Help Needed For Fast Idle Adjustment For a Motorcraft 2150 Carburetor

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Oct 28, 2017
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Location
Wisconsin, USA
I installed a new 2150 carburetor that I purchased from Amazon for my 1986 Jeep CJ7. The out of the box quality of the carburetor is excellent. It installed easily and I have all new vacuum lines.

The engine started immediately but goes to 2,800 rpm so I shut it down for fear of damage.

I believe there is a setting I need to make on the fast idle adjustment and need help!


Additional Facts:
  • This is the second 2150 I tried from Amazon. The first one did the exact same thing so I’ve concluded I need to make an adjustment for the carb to work.
  • The carburetor I replaced is a Motorcraft 2100, almost identical to this 2150. It does the fast ide perfectly at 1,500 rpm but can no longer hold low idle when warmed up even after a rebuild.

I'm grateful for all ideas and directions you can provide me on how to set this fast idle.
 
Assuming this carb has a choke assembly….there is a cam and an adjustment screw on the right side that can be adjusted. Is there an electric solenoid on the left side of carb? Idle speed is adjusted there also. You’re sure your throttle linkage isn’t the issue? 2,800 would have the throttle plates 40% open all the time. Back off the chock coil to release any tension so the chock flap is loose and standing straight up. Disconnect the actual throttle linkage and look down inside the carb to see where the throttle plates are.
 
Hi PontiacHO,
It does have a choke assembly and a cam and an adjustment screw on the right side (passenger side) beneath the electric choke. There is no electric solenoid on the opposite side.

The pictures are of the old carb but the new one is setup the same way.

The question is which way do I turn the screw to lower the fast idle. Do I turn the screw inward toward the cam or away from the cam?

Thank you for your help!
 

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Turning the screw out will let the throttle plate close more. But since there is only one screw it should be adjusted for normal idle after warm up. Make that adjustment then the fast idle will be what it is. This is not an elaborate system.

Usually it is designed to drop off of the highest spot on the cam the first time the gas is pressed after starting.
 
Looks like an electric choke. Usually the black housing gets rotated until the choke blade just fully closes then one mark more to the closed side then lock it down. Set idle speed with choke fully open and cold idle will be set by the cam.
As said cold idle will drop down after hitting the pedal to a lower warm up speed. Speeds drop as choke opens.
Cold choke setting needs to be correct so choke pull off will operate correctly after starting to open the choke and idle speed cam goes to its correct running position and steps down until fully warmed up position.
 
PontiacHO, mk378 & repairman54,

Thank you for all the detailed actions to take. I'll remove the carb over the weekend, make all checks and adjustments and reinstall. I'll keep you posted!
 
This can all be done with the carb installed. Some of the adjustments are with the engine running.

Make sure the carb base gasket has a good seal to the manifold. If there's an air leak it will be a problem to get a good idle.
 
Turn the screw to the left to reduce idle speed on the choke cam. Most can be adjusted with carb mounted on the intake. Just a suggestion…back off that adjustment screw about 11/2 turns and then crank it cold. Maybe a starting point and at least you’ve reduced that extreme high RPM scenario.
 
I'll give it a shot with the carburetor installed. On an in-line six, getting to the fast idle screw is really difficult. Its a long reach from the passenger side to the carb and its buried under a heat shield. Some scaffolding across the engine bay would work really well!

Thanks again to all of you for the precision directions.
 

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If you can't get it ironed out look at going with a Weber carb - I have had one on my 78 CJ7 with the same 258 for the last 30 years and its a great carb - easy to adjust and runs better than the stock one it had.
 
Warm up the Jeep so that the choke plate is fully open. With the engine running, open the throttle slightly so that you can lift the fast idle cam to the second highest step and opposite the fast idle screw. Stop holding the throttle open. The fast idle screw should now be touching the fast idle cam. Move the fast idle cam so that the fast idle screw is on the second highest step of the cam and against the highest step. Then adjust the fast idle screw to get 1500 RPM. Clockwise to increase RPM; CCW to decrease. After you set the 1500, when the throttle is blipped, the fast idle cam should drop and the engine speed should return to normal idle. I worked in St. Louis at Carter Carburetor R&D from 1967 until 1972 and that was the procedure. It will work well on your Motorcraft.
 
