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Modifying Your Honda Engine for High Altitude Use
In most Honda engine manuals, there’s a section called “Carburetor Modification for High Altitude Operation.” This simply states that “specific modifications” need to be made for the motor to run correctly at higher elevations. What are these modifications, and why do they need to be made?
What Does Altitude Have to Do With My Honda’s Carburetor?
The higher you are, the less dense the air is. Local air density can vary depending on temperature and humidity, but, all things being equal, the air at an elevation of 10,000 feet is one-third as dense as the air at sea level. During the intake stroke, the piston moves down, filling the cylinder with air. If the air is less dense outside the motor, there will be less air inside the cylinder, even if the volume stays the same.
If there’s less air in the engine, there needs to be less fuel to get the right air/fuel ratio. The main jet and pilot jet determine how much fuel is added with each intake stroke, and it doesn’t vary even when the amount of air does. Honda sets up their carburetors from the factory for use at low altitudes, which makes the mixture much too rich at higher elevations.
How Do I Get My Engine to Work at High Altitudes?
The pilot jet, which provides fuel when the engine is idling, can be adjusted by screwing it into or out of the carburetor body. The main jet, which provides fuel when the engine is running at speed, needs to be replaced with a smaller jet.
What size of main jet do you need? Fortunately, Honda has already figured that out for you and offers carburetor jet kits designed for specific altitude ranges. For most engines, there are three jet options: one for sea level, one for elevations starting at 5,000 to 6,000 feet and one for elevations above 7,500 to 10,000 feet.
How Do I Change Jets?
If you’ve changed jets on a car or motorcycle carburetor, this process should be familiar. First, remove any gas from the carburetor. Depending on the model, this may be as simple as closing the fuel line and running the engine until the fuel in the carburetor is used up, or it may require draining the fuel system through a port on the bottom of the carburetor float bowl; consult your engine manual for instructions.
Once the carburetor is empty, unbolt it from the engine. The screws on top of the carburetor can be removed and the unit can be separated into two halves. On one-half, you’ll see the main jet, located in the center of the body, and the pilot jet, located near the side. Both jets are brass with a wide slot designed for a flathead screwdriver. Unscrew the main jet and screw in the jet included in the kit. Turn the pilot screw to the position specified in the service manual to match the jet kit you are using. Reassemble the carburetor and reinstall it on the engine.
Why Does My Engine Make Less Power at High Altitudes?
Internal combustion engines make power by detonating a mixture of fuel and air. Even with the correct jetting, there is less fuel and air in the engine at higher altitudes, which means less power can be made. On average, the motor will lose 1.5% of its output for every 1,000 feet above sea level.
I Use My Honda Engine Near One of the Altitude Limits. Which Jets Should I Use?
Using jets that are too small will cause the engine to run lean, leading to high combustion temperatures that will cause the engine to overheat. It’s safer for the engine to run rich using the larger (lower altitude) jet, although Honda designs these kits with some wiggle room: a 5,000 ft. kit should work at altitudes as low as 3,000 feet.
How Do I Tell if I Have the Wrong Jets?
Honda recommends that installers place a tag on or near the carburetor that notes the change in jet size. If you’ve bought a used engine that is running lean or rich and you suspect the jet has been changed, open up the carburetor and remove the jet. There should be a small number on the side that specifies the jet size; if it doesn’t match up with the jet kit for the altitude you operate at, it should be replaced.
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