Carburetor tuning issue

Tiny
CJA31186
  • MEMBER
  • 1983 JEEP CJ7
  • 4.2L
  • 6 CYL
  • 4WD
  • MANUAL
  • 112,000 MILES
I have a new carburetor that is a remake of the carter bbd on my vehicle. I have timing set to 9 BTDC. I am trying to tune the idle on the carburetor, but the highest vacuum I get is with the mixture screws all the way in (lightly seated). When the mixture screws are all the way in it reads 20 and has the smoothest idle. This should not be happening. Why is the engine idle not peaking at a point well before the idle mixture screws are turned all the way in? What is causing this to happen?
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Tuesday, October 23rd, 2018 AT 6:34 PM

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Tiny
CARADIODOC
  • EXPERT
You are making the mixture too lean. That is similar to having a vacuum leak. With a vacuum leak, idle speed will go up, but without a corresponding increase in power. In fact, if you go out and drive it, you are likely to find it will fall flat on its face when you try to accelerate off of idle.

I never heard of using vacuum readings to set the mixture. Normally we would run one screw in until the engine began to run rough, run it out until it started to run rough again, then put in it the middle of those two settings. Those mixture screws are only for at idle and very low speeds. For mid-range and highway speed, the jets and rods control the mixture. You can also tweak the mixture by adjusting the floats a little higher or lower. If those are too high now, it could explain why you cannot get it to go lean enough to misfire now at idle. It sounds like by running the screws all the way in, you are just coming up on the correct mixture, but you should be able to go beyond that to where you get lean misfires.
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Tuesday, October 23rd, 2018 AT 6:56 PM
Tiny
CJA31186
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Yes, that is exactly what it seems like. When they are all the way in it runs smooth and with the strongest vacuum. So do you think I should adjust the float lower? What is the best way to do that?
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Tuesday, October 23rd, 2018 AT 7:06 PM
Tiny
CARADIODOC
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Remove the six screws on top, then lift the cover off. The accelerator pump piston will come with it. As I recall, I think you will need to unscrew the inlet valve for clearance. The float is held in place with a 1"-long metal rod. Pull up on the silver horseshoe-shaped metal strip. Once the inlet valve is out of the way, the float just lifts up. My nifty red arrow is pointing to the tab to adjust to change the mixture.

Making this adjustment is impossible by eye because a very tiny change makes a big difference in the mixture. If you get it too high, the float will hit the cover before it pushes the inlet valve closed. That will result in gas overflowing into the intake manifold.

The rebuild kits used to come with a hard paper gauge for checking the float height, but I never had much luck with them. If you clamp something to the bowl to hold the metal strip in place, which holds the float's pivot rod in place, you can run the engine with the cover removed. You cannot adjust the mixture this way with an exhaust gas analyzer because some extra gas will enter through the accelerator pump's port. All you can do this way is observe the height the float is sitting at, and you can adjust the tang on the float to change its height. This will get a little messy because the gas will spray from the inlet valve. When I did this, I would hold the float's tang against the valve to keep the valve closed, then wait for the gas in the bowl to be used up. Wait for the engine to stall, or have a helper stop it. Release the float, then make the adjustment. Only a few drops of gas will run in once the fuel pump is no longer running.

As a point of interest, in an attempt to get better fuel mileage when driving two hours to college, I modified the rods in the carburetor for my 1972 Dart. First I sanded them down to a slightly smaller diameter, then, if you remove the piston they are attached to, you will find a spring inside it. I cut off about 1/8" of that spring. The piston is held down by intake manifold vacuum. That keeps the jets closed at idle and during normal cruising speeds. When it becomes necessary to pass a bunch of cars, you need to push further on the accelerator pedal before you lose enough vacuum for the spring to push the piston up and open the jets. This gave me better fuel mileage since the jets did not open so often. Once they did finally open, the sanded rods let more fuel in, and off I went into the sunset with my hair streaming straight back! That car with the 318 c.I. Was already fun to drive. This modification was by no means scientific, but it sure had an effect on how many cars I passed.
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Tuesday, October 23rd, 2018 AT 8:20 PM
Tiny
CJA31186
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Thanks for the information. Do you have an idea on any other tests I might do to make sure it is the float and not another issue?
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Tuesday, October 23rd, 2018 AT 8:25 PM
Tiny
CARADIODOC
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Since this is a new carburetor, I would be very surprised if the float is set wrong.

