Another question on timing-

[SkinnyG]

If your choke is setup properly yes it will automatically kick down for you.. That is the whole point of the choke.

How does the stepped/weighted part of the fast idle cam overcome the dual throttle return springs to kick down automatically for me?  I don't see how this works without touching the gas pedal manually.

[Greybeard]

Ps. I'm convinced manifold vac is where its at!

http://forum.73-87chevytrucks.com/smforum/index.php?topic=32085.msg270444#msg270444

Care to say that again please.. You mean the idle dropped on its own without user intervention Of course it did. That is how they are SUPPOSED to work

I still disagree that the ckoke should fall off on it's own with two caveats...1) the carb has a kicker solenoid to knock the choke off the high step of the carb; 2) the carb is not a Holley. OK, OK, I know we have been talking about Q-jets here and the Holley is a different beast entirely by using a vacuum pull-off canister. But otherwise, as was mentioned by SkinnyG, the pressure of throttle return springs (only) on the high idle (I believe) would overcome the ability of the vacuum pull-off keeping the engine at a high idle.

Point of fact- just because I've done something the same way for forty years does not make it right. The person that I learned it from could have possibly been an idiot.     I admit I have little experience with Q-jests or or any newer carbs of the same style. I have always used Holley's (except my 74 Monte Carlo w/454 and T400, it had a Q-jet and ran awesome for a reasonably stock smog engine [15seconds flat in the quarter consistently]). 

[enaberif]

If your choke is setup properly yes it will automatically kick down for you.. That is the whole point of the choke.

How does the stepped/weighted part of the fast idle cam overcome the dual throttle return springs to kick down automatically for me?  I don't see how this works without touching the gas pedal manually.

First I'm no expert on these things but I know a few things from reading and listening.

First you are putting the fast idle cam under tension in the opposite direction of the throttle springs. When you push on the throttle you have set the fast idle as well as put tension on the springs. Measure the throttle springs when the fast idle isn't set to when it is and you will see that you have extended the throttle springs.

So now that the fast idle is under tension from your throttle springs when the truck is running and the choke is opening up because of the heat coil its pull the fast idle cam around and will cause it to fall down a step decreasing the idle and decreasing the tension on the throttle springs.

Once the choke is fully open it has pulled the fast idle cam off its last step and onto the area that returns the truck to its normal idle and no tension is left on the throttle springs. The reason the fast idle doesn't occur during normal usage is because the choke is fully open not pulling on the fast idle cam.

TLDR: Fast idle set = throttle springs placed under tension. Choke rod pulls up on fast idle via choke coil taking the tension off and decreasing idle back to normal.

[Greybeard]

Know what works the best and is pretty fool proof once the concept is learned? A manual choke. 

However, I wonder if we have been on the same page? As the engine heats up and the choke opens that does not mean the high idle is reduced automatically. But the choke will pull off all the way to an open state. In order to get the idle down it requires operator input....

I had to double check what I was saying and I found this at Hemmings Muscle Machines>>http://www.hemmings.com/mus/stories/2009/10/01/hmn_tips2.html

When cranking the engine, you do not want to "pump" the throttle, since this will decrease the signal to the booster and limit or stop fuel flow. If the engine does not start when cranked over for a few seconds, release the key and press the accelerator to the floor one more time to evoke an additional accelerator pump shot.

Most automatic chokes have a "clear flood" mode or "unloader" that is carburetor-linkage operated. If the engine does not start and becomes flooded, holding the accelerator to the floor will force the choke plate open, which will decrease the booster signal and provide more air to lean the mixture.

As the engine warms up, the choke spring tension will be reduced; if the accelerator is moved slightly open, the fast-idle cam will drop a notch and move toward curb (warm) idle RPM. You do not want to aggressively throttle the engine to get the fast idle to drop. As the spring tension is released, the pedal only needs to be moved enough to allow the fast-idle screw to clear the next step in the cam.

If the idle speed does not want to drop, the choke has not received enough heat to relax from that position. You do not want to "blow it open" by whacking the throttle hard, which brings us to a significant point: Many cold-start issues are simply driver error and not a problem with the choke circuit.

The entire article can be read at the above link. It is easier than quoting out of my carburetor books here.

[enaberif]

Exactly and read what you put in italics.

It states exactly what I've been saying all a long. All components of the choke need to be working correctly or else it will not step down.

That means proper choke adjustment, properly setting the lean/rich dial, heat stove in place, the snorkel valve working properly.

