See Figures 1 and 2
Diesel engines have both idle and maximum speed adjustments. The maximum speed adjustment prevents the engine from over-revving when the control lever is in the full speed position and there is no load on the engine. No increase in power is available through this adjustment. The adjusters are located side-by-side on top of the injection pump. The screw closest to the engine is for low-idle, the outer screw is for high speed.
Because the diesel engine does not have a conventional ignition system, you may need a special adapter (VW 1324) to use some tachometers. Do not rely on the tachometer in the instrument panel (if so equipped), as they are notorious for being inaccurate. You should check with the manufacturer of your tachometer to see if it is compatible with diesel engines.
- Connect a suitable diesel engine tachometer as per the manufacturer's instructions.
- Run the engine to normal operating temperature and turn off all electrical accessories.
- If equipped, make sure the manual cold start knob or idle boost is pushed in all the way.
- Loosen the locknut and set the low idle to specification (refer to the tune-up chart or the underhood sticker). When set properly, there should be little or no vibration.
- When tightening the locknut, apply a thread sealer (Loctite® or similar) to prevent the screw from vibrating loose.
- Advance the control lever (throttle) to full speed. The maximum speed for engines through 1980 is 5,500-5,600 rpm. From 1981 on, the maximum speed is 5,300-5,400 rpm for normally aspirated engines or 5,050-5,150 on turbo engines. If not in this range, loosen the screw and correct the speed (turning the screw clockwise decreases rpm).
- Lock the adjusting screw nut and apply thread sealer.
Do not attempt to squeeze more power out of your engine by raising the maximum speed. This will not increase power, and will more likely result in the need for a major overhaul in the not too distant future.