There was a procedure that involved introducing an artificial lean and rich condition, but that was something I never got involved with, and few mechanics ever did. The lean condition was created by removing a small vacuum hose, and the rich condition was to inject propane into that hose. You can find this procedure in older Chrysler service manuals from the late 1960's and early 1970's.

I think what I would pursue is what I mentioned earlier about setting the mixture screws midway between the two points where the engine runs rough. In this case you cannot reach one of those points, so I would start with each screw out one turn, then see how the engine performs. The mixture screws only pertain to idle and the transition to off-idle, not highway speed.

I tried fiddling with the one mixture screw on my 1980 Volare when that car was only a year old. What I accomplished was changing it from running smoothly all the time to bucking when lugging it in too high a ear. I think I broke off a little chunk of corrosion or debris that partially blocked the passage. It took many years for that to clear up on its own.
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Tuesday, October 23rd, 2018 AT 10:15 PM
Tiny
CJA31186
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So it was definitely something wrong with the new carb. I have another rebuilt carb, so I installed that one. Now I can tune the engine properly. I tuned it tonight. However the engine was running so rich from the other carb that my plugs are now fouled up. I took it for a test drive and it runs well. However, I do notice that after I drive it for a bit then stop at a light it starts to run rough. I went back parked then turned it off for less than 1 min and restarted to check things again and it was back to idling smoothly. I took it back out and it started running rough at the same spot on my test drive route. What could be causing this? My guess might be either the spark plugs, sticking secondary throttle, or malfunctioning electric choke? Any ideas?
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Friday, October 26th, 2018 AT 8:38 PM
Tiny
CARADIODOC
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The electric-assist choke was added specifically to address the problem of the choke blade not opening. That was due to the passage under the thermostatic spring became plugged with carbon, then that spring wouldn't get warmed up. The easiest way to tell if that is working is to set the choke when the engine is cold, turn the ignition switch on, then watch if the choke opens. It should be fully open in three minutes, although that might take a little longer when the engine isn't running.

Be sure the intake air preheat hose is attached to the air filter housing and that system is working. A vacuum motor opens the passage to that tube in cold weather.

If your engine uses an EGR valve, sticking open due to a chip of carbon is a good suspect. That valve must never open at idle. When it leaks, it will cause a rough idle. You can loosen it, then slip in a thin sheet metal shim to block it temporarily as a test. For intermittent problems, disconnect the vacuum hose at the valve and plug it. That way it will never open. If the rough running occurs, it is due to something else.
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Saturday, October 27th, 2018 AT 11:09 AM
Tiny
CJA31186
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I will take a look at the choke. The EGR is attached to the manifold but is plugged cause I have modified the engine. Vacuum caps are on the ports. Even if it is plugged off, can it still cause a problem? If I remove it I need something to plug the holes in the manifold.
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Saturday, October 27th, 2018 AT 11:23 AM
Tiny
CARADIODOC
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That's where the thin sheet metal shim comes in. Some designs don't allow for that. This works on valves that have recessed valves and the mounting surface is perfectly flat.
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Saturday, October 27th, 2018 AT 11:40 AM
Tiny
CJA31186
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So do you think its causing a problem even though there's no vacuum lines connected to it and the ports are capped? If so, I'll start on the egr as there is a high probability it could be causing this issue.
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Saturday, October 27th, 2018 AT 11:48 AM
Tiny
CARADIODOC
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The rough idle can be caused by the valve not closing off completely, then exhaust gas can sneak into the intake system and dilute the fuel. That usually isn't intermittent though when the actuating hose is already disconnected. If the valve is sealing, it isn't likely to suddenly start leaking while you're driving. They typically open up when they're supposed to, at highway speed, then they fail to reseal when you slow down. That is when they stick partially-open and cause the rough running.
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Saturday, October 27th, 2018 AT 5:10 PM

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