# Product Requirements Document
## Yamaha YL1 Twin Jet 100 — Restoration Project

**Version:** 1.0
**Date:** 2026-02-21
**Status:** Active — M1 in progress

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## Current Build Status

| Milestone | Status |
|---|---|
| M1 — Assessment complete | **In Progress** — disassembly substantially complete; measurements and parts list outstanding |
| M2 — Engine complete | Not Started |
| M3 — First start | Not Started |
| M4 — Break-in complete | Not Started |
| M5 — Project complete | Not Started |

**As of 2026-02-21:** The bike is substantially disassembled. Photographic documentation of disassembly covers Nov 2024 – Jan 2025 (16 photos in `06_Reference_Images/`). Formal measurement logging, parts assessment, and parts ordering have not yet been started in this tracking system.

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## Purpose

Get a Yamaha YL1 Twin Jet 100 (engine serial L1-47603) running well and safe to ride on the road, with the work recorded. Parts are checked and reused where they pass, and replaced or machined only when a check fails. Restoring the bike to new is not a goal.

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## Goals

1. **Mechanical integrity** — All engine, fuel, electrical, brake, and chassis systems are checked against factory specification and repaired where they fail. No systems left in unknown condition.

2. **Safety** — The bike passes a self-administered safety inspection (see Success Criteria) and, where applicable, a state vehicle inspection. No deferred safety items.

3. **Documentation completeness** — Every major disassembly step is photographed, all measurements are recorded, all parts substitutions are logged with reasoning, and a complete work log exists from start to finish.

4. **Rideable end state** — The bike starts, idles, runs through all gears, and is capable of being ridden on public roads.

5. **Authentic character** — The restoration preserves the bike's original specifications and appearance to the extent that parts availability allows. Modern substitutions (e.g., AGM battery, ethanol-compatible rubber) are acceptable where safety or availability requires, and must be documented as ADRs.

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## Non-Goals

- **Show/concours restoration** — cosmetic perfection and date-correct parts matching are desirable but secondary to mechanical correctness and safety.
- **Performance modification** — no porting, re-jetting for power, or ignition advance beyond factory spec.
- **Immediate completion deadline** — this is a quality-first project; no fixed completion date.
- **Full cosmetic restoration of bodywork** — paint and chrome are deferred to after mechanical completion unless they block mechanical access.

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## Stakeholders

| Stakeholder | Interest |
|---|---|
| Owner (primary) | A running, reliable, safe machine; documented history |
| Future owner (potential) | Complete provenance record, known-good mechanical state |
| Vintage motorcycle community | Accurate documentation of YL1-specific procedures that others can reference |

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## Success Criteria

The project is complete when **all** of the following are true:

### Engine
- [ ] Crank seals replaced, cases sealed, no air leaks confirmed
- [ ] Bores and pistons checked; a cylinder is machined and fitted with a new piston only if it is scored
- [ ] Rings fitted and end-gap confirmed in spec
- [ ] Head torqued to spec; compression checked (both cylinders within 10% of each other)
- [ ] Autolube system primed, bled, and delivering oil (or premix system documented as ADR)

### Carburetion
- [ ] Both VM16 carbs rebuilt (new jets, needle, float valve, O-rings)
- [ ] Float heights set and matched
- [ ] Pilot screws set and synchronized
- [ ] Carbs synchronized by vacuum gauge

### Electrical
- [ ] Points replaced and gapped (both cylinders)
- [ ] Ignition timing set and verified by timing light
- [ ] Battery 6V, charged, and holding charge
- [ ] Rectifier tested; charging voltage confirmed 6.5–7.5V at 3000 rpm
- [ ] All wiring inspected, corroded connectors replaced, all grounds clean

### Chassis / Safety
- [ ] Tires: age ≤ 6 years, no cracking, correct spec
- [ ] Brakes: shoes above minimum thickness, drums within wear limit, adjustment correct
- [ ] Wheel bearings: no roughness or play
- [ ] Fork seals not leaking, fork oil replaced
- [ ] Frame: no cracks at welds, no through-rust in structural areas
- [ ] All lights functional (headlight, tail/brake, signals if present)

### Documentation
- [ ] All photos indexed in `photo_index.csv`
- [ ] Measurement log complete for all engine components
- [ ] Parts log complete with OEM numbers and sources
- [ ] Work log covers all sessions
- [ ] CHANGELOG current through completion
- [ ] All "verify from manual" flags in best practices resolved

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## Constraints

| Constraint | Detail |
|---|---|
| Budget | TBD by owner |
| Parts availability | Many OEM parts are NOS-only; project timeline is gated on sourcing |
| Service manual | `2Cyl2StrMan.pdf` — confirm whether text-extractable; if image-only, all specs must be manually transcribed |
| State registration | Bike must meet applicable state vehicle code for vintage registration |

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## Milestones

| Milestone | Definition |
|---|---|
| M1 — Assessment complete | Full disassembly done; all measurements taken; all systems assessed; parts list final |
| M2 — Engine complete | Engine rebuilt, installed, sealing confirmed |
| M3 — First start | Engine runs; both cylinders fire; no leaks |
| M4 — Break-in complete | 300km break-in done; all post-break-in checks passed |
| M5 — Project complete | All success criteria checked; documentation finalized |

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## Open Questions

| # | Question | Owner | Status |
|---|---|---|---|
| 1 | Is `2Cyl2StrMan.pdf` text-extractable, or image-only? | — | Open |
| 2 | Is the Autolube pump functional, or will this run premix? | — | Open |
| 3 | What is the current cylinder bore condition (standard vs. already bored)? | — | Open |
| 4 | What state is the bike registered / to be registered in? | — | Open |