To aid in troubleshooting, common symptoms are noted and their most common causes are identified.

Problems when driving over 30 to 35 MPH

Common Causes

• Float is set too low.
• Lower Casting Bowl Passage is restricted to Main Jet & Drain Plug.
• Main Jet Orifice contains dirt or rust, or is too small.
• Upper Casting Fuel Line Passage is restricted to Float Valve via Filter Strainer.

ˆ Top of Page  •  Float Valve and Float  •  Lower Casting Passages  •  Compensator Jet  •  Upper Casting Passages  •  Main Jet

Problems when driving 5-35 MPH

Common Causes

• Gas Adjusting Valve is open too much or seat is not honed to let needle seal.
• Lower Casting Bowl Passage is restricted to Secondary Well via GAV Seat Hole.
• Compensator Jet Orifice contains dirt or rust, or is too small.
• Compensator Jet Orifice is too large.
• Gas Adjusting Valve Brass Seat is missing from carburetors built through 1929.
• Lower Casting Comp. Jet Passage is restricted to Cap Jet via Secondary Well.

ˆ Top of Page  •  Lower Casting Passages  •  Compensator Jet  •  Gas Adjusting Valve (GAV) and Seat  •  Gas Adjusting Valve Seat Honing Tool & Guide

Engine won't idle smoothly at 350 to 450 RPM

Common Causes

• Vacuum leaks at manifold gaskets, wiper line, or throttle shaft.
• Idling Jet Orifice contains dirt or rust, or is too small.
• Idle Air Adjust Screw is mis-adjusted or too short, collapsing spring before it seats.
• Idling Jet Orifice is too large.
• Throttle Idle Adjustment Screw is mis-adjusted.
• Upper Casting Idle Jet Passage is restricted to Idle throat port.
• Upper Casting Air Adjust Screw Passage is restricted to vent hole.

ˆ Top of Page  •  Throttle Assembly  •  Idle Air Adjustment Needle Valve  •  Carburetor Idle RPM and Idle Air Mixture Adjustment  •  Upper Casting Passages  •  Idle Jet

Engine dies when vehicle comes to a stop

Common Causes

• Float is set too high or a pin hole absorbs gasoline into float.
• Compensator Jet Orifice contains dirt or rust, or is too small.
• Idling Jet Orifice contains dirt or rust, or is too small.
• Idle Air Adjust Screw is mis-adjusted or too short, collapsing spring before it seats.
• Throttle Idle Adjustment Screw is mis-adjusted.
• Upper Casting Idle Jet Passage is restricted to Idle throat port.

ˆ Top of Page  •  Float Valve and Float  •  Compensator Jet  •  Idle Air Adjustment Needle Valve  •  Carburetor Idle RPM and Idle Air Mixture Adjustment  •  Idle Jet  •  Upper Casting Passages

Gas Leaks from Carburetor

Common Causes

• Float Valve does not seat properly due to dirt, rust or material failure.
• Float is set too high or a pin hole absorbs gasoline into float.
• Main Jet or Cap Jet Tip is not "dished" / "indented".
• Loose fuel line fitting, drain plug, filter strainer, GAV, Passage plugs, etc..
• Gaskets are not sealing properly.
• Gasket at Main Jet is too thick or multiples are used, resulting in tip being too low.

ˆ Top of Page  •  Drain Plug  •  Filter Strainer  •  Float Valve and Float  •  Gaskets  •  Main Jet  •  Cap Jet

Low mileage / Sooty plugs / Mixture rich / Smell gas

Common Causes

• Gas Adjusting Valve is open too much or seat is not honed to let needle seal.
• Compensator Jet Orifice is too large.
• Gas Adjusting Valve Brass Seat is missing from carburetors built through 1929.
• Air Filter is restricting air flow.

ˆ Top of Page  •  Gas Adjusting Valve (GAV) and Seat   •  Filtering Air  •  Compensator Jet

Mixture lean and over heating

Common Causes

• Float is set too low.
• Lower Casting Bowl Passage is restricted to Main Jet & Drain Plug.
• Compensator Jet Orifice contains dirt or rust, or is too small.
• Gasket at Main Jet is too thick or multiples are used, resulting in tip being too low.
• Main Jet Orifice contains dirt or rust, or is too small.
• Upper Casting Fuel Line Passage is restricted to Float Valve via Filter Strainer.
• Lower Casting Comp. Jet Passage is restricted to Cap Jet via Secondary Well.

ˆ Top of Page  •  Float Valve and Float  •  Lower Casting Passages  •  Main Jet  •  Compensator Jet  •  Upper Casting Passages

Back fires at high speeds

Common Causes

• Float is set too low.
• Main Jet Orifice contains dirt or rust, or is too small.

ˆ Top of Page  •  Main Jet  •  Float Valve and Float

Miss and jerks on hard pulls at low speeds

Common Causes

• Compensator Jet Orifice contains dirt or rust, or is too small.
• Lower Casting Comp. Jet Passage is restricted to Cap Jet via Secondary Well.

ˆ Top of Page  •  Compensator Jet  •  Lower Casting Passages

Opening GAV does not provide extra fuel at low speeds

Common Causes

• Gas Adjusting Valve Seat is blocked with dirt or rust.
• Lower Casting Bowl Passage is restricted to Secondary Well via GAV Seat Hole.

ˆ Top of Page  •  Gas Adjusting Valve (GAV) and Seat  •  Lower Casting Passages