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post #1 of (permalink) Old 12-09-2001, 12:05 AM
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Carb Rejetting Help (TeamRush)

Hey TeamRush,

Thanks a bunch for your help back there --- I went and got that information that you requested and the results are...

Idle Vacuum: 17.5 - dead steady.
Operating Vac: 19.2 - also steady (2500 RPM)
Normal Operating RPM: 2750
Top RPM: 5500
Engine: 1975 AMC 360, with 10,000 miles since complete rebuild involving 0.030 over bore, crank ground 0.010. Otherwise completely stock specs, including cam. The carb is a Motorcraft 2150 (Altitude Compensating) 1.21 version (I believe that's venturi diametre) jetted with "55F" jets, but I also have 53's and 47's on hand.

This engine is in an 1983 CJ7, heavily modified for off-roading. So my requirements are: smooth idle to bottom end torque, not high RPM screaming horsepower.

Thanks again,


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post #2 of (permalink) Old 12-09-2001, 10:21 AM
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Re: Carb Rejetting Help (TeamRush)


1. Run the vehicle low on fuel.
Go get a tank full of 90 octane so we have a base line.

Stop by the auto parts store and get a couple of sets of extra spark plugs.
(the $1.25 each Autolite plugs work just as well as anything else...)

Don't forget two or three cans of carb cleaner.
Use name brand carb cleaner, not WD-40 or 'penetrating oil'.

2. Set your idle adjustment by hooking up a vacuum gauge to the carb base plate.
Turn in both idle screws until they bottom out LIGHTLY, DON'T CRANK DOWN ON THEM,
IMMEDIATELY TURN THEM OUT 1-1/2 turns. (heat expansion can lock them into place when all the way in...)

Start the vehicle, and turn the screws in TOGETHER 1/4 turn at a time until you get lean misfire.
Back the screws out TOGETHER until you get the highest vacuum reading at idle.
Both screws should be backed out the same amount when you are done.

3. Mark off a 1/4 mile 'drag strip' near the pace where you work on your vehicle. This will be your wide open throttle testing ground.
It's a good idea to make it somewhere there are no police or neighbors to get upset around.

4. Have someone work the throttle pedal for you while you look down the carb throat (with the engine off).
When they push down on the throttle pedal, you should see a thin, clean stream of fuel being pumped into the venturis.
(If there are any bubbles, sputtering, delayed start, ect. you have an accelerator pump problem)

Make sure the throttle blades open fully, straight up and down at a perpendicular ( perfect 90 degrees) from the base plate.
If they don't open, or the blades go over center, adjust the linkage until they do the perpendicular thing.

Use carb cleaner on any and all air bleed holes you find in the throttle bores. Don't use wire brushes, just carb cleaner in the can.
Clean them as good as you can. Starting the engine and spraying them is just fine, just watch out for back fire.

5. With the engine running, spray carb cleaner all around the base plate of the carb, around the throttle shafts, all the vacuum line connections, ect.
You are looking for the idle to go up or down. That would indicate a vacuum leak. If you find one, fix it first.
(Although more volatile, people have used ether and propane for leak testing before)

6. Change the spark plugs, use new ones with white center electrode insulators.
Make a full throttle pass as close to the place where you changed plugs as you can.
Kick the vehicle into neutral and turn the engine off when you end your pass. (To preserve the plug reading)
DO NOT USE CHOKE TO START ENGINE... This will foul the reading before you even get moving!

Pull #1 and #8 plugs and have a look at them.
You are looking for a light 'caramel' tan color. (unleaded gas doesn't leave the color that leaded fuel used to, so you have to look close)
Give it hell! Drive it at full throttle (your upper RPM limit) for as long as you can in your 1/4 mile pass, and get to high gear, you want it pulling.
(Doesn't hurt to add weight to the vehicle to load it even more)

Black with soot, Ignition or Compression problems.
(Compression problems could be hole in piston, bad rings, bad valve seats, bent valves, leaking head gasket, ect...)

Black, just way too rich.
Dark brown, just a little bit too rich.
Light Brown, just a smidgen too rich.
Very Light tan, just perfect.
'Bone' Yellow (Almost White), just a tiny tad too lean, but probably all right.
'Dirty' White, Too lean.
White, WAY too lean.

Make two or three passes, checking the front and rear (#1 & #8) plugs each time, then pull some plugs in the middle and see what they are doing.
Take rags, and an extension for the plug socket. (the plugs are going to be very hot)
Take a 'Sharpie' magic marker, you will need to mark what hole the plugs came out of. (you WON'T remember, trust me!)

7. Then, drive around on the highway a while (1/2 hour or so) and pull plugs at random, seeing what they are doing (part throttle operation).
If the Plugs look OK, you probably have the correct jetting.

The idle circuit, (controlled by the idle screws) is where you will do the vast majority of your driving.
Idle circuit controls up to about 1,500 RPM on most vehicles, then the primary (jetted) circuit takes over.
There is usually some kind of transfer circuit from Idle to Primary in most carbs, and these can be a source of problems if they are dirty.
Use a good name brand carb cleaner on the parts you can see, and a good name brand carb cleaning fuel additive in the tank.
Lots of good quality fuel filters are a must, and change them often.

Good indicators are:
Your tale pipe. If it's black and has a lot of soot, you are probably too rich on either idle or primary.
Your plugs. If they are black after a couple of hundred miles, you are probably rich on the idle or primary.
You gas gauge. If you don't get at least 16 or 17 miles to the gallon, you are probably rich.
Your nose. If you can't stand to be in a closed garage with the vehicle running, without your eyes and lungs burning, you are probably rich.
Your eyes. If you hammer the throttle, and a black fog pursues you down the road, you are probably rich.

You can also buy a Rich/Lean indicator that uses an O2 sensor in the exhaust head pipe that takes the guess work out of tuning. They are about $100, and available at or

So many cats, so few recipes...
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post #3 of (permalink) Old 12-09-2001, 10:43 AM
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Re: Carb Rejetting Help (TeamRush)

Hey thanx alot TeamRush,With an answer like that,I guess thats why they call you the grand pooh-bah [img]/wwwthreads_images/icons/smile.gif[/img]

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post #4 of (permalink) Old 12-09-2001, 11:23 AM
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Re: Carb Rejetting Help (TeamRush)

Hey TeamRush,

I tried to find a O2 rich/lean indicator/tester, but there's a lot there, and I didn't know what I needed.

Do you suggest a specific item that would work for me (in the $100 range)?

Thanks again for the help,

post #5 of (permalink) Old 12-09-2001, 11:55 AM
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Re: Carb Rejetting Help (TeamRush)

Better than the 'Grand Doo-Doo' I guess [img]/wwwthreads_images/icons/tongue.gif[/img]....

Let me know how things turn out...

So many cats, so few recipes...
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