The Stihl MS 390 is a powerful, top end saw intended for ranchers, professionals and homeowners with heavier cutting needs. These chainsaws can be used for large tree felling, bucking and cutting firewood. To maintain the engine at its highest level, use the manufacturer's specifications for tune-ups, and while performing other maintenance work on the MS 390.
The single piston, two-stroke engine on the MS 390 uses a much larger engine than most mid-range saws. The piston, at 64.1 cubic cm, moves inside a 49mm bore, producing a stroke that's 34mm. When the MS 390 is running at 9,500rpm it will generate about 3.4kW of power. The MS 390 should idle at 2,800rpm and has a maximum permissible speed of 13,000rpm before damage begins; the clutch will engage at 3,500rpm.
The MS390 requires a mixed fuel for combustion, combining two-stroke engine oil with regular unleaded gasoline. Mix fuel with an 89 octane or higher grade with high-quality engine oil at a ratio of 50-to-1 gas to oil. The all-position carburettor uses an integral fuel pump with a diaphragm. The carburettor must maintain 11.6 psi to operate and pump fuel. The fuel tank holds 1.2 pints of fuel.
The ignition system on the MS 390 operates with an electrical coil housed inside a module. This electric coil acts like a capacitor, holding and discharging the spark for combustion. The ignition system--called an electronic breakerless magneto ignition--must maintain an air gap of .15 to .3mm between itself and the flywheel. The MS 390's spark plug is a Bosch WSR 6 F or an NGK BPMR 7 A, with an electrode gap of .5mm.
Bar and Chain
The MS 390 uses a bar and chain lubrication that's fully automatic. A speed-controlled oil pump uses a rotary piston to deliver oil from the tank to the bar and chain, which will start as soon as the clutch engages at 3,500rpm. The oil tank's capacity is .68 pint. The MS 390 can operate with a 16-, 18-, 20-, 25- or 30-inch bar. Stihl recommends using a chain with 3/8-inch pitch.