Yes, the signal on the stator is actually a sine wave that peaks at battery voltage, but I don't know many people who probe around the engine compartment with their meter set to AC. The alternator is simply a filterless 3-phase drive with a rectifier, so yes you will see a sine wave on each stator. But most people keep their meter on DC volts when they are working with this. And when you look at the stator terminal measuring DC, you will see a 0-Hz component between 6 and 7 volts.

 

Now I have new problems.

The carb is back on the engine, and everything went smoothly. I replaced some broken parts (the choke pull down actuator, for example) and put everything back in. The first time I tried to start it, it fired right up immediately. I was happy at first, till I heard the racing engine. It's going at least 3000 RPMs. I put the idle mixture screws on the bottom of the carb all the way in (not too hard). And it still is racing. I have trouble starting it sometimes. I found that by flooring the pedal, I could start it after a few cranks. But, it's off to the races afterwards. The idle screw on the throttle isn't even engaging. I don't know what else to do to lower my truck's idle.

Is there something obvious I'm forgetting here? The vehicle seems to respond to the choke. If I move the choke manually, if I close the valve, it seems to idle smoother and faster, but still way too fast.

I've checked around for vacuum leaks, and haven't noticed any so far. But, I hear a loud hissing. Is that from the intake itself? If I cover the choke plate with my hand, the hissing diminishes.

Any help will be greatly appreciated.

Thanks,
Dan

 

Alright, I just went out and inspected the truck again. The choke seems to not be working. It wants to stay in "fast-idle" mode. If I manually take it out of fast-idle, it will go into an idle that I can adjust. But, if I give it any gas, it goes right back into fast-idle. I'm sure a number of adjustments are out of whack, knowing my luck. I tried to follow the instructions with the carb kit, but apparently I messed it up. Those instructions weren't the most clear, I must say. What can I do to prevent it from going into fast-idle after giving it gas?

 

Did you hook up the wire to the heater? If so, does the choke eventually open? The choke will never open fully without the heat to the spring.

You could have the choke spring too tight, the heater for the coil could be faulty. You can't make adjustments to the idle speed and mixture screws unless the choke is open fully.

For the moment, back off the choke spring until the choke plate is fully open. Warm the engine up and set your curb idle and idle mixture. Check the choke heater with an ohm meter. I don't remember the spec, but they usually fail open, so if there is contimuity it is ok. Make sure the metal ring holding the spring in place is clean on the back side and the metal tabs where the ring clamps are clean. This is the ground path for the heater.

 

Your fast idle index is too high, or the choke is not working. How long are you letting it warm up? It will take a little bit. If the choke opens and you still have this problem, then you need to set the fast idle index and choke pulloff. You should check these anyway, with a new carb rebuild. Here is the procedure:

This has to be made with the truck cold, so let it sit overnight. Loosen the three choke retaining screws and rotate the choke 90 degrees rich, then tack it back down. On the choke pulloff, push the diagraphm all the way down so that the pulloff opens the choke slightly. Somewhere in your rebuild sheet, there will be a specification for pulloff clearance, something like 5/16". You need to get a drill bit of whatever size the sheet calls for, and gauge it between the slightly open choke plate and the choke tower, on the long edge, on the back-side. If the clearance is not within spec, there is a small screw within the choke housing that you can adjust to get it within spec.

Once you have the pulloff at the right clearance, you need to set the fast idle index. Let off the choke pulloff so that the choke is fully closed again. Now with one hand, depress the choke pulloff again so that the choke pulloff pulls the choke open to spec, then with your other hand, open the throttle (on the drivers side) just slightly, then close it back up. You can then take your hand off of the choke pulloff. Look down underneath the choke housing and you will see a small tab on the passenger-side end of the throttle shaft, resting on a small cam. This is called the fast idle cam. There is a small V on that cam. You want that tab to be directly on that V. If it is not, there is a small hex-head screw on the plastic fast idle arm that you can adjust. It is trail and error - you will have to repeat the above process over a few times until you get the fast idle tab to line up on the cam.

Once you have the index set correctly, you can use the fast idle speed screw (passenger side, rear) to control the fast idle speed. Put the choke back to the normal position and tack it down.

There are three stages to fast-idle: Startup, fast-idle, and normal operation. When you first pump the gas in the morning before you first start the truck, the throttle tab is on the highest step of the cam. Once you crank the engine, vacuum pulls the choke pulloff to let just enough air into the throat of the carb for the motor to run. The choke is now in the pulloff position. Once you tap the gas again, the throttle opens and the fast idle linkage falls, and the throttle lands on the V. As the engine warms up and the choke opens slightly and you drive, the fast idle cam falls with the choke, until the choke is all the way open, and the throttle tab clears the linkage. This is the last stage - normal operation. The idle speed is now controlled by the curb idle screw on the driver's side carb.

