Fuel Adjustment on a John Deere 6602 Combine with a Roosa Master Pump

[MikePhua]

The fuel setting screw on a Roosa Master injection pump used in a 6602 John Deere combine is located beneath a small cover plate on the side of the pump body. Adjusting it alters the maximum fuel delivery rate, but improper tuning can lead to engine damage or poor performance.
John Deere 6602 combine background and engine configuration
The John Deere 6602 was a mid-1970s pull-type combine designed for small grain harvesting. Unlike self-propelled models, the 6602 was powered by its own inline six-cylinder diesel engine, typically a naturally aspirated John Deere 404 or 466 series. These engines were known for reliability and torque, especially in dusty harvest conditions.
The fuel system used a Roosa Master (also known as Stanadyne) rotary injection pump. This pump was widely adopted across agricultural and industrial engines due to its compact design and mechanical simplicity.
Roosa Master pump overview and adjustment principles
The Roosa Master pump regulates fuel delivery through a rotating distributor rotor and a cam ring that actuates plungers. Fuel quantity is controlled by a metering valve linked to the governor and throttle.
Key components include:

• Fuel setting screw: Adjusts maximum fuel delivery
  
• Governor linkage: Controls fuel based on engine speed
  
• Advance mechanism: Alters timing based on RPM
  
• Transfer pump: Supplies fuel to the rotor cavity
  

The fuel setting screw is typically located under a tamper-resistant cover on the side of the pump. It may be sealed with a lead plug or riveted plate to discourage unauthorized adjustment.
Adjustment procedure and safety considerations
To adjust the fuel setting:

• Remove the cover plate carefully
  
• Use a calibrated screwdriver to turn the screw clockwise to increase fuel, counterclockwise to decrease
  
• Make small adjustments—typically no more than 1/8 turn at a time
  
• Monitor exhaust smoke and engine temperature during testing
  
• Avoid over-fueling, which can cause excessive black smoke, overheating, and piston damage
  

It’s recommended to use a dynamometer or field load test to verify performance gains. For combines used in demo derbies or non-harvest applications, fuel delivery may be increased slightly for more aggressive throttle response.
Field anecdotes and tuning advice
One operator in Washington used his 6602 combine for a local combine demolition derby. He sought to increase fuel delivery for better acceleration and torque during competition. After locating the fuel screw and making a modest adjustment, he noticed improved throttle response but also increased exhaust smoke.
Another mechanic noted that older Roosa Master pumps may have worn plungers or sticky advance mechanisms. Before adjusting fuel, ensure the pump is clean and the governor linkage moves freely.
Recommendations for long-term reliability

• Replace fuel filters regularly to prevent pump wear
  
• Use clean diesel with anti-gel additives in cold climates
  
• Inspect pump housing for leaks or corrosion
  
• Avoid aggressive fuel adjustments without monitoring EGT (exhaust gas temperature)
  
• Consider professional pump calibration if using the combine for harvest work
  

Conclusion
The fuel setting screw on a Roosa Master pump in a John Deere 6602 combine can be adjusted to increase fuel delivery, but care must be taken to avoid engine damage. For non-agricultural use like combine derbies, modest tuning may improve performance. Always verify adjustments with load testing and monitor engine health closely.