Fuel Deprivation

• Power loss associated with fuel deprivation typically displays itself as an inability to rev completely though the engine's rpm range. Instead of revving cleanly, the engine will bog near the half-to-full throttle range. In most cases, fuel deprivation is caused by an obstruction anywhere in the fuel tank valve, the fuel hose and the carburetor, reducing the maximum fuel flow required at higher engine speeds. A pinched vent tube will severely limit the fuel flow. Fuel deprivation can also occur if the carburetor's high-speed fuel jet is clogged. In either case, cleaning the carburetor jet, fuel valve or fuel hose can restore the flow of fuel to its maximum volume.
  
  

Loss of Spark

• Power loss can be caused by a decline in the spark plug's ability to generate a spark. In most cases, the spark plug's electrode is the main culprit. The electrode tip should appear light tan in color and must have a gap of 0.024- to 0.028-inches between the outer and inner electrodes. A gap larger or smaller than the specified gap range can change the ignition timing, leading to a loss of power. Additionally, a loose spark plug cable is enough to reduce the spark plug's efficiency through the rpm range. Test the ignition system's ability to generate a spark by plugging the spark plug into its cable and holding the electrodes against the engine cylinder. Kick the engine through a full revolution, using the kick-start lever and observe the electrodes. Ideally, the spark should appear bright and blue in color. Try a different spark plug, if a spark does not appear. You will need to inspect the capacitive-discharge igniter box and the ignition coil, if spark is still not present.
  
  

Air Infiltration or Deprivation

• Oxygen is a key element in creating a highly-combustible fuel mixture and must only be introduced through the air filter-protected carburetor inlet. The infiltration of air through leaks in the carburetor manifold can affect the motorcycle's power output by radically altering the precise air-to-fuel ratio entering into the engine. Air leaks are usually identified as a hanging idle at a fully-closed throttle position and will cause the engine to overheat. Alternatively, a deprivation of air can be caused by a clogged or dirty air filter. As the amount of air flowing into the engine is restricted, the potential combustible force of the air and fuel mixture is reduced, creating a loss of power. Air deprivation usually appears as excessive backfiring, caused by the ignition of unburnt fuel as it enters into the oxygen-rich atmosphere.
  
  

Mechanical Issues

• Assuming that the XR70's air, fuel and spark requirements are met, a loss of power can be only be attributed to mechanical issues within the XR70's engine, transmission and chassis. Over-filling the engine with oil will generate pressure within the crankcase, preventing the engine from operating at its peak and is felt as a sluggish response to throttle inputs. A slipping clutch is normally exhibited as a lack of power regardless of rpm and throttle input. Likewise, a locked brake caliper can create the same problem as it prevents the wheels from turning.