Small engines
Workshop sums for small petrol engines and more: two-stroke fuel mix in litres, gallons and fluid ounces; antifreeze strength and freezing point for ethylene and propylene glycol, and how much to drain to get there; rejetting a carburettor for altitude and temperature by air density; reading spark plugs and a compression test; identifying chainsaw chain by pitch, gauge and drive links; mower blade tip speed and belt length; displacement and power; and a seasonal maintenance and winter storage checklist.
Added
Works with
- Engine calculator Cars and EVscompression ratio, rod ratio and gearing for the same bore and stroke
How to use it
The small jobs that keep a mower, a chainsaw, a generator or a motorbike going each come with a sum or a table: how much oil goes in the can, whether the coolant will survive the winter, what jet it wants at the cabin in the mountains. This page puts them in one place.
How to use it
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2-stroke mix — the ratio and the amount of petrol, in litres or US or UK gallons, give the oil to add in millilitres or fluid ounces. The second box works backwards: how strong is a can already mixed? A table lists the usual can sizes.
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Coolant — the glycol type and the strength now (from a refractometer or hydrometer) give the freezing point; a target temperature gives the strength needed. With the system’s capacity it shows how much to drain and refill with concentrate (or a 50/50 premix) to get there.
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Jetting — the conditions the carburettor was set up in, and the conditions now, give the air density ratio and the main jet that keeps the same mixture.
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Compression & plugs — type a compression reading per cylinder (and wet readings, if you did them) for the spread and what it points to; convert spark plug gaps; read what a plug’s colour says; and see what heat range means.
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Saw chain — pitch, gauge and drive links, the chain’s length, the round file it takes, the bar length that count usually goes with, and the pitch of an unknown chain from a measurement.
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Mower blades & belts — blade tip speed against the safety limit, and the length of a belt round two pulleys (or the centre distance for a belt you have).
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Displacement & power — swept volume, power, torque and piston speed from bore, stroke and mean effective pressure. The engine calculator does compression ratios, piston motion and gearing.
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Seasonal checklist — start-of-season and winter-storage jobs for a mower, chainsaw, trimmer, generator, snow blower or tiller, with ticks remembered in this browser.
The sums
Two-stroke mix. "50:1" means 50 parts of petrol to 1 part of oil, so the oil is the petrol divided by 50: 100 ml in 5 litres, 2.6 US fl oz in a US gallon, 3.2 UK fl oz in a UK gallon (a UK gallon is 160 UK fluid ounces, a US gallon 128 US ones).
Coolant. Freezing points come from the standard property tables for ethylene and propylene glycol in water (ASHRAE Handbook — Fundamentals), which give them by mass. The page converts from the volume percentage you measure, using the glycols' densities (1.113 and 1.036). A 50/50 ethylene glycol mix freezes at about −37 °C. To change strength, drain x = capacity × (target − now) ÷ (coolant added − now) and refill: from 30 % to 50 % in a 6 L system with neat concentrate is 6 × 20 ÷ 70 = 1.7 L. The tables stop at 60 % by mass, which is about 57 % by volume. Above about 60–65 % protection gets worse again, so do not run neat glycol.
Jetting. A carburettor meters fuel by the pressure drop the air makes in the venturi. For the same volume of air, that drop is proportional to the air’s density. Fuel through a jet goes as the jet’s area × √(pressure drop), so the fuel mass goes as area × √density while the air mass goes as density. To keep the same mixture, the jet’s area must change as the square root of the density ratio. Jets numbered by their bore (Keihin, Dell’Orto and most small-engine jets: a 120 is 1.20 mm) change as the fourth root, and jets numbered by flow as the square root. Air density comes from the pressure (the International Standard Atmosphere at your altitude), the temperature and the humidity, as a mixture of dry air and water vapour. Going from sea level at 20 °C to 2000 m at 10 °C the density falls to 0.81 of what it was, so a 120 main jet becomes about 114. The engine also makes about 19 % less power.
Compression. A spread of up to 10 % between cylinders is normal; 10–20 % is worth investigating and over 20 % is a fault. A low cylinder that rises a lot when wet has worn rings or a scored bore. One that barely changes has a valve not sealing, a head gasket leak or a holed piston. Two neighbouring cylinders low together suggest the gasket between them. The estimate from the compression ratio is atmospheric pressure × CR^1.2, less atmospheric pressure. That is an upper bound for slow cranking: real readings are lower because the intake closes after the piston starts to rise.
Chain. Pitch is the distance between three consecutive rivets divided by two, and the chain’s length is twice the pitch for each drive link: 72 drive links of .325″ is 46.8″ round the loop. The typical drive-link counts are common pairings, not a standard — bars vary between makes.
Mowers. Tip speed is π × blade length × rpm. ANSI/OPEI B71.1, the US safety standard for mowers, limits it to 19,000 ft/min (96.5 m/s): a 21″ blade reaches that at about 3460 rpm, which is why mower engines are governed near 3000–3300 rpm. A belt over two pulleys of pitch diameters D and d at centre distance C is 2_C_ + π(D + d)/2 + (D − d)²/(4_C_) long.
Power. Power = brake mean effective pressure × displacement × power strokes per second: a four-stroke fires each cylinder every other revolution, a two-stroke every revolution. A 196 cc four-stroke at 3600 rpm and 7 bar makes 4.1 kW (5.5 hp).
Limits
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The ratios, gaps, intervals and typical figures here are general. The engine’s manual, label or maker comes first, especially for oil ratios, plug gaps and coolant type.
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Coolant freezing points are approximate, and only for plain glycol in water; commercial coolants add inhibitors that shift them slightly. Measure with a refractometer for the right glycol.
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Rejetting assumes the engine was jetted correctly to begin with and changes only the main jet; idle circuits, needles and the exhaust matter too. Confirm by reading the plug.
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Compression figures vary with the gauge, the cranking speed and the engine’s temperature. Compare cylinders and tests on the same gauge.
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Nothing here is sent anywhere; the checklist’s ticks stay in this browser.
Linking to an example
Share link copies a link with every setting except the checklist’s ticks. A hand-written link can set any of these:
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Mix ratio ( |
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Petrol amount, its unit ( |
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Oil already in the can, and its unit |
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Coolant: |
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Capacity, its unit ( |
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Temperatures in |
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Jetted at: altitude, temperature, humidity |
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Running at: altitude, temperature, humidity |
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Altitudes in |
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Main jet number, and whether it measures |
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Compression readings (comma-separated), wet readings, unit ( |
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Compression ratio and altitude (m) for the estimate |
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Chain: pitch ( |
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Mower blade length (mm) and rpm |
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Pulley diameters, centre distance, belt length (mm) |
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Bore, stroke, cylinders, cycle ( |
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Checklist machine ( |
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A preset (below) |
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Presets:
example= |
Shows |
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50:1 in a 5-litre can |
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40:1 in a US gallon, in fluid ounces |
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Bringing a 6 L system at 30 % up to −40 °C protection |
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A 120 main jet set up at sea level, taken to 2000 m |
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A four-cylinder with one low cylinder that rises when wet |
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.325″ .050″ × 72 on an 18″ bar |
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A 21″ blade at 3200 rpm and a deck belt |
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A 196 cc four-stroke mower engine |
For example ?example=carb-mountain or
?mr=50&mf=1&mfu=galUK&mou=flozUK.
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