Page last updated July 17, 2008



Mercury Carburetor Adjustments

For carburetor adjustments to these motors, the repair manual suggests to loosen the idle screw to 1 1/4 turn out. The actual procedure will differ or the amount that is suppose to be out is slightly different for each different motor.

I find that the process is the same. I will go over the process with a few steps below. The steps assume the proper plugs are in palce.

For my particular engine, the idle must be adjusted while in FORWARD gear and with water to act as resistance. If you adjust the idle while in neutral, you will succeed in setting the idle but it will only idle while in NEUTRAL. When you switch gears, it will stall due to the additional resistance, so that is why it needs to be adjusted while in FORWARD.

SOO..... Make sure you adjust idle while in FORWARD and that you also have WATER to act as the resistance (flush device will not provide resistance). You can use a large rubbermade trashcan filled with water. Ensure the hose is still running as your engine will expel water from the trash can through waterpump / peehole. It is best to adjust the carburetor while your boar is in the water.

1. Loosen idle screws counter clock wise or from a lightly seated position 1 to 1 1/4 out.

2. Start up engine and ensure the engine is idling properly (not backfiring, warmed up). If it is not correct, you may have to adjust the idle stop screw. My Merc 650 is suppose to idle at around 600 RPMs. Check the service manual for your particular engine.

3. Counting the number of times you turn, turn the idle screw Counter Clock wise or loosen it further until the engine seems as if it is about to die out due to fuel and air mix ratio being too RICH (too much fuel).

4. Then, turn idle screw clockwise / tighten or close until the mixture starts getting to lean (too much air) and the engine will stutter / backfire, seem as if it is about to give out. Loosen the idle screw from that point 1/4 to 1/2 turn out.


Please note I take no responsibility if you burn your engine out. It is better to be slightly rich than lean. If you are running lean, you run the risk of not supplying enough lubrication to the pistons. You be the judge of what is too lean. If the plugs are white, or no fuel / oily substance on the spark plug, then you are more than likely lean. Again better to be rich than lean.

For the idle, you will need a proper tool to find the correct idle reading unless your motor is equipped with a tachometer. The Mercury 650E has a wire but was needing to be rewired in order for me to use a modern tachometer. See Tach page for more info.

The sound that your engine makes while idleing in neutral will be based on numerous factors. Here are a few.

Propeller size - the larger the propeller size, the more resistance it will transfer to the engine, so the final idle sound may be high if your prop is too big.
Propeller pitch - the higher the pitch, the higher the resistance and vice-versa; this also may increase the RPMs and isle noise if you have an incorrect prop size.

If you are in the process of buying a new prop, try your local marine dealer; they may have different size props for you try before you commit to buying. This is the best option as it gives you a chance to determine which one offers the best match for your engine / hull design. This will allow you to increase fuel efficiency and performance based on your specific needs.

This engine should be idled at around 600 RPMs. Any higher and you risk damaging gears when shifting unless you have adjusted the idle to match the new prop.. When you shift, you should be as close to the recommended idle as possible. If you have a prop that has a pitch that is too high, you will have an idle that is higher in neutral and correct while in first gear. SO essentially, when you switch from neutral to first, you will be shifting gears while at high RPMs. This is bad for you gears.

This engine's prop size is suppose to be 10.5 Diameter with an 11 pitch prop. Any adjustments may cause uneeded stress on gears due to higher RPMs while shifting. Try to stay near the recommended size and pitch unless you have adjusted idle RPMs to match the new propeller..

Rough Start

If you find your engine to be tough to start:

  1. make sure you are squeezing priming bulb enough times.
  2. Ensure you are ventilating fuel can; if you have it sealed, the vacuum will suck the fuel back to the can.
  3. Never spray anything into the throat of the engine to help startup ...this will usually remove the lubrication the fuel is providing.



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