)
- Hard cold start / cranking but won’t start: choke not operating/air enrichment or no fuel to bowl (needle stuck/empty).
- Rough idle / stalls at idle: clogged idle jet, wrong idle mixture, vacuum leak, wrong float level.
- Hesitation/flat spot on acceleration: accelerator pump failure, clogged main discharge, sticky metering valve.
- Black smoke/sooty plugs / poor economy: excessively rich (choke stuck closed, high float level, oversized jets).
- Flooding or fuel leaking from carb: stuck float/needle, worn seat.
- Surge at cruising speed: vacuum leak, incorrect idle mixture, or worn metering system.

3) Tools, parts, safety (brief)
- Basic hand tools, carburetor rebuild kit (gaskets, needles, jets, diaphragms), compressed air, carb cleaner, small picks, feeler gauges, new intake manifold gasket, torque wrench.
- Battery disconnected when working, fuel shutoff if present, work in ventilated area, catch fuel, no open flames.

4) Ordered diagnostic + repair procedure with theory & how repair fixes the fault
1. Safety & baseline checks
- Turn off engine, disconnect battery, inspect for obvious leaks, visually check choke linkage, fuel lines, vacuum hoses.
- Why: rules out simple external faults; vacuum or fuel feed issues mimic carb problems.

2. Confirm fuel supply to carburetor
- Verify fuel pump operation/pressure and that fuel reaches the bowl (open drain or remove bowl plug briefly).
- How it fixes fault: If no fuel or low pressure, carb cannot meter fuel; replacing/fixing pump or filter resolves fuel starvation/no-starts.

3. Check for vacuum leaks and intake sealing
- Inspect intake manifold gasket, vacuum hoses, carb base gasket; spray brake cleaner around base while engine runs (if running) to detect rpm change.
- How it fixes fault: Seals ensure metered air/fuel ratio. Fixing leaks restores designed vacuum signals and mixture; eliminates lean symptoms, rough idle.

4. Float level and needle/seat inspection
- Remove float bowl, inspect float for fuel seepage, check needle/seat for wear, measure float level to service spec and adjust.
- How it fixes fault: Correct float level ensures correct fuel head at jets. Too high → rich/flooding; too low → lean, poor performance.

5. Clean all jets, passages, and air bleeds
- Disassemble air/fuel passages, remove jets, soak in cleaner or use ultrasonic cleaning, blow out with compressed air; don't enlarge jets.
- How it fixes fault: Blocked idle/main/bleed passages restrict fuel flow causing hesitation, rough idle, or no-start at certain throttle positions. Cleaning restores metering.

6. Inspect & service accelerator pump and throttle shaft
- Check pump diaphragm, check for worn shaft bushings or air leaks at throttle shaft; replace pump or diaphragm if weak/leaking; bush shafts or replace carb if worn.
- How it fixes fault: Accelerator pump provides transient fuel. Replacing fixes flat spots on throttle. Shaft air leaks cause lean surge; bushing fixes restore proper idle/transition.

7. Choke system: operation & heat element
- Verify choke plate opens/closes properly, thermostatic or electric choke heating operates; adjust choke free play and fast idle per spec.
- How it fixes fault: Proper choke enriches for cold start only. A choke stuck closed keeps mixture rich causing black smoke and fouling; stuck open causes hard cold starts.

8. Replace gaskets, metering diaphragms, check linkage
- Rebuild with kit: gaskets, needle/seat, metering diaphragms, valves that are degraded. Reassemble and ensure smooth linkage return.
- How it fixes fault: Worn seals and diaphragms change pressures and leaks inside carb, altering metering; replacing restores original control.

9. Reinstall, set float & idle mixture/speed
- Start engine, warm to operating temp, set idle speed first (rpm), then adjust idle mixture screw for best steady rpm, re-check speed, and final mixture fine-tune.
- How it fixes fault: Idle mixture screw adjusts pilot circuit richness. Proper setting cures rough idle/stalling and ensures acceptable emissions/fuel economy at idle.

10. Road test and re-check
- Test acceleration, cruise, cold start, and warm idle. Re-inspect for fuel leaks and vacuum leaks. If surging persists, re-check for vacuum leaks or worn internal metering parts.
- How it fixes fault: Confirms repairs address original symptoms and reveals intermittent issues.

5) Quick mapping: common symptom → immediate first part to check
- No fuel in bowl → needle/seat, fuel pump, clogged filter.
- Rich at all throttle positions → float level, stuck choke, oversized main jet, leaking needle valve.
- Lean at part throttle → clogged main jet or air bleed, vacuum leak, worn metering diaphragm.
- Flat spot on throttle → accelerator pump or clogged discharge.

6) Final notes on limits and when to replace
- Excessive wear of throttle shaft bores, distorted castings, or multiple internal parts worn — rebuilding may be ineffective; replacement carb or conversion to fuel injection (major) may be needed.
- Always set float and mixture per the factory spec for that carb model; small deviations change A/F significantly.

This orders diagnosis from fuel supply and vacuum integrity outward to internal metering, explains the function of each subsystem, the effect of its failure on symptoms, and why each repair step corrects the fault.
rteeqp73

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