Starting & Running Troubles — No-Start to Knock
Dead on air, slow on air, silent on fuel, smoking, sagging, shaking, knocking, overheating — each with its hunt list.
Key Principles at a Glance 5 points
- No-start hunts air first (pressure, valves, distributor, leaks, brakes, propeller) and only then suspects tripped safety cut-outs.
- Slow turning means weak or leaking air delivery; no fire means fuel never arrived combustible — pressure, purity, timing, compression.
- Smoke colour convicts: grey/black is fuel-air mismatch across a dozen causes; blue is lube oil burning past a piston leak.
- Knock diagnosis is one test: cut fuel to the suspect cylinder — silence means combustion, persistence means metal.
- A hot piston is cooled down, never stopped dead — dead-slow, cut its fuel, flood coolant and oil, bar over if stopping is unavoidable.
1. Dead, Slow, or Silent — the Starting Ladder
Idea in one line: starting fails in layers — air moves it, fuel fires it — so hunt the layer the symptom points at.
Kicks in the wrong direction mean one or more starting valves leaking or sticking. Every list below sits on top of any tripped safety cut-out — check cut-outs too.
| Symptom | Hunt in order |
|---|---|
| Will not start on air | Starting pressure too low; stop valves between reservoirs and engine shut; sticking starting, distributor or pilot valves; starting valve leaking back against compression on the upstroke; mains or bottom ends too tight, braking the shaft; fouled propeller. |
| Turns slowly, no ignition | Air pressure low; automatic starting valve not lifting fully or leaking; starting valves leaking; pistons above starting valves leaking; pilot/distributor valves sticking; air-distributor chain slack or stretched; starting stop valve half-open. |
| Turns, will not fire on fuel | Valves between service tank and booster shut; fuel pumps air-locked; leaking fuel valves; fuel too heavy or watery; fuel-valve bypass open; air in the fuel system; choked filters; empty service tank; dead service pump; compression lost to rings or scavenge pressure. |
| One cylinder dead running | Air in pump, line or valve — open the priming valve on the fuel valve and the air vent on the pump till solid fuel flows; else a choked fuel valve needing renewal. |
2. Smoke, Sag & Shake
Idea in one line: exhaust colour, revs and rhythm are the engine's confession — each points at a different half of the combustion bargain.
Grey / black smoke
Fuel-air mismatch: overload, unsuitable fuel, wrong or choked nozzle holes, low injection pressure, leaking valves, air in fuel, wrong lift, late ignition, dirty strainers, mistimed or slackened pump cams overloading sister cylinders, weak compression (rings, blower), high back-pressure, overheated thin fuel.
Blue smoke
Lube oil burning in the combustion space — suspect a leak in an oil-cooled piston.
Revs sagging
Handle fixed, revs falling: seized or leaky pump, slackened regulation, air locks (leaking joints, priming valves, burst pipes, choked holes, small lift, choked filters, water, bypass leaks, empty tank, suction air), choked air strainer, weak compression, seized parts, hull/propeller/weather overload.
Irregular running
Governor and gear first, then fuel contamination: air or gas in the line, water, excess viscosity, choked filters, faulty fuel valves.
3. Knock Test & Hot-Piston Drill
Idea in one line: a knock is either fire arriving wrong or metal moving wrong — starving one cylinder of fuel tells you which.
Slow-speed or cold-start knock that clears with warmth is usually characteristic of compression ignition and needs no action. Persistent knock, or knock at normal revolutions, gets investigated immediately.
One-cylinder fuel cut decides the case:
| Result | Verdict |
|---|---|
| Knock dies with its fuel | Combustion fault — overload, play in oscillating parts, early or late ignition, high injection pressure, leaking/sticking valves, bad atomisation, uneven distribution between cylinders |
| Knock survives | Mechanical defect in the working parts |
Scuffing or an afterburning injector overheats the piston: dead-slow at once, cut that cylinder's fuel, pass maximum coolant through the piston and raise cylinder lubrication, hold cooling high but inside limits. Cutting fuel alone already drops temperature generally. Stopping seizes it — and if stop is unavoidable, engage turning gear instantly and keep barring till the heat walks out, investigating only on cold metal.