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Posted on May 30, 2010
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Carb and throttle adj.

I have a 82 wellcraft with a 260 mercruiser i just got a new carb rodchester 4bbl i start the engine it idles at 2000 rpm i need to know how to adj. the carb and cable

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fordexpert

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  • Master 5,692 Answers
  • Posted on Jun 04, 2010
fordexpert
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Joined: Apr 20, 2009
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Here is how to do it. First disconnect the throttle cable to the carb and let it hang loose. Make sure the outdrive is in neutral and the boat is secured to the dock or on a trailer with a water ****.

Pull back on the throttle by hand directly on the carb throttle shaft. Observe the Choke cam on the opposite side of the Carb. Let go of the Carb throttle shaft and see where the Stepup Cam is set. The Choke plate should be closed most of the way. You will not have very much time to make an adjustment.

Start the engine and let the Choke work and observe the RPM. There are usually adjustments you can make to put the Stepup Cam on a lower step by bending the Link Rod if it is running too high in Choke. Sometimes the Cam is left alone and a screw similar to the Curb Idle screw on the Throttle side just needs reduction to ride lower on the Choke Cam.

After the Choke is warm, it disengages with a pull on the Throttle. Now you are using the Throttle side of the Carb to operate the engine RPM. You want a low RPM for sake of the outdrive gears and NO WAKE zone. The adjustment screw opens the Throttle shaft, You set it for the Manufacturers Spec. probably 550 RPM.

The Throttle cable is now adjusted so it pushes slightly against the Carb. This forces the throttle closed. The Throttle cable has threads and a sort of Knurled knob which runs on these threads. This device is used to lengthen or shorten the cable so that pressure is applied against the Throttle.

When the outdrive is in water you will has resistence on the Exhaust. This will slow the RPM. If the motor stalls you will need to adjust the motor to idle Specs while in the water.

For best economy, find a Vacuum port and attach a Vacuum guage to it and adjust the air mix to the highest Vacuum reading then turn in the Mixture screws slightly until the vacuum starts to drop.

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I need to adjust the idle on my 1988 celebrity boat. It is a 140 hp 3.0 l mercruiser engine. I am not sure what screw to turn and which way. it only idles at 500 rpm. i believe it needs to be at 800 to...

First, as you look at the motor you will see cables attached to the carb. When you have the motor off move the throttle cables forward and see which cable moves the carb part. The other cable moves the outdrive gears and thats not what you want.

When you find the correct cable unhook it from the carb. Now the carb is free to move by hand. When you pull back on the carb "arm" you will find it is spring loaded. The carb arm will return to where it is set. BUT it must be done after the engine is warmed up or you will be fighting the choke.

As you let the arm return several times you will see a screw that part of the arm always hits when it stops.
Do this until you see this contact. It is this screw which is turned to make the arm stay open wider.
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You do not want the engine to run very fast at idle. It causes the outdrive to BANG into gear and will cause problems with docking and NO WAKE zones. You may increase it to 600-625 RPM. There are also different effects with different props concerning idle speed.

Finally, after you set the carb where you want, you need to adjust the cable you removed earlier so it is pressing the carb arm slightly and helping the return spring close the throttle.

Another reason for lower RPM at idle is that under the right circumstances the fuel can bleed into the engine because the throttle is cracked open too far. This can cause a wash down of the cylinders and you end up with gas in the oil.
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RPM at idle, and T.D.C degree

5.0 L with CFI = 550 rpm

5.0 L with CFI = 10 degrees @ 550 rpm

5.0 L with a 4bbl carb = 700 rpm

5.0 L with a 4bbl carb Calif. = 10 degrees btdc @ 800 rpm

5.0 L with a 4bbl carb Fed. = 12 degrees btdc @ 800 rpm

cheers mate !
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