[philo_beddoe]

I did have to tap the pedal, but it did not take nearly as long. After all, go back to post 20, look at the bullet "cold engine" see the paragraph just above the shaded area-- "after 30 seconds the idle speed may be reduced"..... I dont think it would say that if idle will drop on its own.

Modern EFI cars drop on their own. My wife always asks me when we are in her car, "why are we just sitting here" and i say, "i'm waiting for the idle to drop".

[SkinnyG]

First I'm no expert on these things but I know a few things from reading and listening.

Do you have a carb on your truck?

[blazer74]

From the same article, doesn't get any clearer than this.

"""As the engine warms up, the choke spring tension will be reduced; if the accelerator is moved slightly open, the fast-idle cam will drop a notch and move toward curb (warm) idle RPM. You do not want to aggressively throttle the engine to get the fast idle to drop. As the spring tension is released, the pedal only needs to be moved enough to allow the fast-idle screw to clear the next step in the cam."""

Otherwise;

As far as the heat stove and air cleaner mechanisms is to prevent carb icing in cold weather. This only has a minor effect on the choke and can easily be adjusted out if not used.  There are plenty of normal operating chokes out there not using the air cleaner heat.

Spring tension sets the choke to closed  and sets the fast idle cam to the high step. After start as the choke heats the spring unwinds and opens the choke flap. It in no way is translated to the fast idle cam dropping to the lower setting.

I guarantee you will not find a video anywhere that shows a choke completely operate by itself from high to normal curb idle UNLESS it has an electrical device such as a throttle kicker activated by a thermal switch or computer.

[philo_beddoe]

With all respect to others, i must agree with blazer. I have my engine set up pretty close to stock factory with NO modifications, and it is operating like the owners manual states.

I have never owned an older carbureted engine other than a chevy, so i cant speak of any other make or brand of carb other than a chevy/rochester/qjet-2/4 bbl. 

Here is a fair challenge- i'd like to see a chevy manual from the 70's that states in some form or fashion that the idle will drop entirely on its own from a cold start. Car or truck.

[Greybeard]

I am confused, are we on the same page? I totally agree with one caveat, no carb will lower it's own idle speed with out operator input UNLESS it has some sort of external kicker (electric solenoid or vacuum) to push/pull the fast idle off. These in fact showed up on smog engines to help quicken up (shorten up) the time the engine was running enriched. Remove that canister and the engine will remain at high idle all day if there is no operator input. The choke will be wide open but the idle will remain high.

The choke circuit is connected to, but independent of, the throttle system. There are two times the choke is impacted by the throttle, once to set it (release the pressure so the springs can set the choke), and once at WFO to clear a flooded state (mostly on Holley's I think). Anything else is simply to let gravity/vacuum/heat do their things. The Q-jet is (I think) unique in that it also uses actually air flow past the upper plate to provide so help with opening of both the choke and the secondary throttle plates. Correct me if I am wrong...   

[philo_beddoe]

I still agree with blazer. Dont be confused, just accept what blazer wrote in post 83. 

[blazer74]

Quadrajet uses a pull off to slightly open the choke on start. Also Has another function but that's another chapter.

Qjet also has the mechanism to clear a flooded state.

[philo_beddoe]

Back in the day, a lot of guys worked on their own vehicles and having a basic understanding of how a carburetor and timing and the simple functions of an engine operated were much more common. You dont see that today, men working on their cars on a saturday in the driveway, even doing an oil change.

Cars go 15-20k miles per oil change, you never touch the coolant, the tranny, timing, plugs, wires, etc. For the most part you only open the hood to add wiper fluid. And after 300k miles you toss it in the dumpster and get a new one.

And many folk like that, having a reliable car or truck that will always start and be safe, with all the comfort and luxeries.

Glad this site still talks about the trucks of the old days and how carburetors work and how to solve problems and such.

Its nice to keep going on these posts just to learn more, but honestly i'm running out of things to say. Probably because my entire engine is running well, for now.

[philo_beddoe]

Ok, back on this again.... What does it mean if you start up a cold engine, (and its real cold out), drive for several miles and get it fully warm, then shut it off to get gas or something, then when you start it back up the idle is high? You actually have to kick it down again!?

Is too much cool air going to the carb through the snorkel? Should i remove it in the winter?

[blazer74]

The choke coil is external to the carb airflow.
Just the choke coil cooling off and contracting due to the extreme cold.