Warm up the Jeep so that the choke plate is fully open. With the engine running, open the throttle slightly so that you can lift the fast idle cam to the second highest step and opposite the fast idle screw. Stop holding the throttle open. The fast idle screw should now be touching the fast idle cam. Move the fast idle cam so that the fast idle screw is on the second highest step of the cam and against the highest step. Then adjust the fast idle screw to get 1500 RPM. Clockwise to increase RPM; CCW to decrease. After you set the 1500, when the throttle is blipped, the fast idle cam should drop and the engine speed should return to normal idle. I worked in St. Louis at Carter Carburetor R&D from 1967 until 1972 and that was the procedure. It will work well on your Motorcraft.
Thank you so much for these detailed instructions.
 
Warm up the Jeep so that the choke plate is fully open. With the engine running, open the throttle slightly so that you can lift the fast idle cam to the second highest step and opposite the fast idle screw. Stop holding the throttle open. The fast idle screw should now be touching the fast idle cam. Move the fast idle cam so that the fast idle screw is on the second highest step of the cam and against the highest step. Then adjust the fast idle screw to get 1500 RPM. Clockwise to increase RPM; CCW to decrease. After you set the 1500, when the throttle is blipped, the fast idle cam should drop and the engine speed should return to normal idle. I worked in St. Louis at Carter Carburetor R&D from 1967 until 1972 and that was the procedure. It will work well on your Motorcraft.

Warm up the Jeep so that the choke plate is fully open. With the engine running, open the throttle slightly so that you can lift the fast idle cam to the second highest step and opposite the fast idle screw. Stop holding the throttle open. The fast idle screw should now be touching the fast idle cam. Move the fast idle cam so that the fast idle screw is on the second highest step of the cam and against the highest step. Then adjust the fast idle screw to get 1500 RPM. Clockwise to increase RPM; CCW to decrease. After you set the 1500, when the throttle is blipped, the fast idle cam should drop and the engine speed should return to normal idle. I worked in St. Louis at Carter Carburetor R&D from 1967 until 1972 and that was the procedure. It will work well on your Motorcraft.
Is there any way to make some of these adjustments without the engine running?

Currently it's screaming at 2800 rpm when it starts and I don't want to be working on it running like that.
 
First just loosen the screw a bunch and if necessary use the gas pedal to keep it running until warmed up. If there's another stop screw on the other side use that for warm idle, making sure the fast screw isn't touching the cam at all.

Realize though that 2800 isn't going to damage anything, turns 2800 on the road a lot.
 
First just loosen the screw a bunch and if necessary use the gas pedal to keep it running until warmed up. If there's another stop screw on the other side use that for warm idle, making sure the fast screw isn't touching the cam at all.

Realize though that 2800 isn't going to damage anything, turns 2800 on the road a lot.
Got it . . . thank you!
 
Hi Everyone,

Good news...I got the fast idle set to 1,500 rpm.

But it is running extremely rich, both at the 1,500 rpm and at 700 hot idle.

Are the idle mixture screws too far out? They're set to 1.5 turns out.

Thank you!
 
Hi Everyone,

Good news...I got the fast idle set to 1,500 rpm.

But it is running extremely rich, both at the 1,500 rpm and at 700 hot idle.

Are the idle mixture screws too far out? They're set to 1.5 turns out.

Thank you!
Run them all the way in until they touch the seat, then back them off the seat half a turn. Once you lean it out though, the RPMs will want to go higher so you'll probably have to repeat the process of setting the fast idle RPMs
 
Run them all the way in until they touch the seat, then back them off the seat half a turn. Once you lean it out though, the RPMs will want to go higher so you'll probably have to repeat the process of setting the fast idle RPMs
Great . . . I'll give it a try tomorrow. Thank you thastinger!
 
The initial turns out on the mixture screws is called a bench setting. It's just to get the engine started. After the engine is running and fully warmed up, you would adjust the idle mixture with either a vacuum gauge hooked up to the intake manifold full vacuum port, or with a tachometer. You're looking for the highest vacuum reading during idle. If using a tachometer, you're looking for the highest rpm during idle. If the engine is set either too lean or too rich, the rpm will drop and that's an indication of inefficient combustion. There's quite a wide margin between too lean and too rich, so it's not like the engine will shut off at the slightest adjustment. You can also use your ears and your sense of feel to a certain degree. Your ears will tell you when the rpm increases and your sense of feel will tell you when the engine smooths out and stops shaking from inefficient combustion.
 
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