Many owners of legacy Mercury two-stroke outboards eventually decide to remove the factory oil injection system and convert the engine to a premix fuel and oil setup. This is common on engines used for fishing, performance boating, and racing, particularly as these motors age and original oil injection components become more difficult to service or replace.
To support these engines, Buckshot Racing #77 now offers Oil Injection Block-Off Kits designed for Mercury two-stroke outboards, including many of the 3-cylinder and 4-cylinder looper engines produced from the mid-1980s through the mid-2000s. These kits allow the oil pump assembly to be safely removed while properly sealing the pump drive location in the engine block.
Why Many Mercury Owners Remove Oil Injection
Mercury’s oil injection system—often called AutoBlend—was designed to meter oil automatically from a remote tank into the engine’s fuel system. While effective when new, many of these systems are now decades old. Over time, oil pumps, drive gears, hoses, and check valves can wear or deteriorate.
Because the oil injection system is responsible for engine lubrication, a failure can result in severe engine damage. For this reason, many owners of older Mercury outboards choose to eliminate the system entirely and run premixed fuel and oil, especially when restoring or rebuilding a classic two-stroke engine.
When the oil pump is removed, the opening that previously drove the pump must be sealed correctly. The Buckshot Racing #77 Oil Injection Block-Off Kit provides a secure way to close this opening and maintain proper crankcase sealing.
How the Block-Off Kit Works
Removing the oil injection pump leaves an opening in the engine where the pump once mounted. If this opening is not sealed properly, it can introduce air leaks into the crankcase, which may cause poor idle, lean running conditions, or contamination entering the engine.
The block-off kit seals the oil pump mounting location, protects the internal pump drive gear, and preserves correct crankcase pressure. This allows the engine to operate reliably using a premixed fuel and oil combination.
Compatible Mercury Two-Stroke Engines
These block-off kits support a wide range of Mercury 3-cylinder and 4-cylinder looper-style two-stroke outboards commonly found in the 75–125 horsepower range. Many of these engines were produced using 3.375-inch and 3.500-inch bore configurations.
Among the most common applications are the Mercury 75 HP, 80 HP, and 90 HP three-cylinder engines produced from the late 1980s through the early 2000s, along with the larger Mercury 100 HP, 115 HP, and 125 HP four-cylinder loopers used on many bass boats, fishing boats, and utility hulls.
These engines are frequently referred to by several naming conventions, including Mercury 3-cylinder looper, Mercury 4-cylinder looper, Mercury 3.375 bore engines, and Mercury 3.500 bore engines. They were sold under both Mercury and Mariner branding, but share the same core engine architecture.
Running Premix After Oil Injection Removal
Once the oil injection system is removed and the block-off kit installed, the engine must run on premixed gasoline and two-stroke oil. For most fishing and recreational applications, premix ratios around 40:1 are common, while heavier-duty or performance applications may use richer mixtures such as 32:1, or 24:1 for boat racing applications.
Proper fuel mixing and consistent oil quality become essential once the injection system has been removed.
Supporting Legacy Mercury Two-Strokes
Thousands of Mercury two-stroke outboards from the 1980s, 1990s, and early 2000s remain in service today. Engines such as the Mercury 75, 90, 100, 115, and 125 HP loopers are still widely used because of their durability, simple mechanical design, and strong performance for their size.
At Buckshot Racing #77, we specialize in components that help keep these engines running. Our Oil Injection Block-Off Kits provide a clean and reliable solution when converting these classic Mercury outboards to premix operation.
With proper maintenance and the right parts, these legacy Mercury two-stroke engines can continue delivering dependable performance for many years to come.
Contact Mike Hill at +1-714-697-1716 or email mike@buckshotracing77.com for technical support.