As for the black choke cap itself, make sure you have voltage on the spade connector harness with the engine running, and make sure that the tang inside the choke actually goes into the choke lever. Other than that, the only adjustment to the cap itself is that the notch lines up as I specified in my other post. The rest of the settings deal with fast idle and the pulloff. As for the hissing on the throat of the carb - that is the sound of the engine trying to suck air through the restricted choke; that's normal. But that's the only other place you should hear hissing - did you use a gasket under the choke cap? The choke housing receives manifold vacuum and should have a gasket (included in the kit) between the cap and the housing.

 

Well, after scratching my head for a bit, I found the fast idle and brought it down to a reasonable level. I re-adjusted the choke pull off and that seems to work properly.

But, the choke plate doesn't open up all the way still. There is no voltage going to the choke. The resistance of the choke is 4.7ohms, which seems like it would do the trick. The wire was probably not hooked up after the swap to an aftermarket alternator. I slapped a big one on there, because the vehicle will be enduring hours and hours of idling. It's the same type of alternator they use for parade trucks. If I have to, I'll run a new wire. But, I'm sure I can get power to the choke.

Thank you very much for your detailed posts. They have helped me out immensely. I've definitely learned plenty about carbs in the past week.

 

Just to keep people informed of my progress, I checked the electric choke for function. Despite having a 5 ohm resistance, it doesn't seem to react to power. I put the choke across the batter terminals and watched the spring. After a good 30 seconds the spring had not moved one bit. It wasn't warm either. This is the proper way to check an electric choke, right?

Other than that, things are going smoothly.

 

No, it's not the correct way. Battery voltage and stator terminal voltage are not the same thing. They are two different signals. The choke is like an inductor, and if you put DC across it, it's just going to fall to a short. You need to wire it up the right way first. Also, it's going to take way longer than 30 seconds to open. It could take anywhere from 5 to 10 minutes, depending on the temperature.

You will need to look at the back of the alternator and see if there is a wire coming off the S terminal. As a side note, just because you have a choke that has the spade terminal, doesn't mean the vehicle was wired for one, so someone could have swapped that choke on. Glad to hear things are looking up for you though.

 

I had assumed the choke is like a resistor. Yowza! I'll give it the stator voltage and see what happens. I'm going to run it off of the "S" terminal properly. Phew, I'm glad I mentioned this to the forum before buying a new choke.

 

My apologies, it is a PTC resistor - I mislead you. Wire it up to the S terminal like you say and see if that fixes it. Either way, just give it a little time to see how long it takes to open.

Is this the stock Motorcraft electric choke? I've read that the electric chokes that also run off hot air have a temperature sensing disc in them that cuts off the electric heating element at temperatures below 60 F. That could be part of it.

 

Well, the aftermarket alternator must be different than stock. The S terminal is putting out 14VDC and 100mVAC.

I'm letting it run for a while right now, and we'll see if there is a difference. so far, after about 5 minutes, I don't see a change.

My choke DID use hot air as well. But the choke stove tubes were rotted out, so I just plugged the vacuum lines and hope to use the electric choke

 

After 20-25 minutes of idling the choke plate was still only open a bit. I pulled the vacuum to the choke pull off, and the choke plate closed tightly and my engine died. So it seems my electric choke isn't working. Well, that gives me something to work on replacing.

I'm still able to drive around, but the engine lopes and misses during idle. The loping and missing may also be due to bad wires or plugs. I'm going to change those soon, just in case.

 

After 20-25 minutes of idling the choke plate was still only open a bit. I pulled the vacuum to the choke pull off, and the choke plate closed tightly and my engine died. So it seems my electric choke isn't working. Well, that gives me something to work on replacing.

I'm still able to drive around, but the engine lopes and misses during idle. The loping and missing may also be due to bad wires or plugs. I'm going to change those soon, just in case.

 

The loping is due to the overly rich mixture caused by the partially closed choke plate.
The choke pulloff should pull the choke OPEN slightly with vacuum applied.?? Just a thought, is there a screw holding the lever onto the choke shaft? The lever that the linkage connects to?
Earlier carbs had the lever held on with a screw, later they were peened onto the shaft. If the lever is on facing the wrong direction, the pulloff would close the choke. The spring would also open the choke plate when cold and close it when warm. This could be a real "gotcha" after reassembly, if you're not familiar with how it's supposed to work.

I'm not trying to step on you fmc400. You're doing a great job of explaining the choke operation and setup!! (I just recall making this very mistake many years ago) I wish I had the patience to type all of that out like you did..
I wasn't aware that the choke spring heater would not work with DC applied. I guess I never had a reason to think otherwise.
You say the heater is a PTC resistor. I'm not familiar with that term. Can you expand on that a bit?

 

Hey no problem! Actually I believe I was mistaken when I said the choke won't run off of DC, so scratch that. PTC stands for positive temperature coefficient, which means it gets more resistive as it gets hotter. Most resistors are NTC (negative temperature coefficient), which means they get less resistive as they get hotter. PTCs are good for thermostats because they draw less and less current as they get hotter to the point where they draw very little current, so they effectively shut themselves off until the temperature drops.