How to Adjust Float Height on Mercury WH, WMH, and WMV Carburetors
- Mike Hill
- 20 hours ago
- 4 min read
Updated: 16 hours ago
Proper float height adjustment is one of the most critical tuning steps when working on 2-stroke Mercury WH, WMV, WMH, and Pumper series carburetors.
Found on Mercury 2.0L, 2.4L, 2.5L, and 3.0 Liter V6 outboards—including race motors like the early Carb 2.4 Liter 7-petal 225 HP, Carb 2.5 Liter 245 HP, SST-120 and SST-140—these carbs rely on precision float settings to maintain optimal fuel levels, throttle response, and overall performance.
Whether you’re building a lake motor or prepping for the racecourse, correct float height calibration ensures you won’t run lean at high RPMs or flood under load.
Adjusting Float Height on WH Series Carbs
When adjusting float height on a Mercury WH carb, the measurement should be taken with the carburetor completely removed and inverted. This means the bottom of the float bowl faces up, and the float is resting lightly on the needle valve without compressing the spring.
Float height is then measured from the carburetor body’s gasket (no gasket installed) surface to the bottom edge of the brass float. For manual fuel pump setups, the float height should be precisely 1/16 inch.
For outboards using an electric fuel pump, we prefer to reduce that clearance to 1/32 inch. These measurements are not arbitrary—they come straight from race-proven experience as well as Mercury’s service recommendations.
To begin the float adjustment process, remove the float bowl by removing the five screws. Once exposed, invert the carb and inspect the float arms for levelness.
Uneven floats can cause inconsistent fuel delivery across cylinders—something especially problematic in high-performance engines like the SST-120.
Use a machinist’s ruler or float height gauge to measure the float drop. If the height is out of spec, gently bend the metal tang that rests on the needle valve.
Bending the tang downward lowers the float and raises fuel level, while bending it upward raises the float and lowers fuel level.
One of the most overlooked aspects of Mercury outboard carb tuning is verifying side-to-side float alignment. Each float arm must sit evenly and move freely—binding or tilted floats can lead to starvation or flooding, especially under hard cornering or acceleration.
After confirming the float height, alignment, and a good seal (by simply blowing air) reassemble the float bowl using a clean gasket to ensure a proper seal.
Adjusting Float Height on WMH and WMV Series Carbs
The float adjustment process for Mercury WMH and WMV carburetors—typically found on later model 2.5L and 3.0L V6 outboards—is very similar to the WH series, but with one notable difference.
Unlike the WH’s dual float design, the WMH and WMV carbs use a single plastic float. Despite the change in hardware, the tuning principle remains the same: float height controls fuel level and must be accurately set.
To adjust float height on WMH or WMV carburetors, invert the carburetor and allow the float to rest naturally on the needle. Instead of measuring a specific gap like with WH carbs, the goal is to set the float to be perfectly level or "even" with the carburetor body.
The float should sit parallel to the gasket surface with no tilt up or down. This ensures consistent fuel delivery and bowl volume during high-speed operation.
Bend the float tab as needed to achieve an even float. Be cautious not to flex the plastic float itself, as it can distort or develop stress cracks.
Just like with WH carbs, always check that the float moves freely, the needle/seat seal and are in good working condition.
If you’re running an electric fuel pump on a 2.5L or 3.0L with WMH carbs, maintaining an even float is critical to prevent bowl overfill under pressure.
For search purposes, this section is key for those looking to adjust float height on Mercury 3.0L outboard carbs, WMH carburetor float settings, or Mercury WMV carb tuning.
🏁 Race Engine Tips (SST-120 / SST-140)
At Buckshot Racing #77, we recommend setting floats tighter (closer to 1/32") when using an electric pump on race engines.
This minimizes the chance of fuel slosh or bowl overflow, maintaining consistent fuel delivery at high G-forces and full throttle.
Also, inspect the needle and seat during every service to prevent wear-related issues that can alter fuel height accuracy.
What Does "Wet Setting" a Carburetor Mean?
Wet setting a carburetor refers to adjusting the float height while fuel is actually present in the carburetor, as opposed to a dry setting where adjustments are made with the carburetor inverted and empty.
The wet set method allows you to fine-tune float height based on the real-world fuel level inside the float bowl.
While a dry set gives you a fast, factory-spec baseline—such as 1/16" for manual pumps or 1/32" for electric pumps on Mercury WH carbs—a wet set helps dial in performance more precisely, especially in high-demand conditions.
This method is particularly valuable when diagnosing fuel-related issues such as starvation, bogging, or flooding. It's also essential when you're trying to match fuel levels across multiple carburetors in a bank, as in many V6 outboard setups.
During a wet set, fuel is delivered via the actual pump setup (manual or electric), and the float height is inspected and adjusted while the bowl is full.
Since WH series carbs don’t have sight windows, race tuners often use transparent bowl adapters.
At Buckshot Racing #77, we recommend starting with a precise dry setting using proper measuring tools, and while necessary, performing a wet set under controlled conditions might be helpful if you're chasing top-end consistency or dealing with tuning issues.
Wet setting is especially effective on multi-carb Mercury outboards where synchronization and fuel balance can make or break your holeshot and high-speed performance.
Just be sure to take proper safety precautions, as you're working with live fuel systems.
Final Checklist
✅ Float height set to 1/16" (manual) or 1/32" (electric) on WH carbs✅ Floats even and parallel on WMH/WMV carbs✅ Floats level and not sticking✅ Needle seats clean and functioning✅ Gasket surface clean, bowl torqued evenly✅ Fuel delivery system verified
For more expert outboard tuning guides, float specs, or carb jetting help, stay connected with Buckshot Racing #77—your source for Mercury race setup and 2-stroke performance.
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