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Gas Boat Engine Tuning

Gas Boat Engine Tuning – Terms

Two terms that will be used to describe the tuning process are Rich, and Lean. When Rich is referred to, it describes a fuel mixture that has excessive fuel in it. To richen a fuel mixture on the carburetor of the engine, adjust the mixture needle by rotating the needle counter-clockwise .
When Lean is referred to, it describes the fuel mixture has an insufficient amount of fuel. ` To lean a fuel mixture on the carburetor of the engine, adjust the mixture needle by rotating the needle clockwise.

Gas Boat Engine Tuning – Break In


Engine break in is a very controversial subject. There are many break in techniques out there including the one in your engines manual. However one of the relatively more simple methods will be described here.

Be certain to use the correct oil and ratio as described in your manual.

Each tank represents 10-15 minutes of run time

Tank 1 and 2 – Run the engine up to around half throttle, while maintaining a slightly rich needle setting. Do not exceed half throttle during the first tank. Vary the throttle constantly to keep the engine operating at various speeds.

Tank 3 and 4 – During the course of tanks two and three, slightly lean out the mixture to achieve good performance. Run the engine up and down, varying throttle much like the last two tanks. However, on these tanks occasionally hit higher RPM’s using wide open throttle.

Tank 5 and 6 – For the last two tanks of fuel, the engine is in its final break in steps. During the duration of these tanks, run the engine harder and allow the engine speed to be maintained for longer periods of time. For the first few minutes of the tank keep the time periods shorter. As the end of the sixth tank is burned, run the engine normally using the full power band.

The break in process is complete.

Needles Found On a Gas Engine Carb

Gas Boat Engine Tuning – High Speed Needle:

 The High Speed Needle often referred to as the HSN. It is best viewed on the carburetor as the taller or longer valve stem. The High Speed Needle is the main control of the fuel mixture in the carburetor.Leaning or Richening this needle will also Richen or Lean the Low Speed needle. The High Speed Needle just as the name implies, controls mainly the top speed mixture of the engine. This is when the engine is in Higher or peak RPM’s. An incorrectly tuned High Speed Needle will not allow the engine to reach peak performance, and it may also harm the engine.

Gas Boat Engine Tuning – Low Speed Needle:

 The Low Speed Needle is often referred to as the LSN. It is typically located 90 degrees from the carburetor venturi opening. The Low Speed needle like the name implies, controls the amount of fuel that enters the carburetor at low RPM’s or slow speeds.

Gas Boat Engine Tuning – Tuning the Engine

Gas Boat Engine Tuning – HSN – High Speed Needle

The best way to tune a gas engine is to run it. Start up the engine and warm it up. Run it for several passes at wide open throttle.
Carefully listen to how the engine responds. If it is bogging down or sounds like it is running lean, richen the High Speed needle valve, If the engine starts to make noises such as gurgling or suggesting to rich lean the mixture.
It is important to listen to the quality of the engine sound. Sound and Performance is your most apparent suggestions to how the engine is tuned. A properly tuned engine will sound strong and perform great.
If you are unsure whether to lean or richen the mixture, it may be a better idea to start by trying to richen the mixture. If performance suffers, bring the needle back to the setting it was at prior to adjustment then go the other way. Always make adjustments in small increments. I would suggest no more than about an 1/8th of a turn.

Gas Boat Engine Tuning – LSN – Low Speed Needle

Once the High Speed Needle is adjusted, it is time to move on to adjusting the Low Speed Needle.
To start adjusting the Low Speed Needle have the engine up to operating temperature and run the boat at its slowest speed. If it slowly bogs out and sounds rich with a gurgling type of noise, lean the Low Speed Needle. If the RPM begin to rise with no input of throttle, richen the Low Speed Needle. Another test for proper tune of the Low Speed Needle is the acceleration test.
Run the Engine around its slowest running speed for around 10-20 seconds. After this time, punch the throttle.
If the engine quits and flames out, adjust the Low Speed Needle according to the noise it makes as described already.
If it’s quite difficult to determine which way to go, try punching the throttle at a slower rate. Another sign of a rich Low Speed Needle under heavy acceleration will be more smoke than usual with a slow acceleration rate. Once the engine hits higher RPM, it will begin to accelerate quickly.