I am asking this question to help in tuning EFI. Does a carb have a start up circuit, or effect aside from the accelrator pump and the choke? I can't think of one, and am just checking out my theories here.

On a fully warmed up engine with a carb, it will start with just a quick turn of the key. If this is just the idle circuit, then in theory, you should be tuning the start up cells for the fuel map in EFI with the saem VE value as the idle cells. One has no vacuum, and the other is the highest vacuum. You carb experts out there, what can you tell me about this?

I realize that the crank RPM is 150-200, and the idle RPM is 750-800. I am not sure how this effects the amount of fuel delivered in the carb at crank.
I keep raising the VE cells for start, and haven't flooded it yet, and can tell it can get richer if I want, since adding pre-start fuel (equivalent of pump shot) does not flood it yet either.

Sam

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As you have noticed that carbed engine when warmed enough, simply fired off right away without choke assistance is the manifold is hot and engine can fire off with leaner mixture.

Choke is what assists COLD engine to fire off by forcing carb to choke up and vacuum pulls very rich mixture from verturi and the idle mixture ports. When engine finally fires off and develops stronger vacuum the choke pull off cracks open the choke a bit to keep engine from choking off air and die on way too rich mixture. Happens within a second or two.

Like I explained before, there is another mechanism that is linked to the choke mechanism is fast idle cam. It's on very fast idle RPM whigh advances the ignition timing once cold engine fires up and build up RPM in a second or two. The for FI engine computer should automatically adjust the idle rpm by way of the idle valve and ignition timing, and fuel mixture ratios depending on coolant temp. Much like
this carb's choke & fast idle mechanisms does.

The VITAL item to consider about, The jets, emulsion tube and idle jets, idle port and transfer port in relationship with the throttle blade edge position and verturi only comes in when air flow get high enough to pull mix in (around 45-50% throttle), DETERMINES the fuel ratios curves.

In fact, 80% of the MPG is from this lowly idle jet and the main and air jets in the carb. For FI, it's in the settings to get best results actually more finer control than carb could.

Steps to setup a FI in that order as I can recall correctly:

1. Adjust for best firing up cold engine and after start (ramp up RPM and kepe engine alive without stumbling and die.
2. Adjust the amount of fuel needed as engine warms up all the way hot without dying. Also this can determine the MPG.
3. Adjust the amount of acceleration % (like the accelerator spray that squirts more fuel every time throttle moves open) so engine don't bog.
4. Adjust the cruise ratios between 20% to 50% throttle so there's no surging, pinging or blowing black or blackening the plugs.
5. Adjust the WOT ratios for best power.

Fuel and ignition curves do matter! there is good threads on the ignition curve for distributor tweaking is worth reading on this slant six forum.

Cheers, Wizard

I would like to know what systems are in operation for a WARM start on a carb engine. A well tuned one starts up immediately, or so it seems. They can get running even before you get your hand off the key. Is it just the idle circuit? It seems a little suprising that enough fuel is pulled through the emulsion tube to get it started at crank speed. Anybody know more about how this start up happens on a warm carbureted engine.

It's not really that important, I just thought a carb expert might like to wax eloquent on the theory. Thanks. I also thought knowing more about the carb would help me understand the EFI tuning.

Sam

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A warm carbureted engine will usually have fuel vapor from the float bowl sitting in the intake tract.

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Joshua

A carb engine with wet manifold flow will leave a lot of fuel on the manifold walls after shut down. With fuel injection, you do leave fuel on the manifold walls, but this is between the injector and the intake valve. Port injection locates the injector just before the intake valve, so there's relatively little surface area exposed to fuel. Central fuel injection works more like carb. As a function of engine coolant temp, run start-up as close to stoich as possible with smooth running. As ECT drops, you will have to add more fuel and use a slower decay rate to keep running.

Mitch

Back to the carb, Is it the fuel left in the walls of the manifold that starts the engine when it is warm? If so, once this has evaporated, then a pump shot would be needed? Will a carbureted engine start without fuel on the walls of the manifold, and no accelrator pump. Anybody know how much fuel the accelerator pump actually squirts in? Has anybody actually calculated that?

This is one of my patented dumb-dumb questions. Actually it is a question tht seeks to explore something that we all take for granted, and maybe don;t understand as well as we might think.

Sam

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Not much in a warm and hot manifold. But the engine is already warm or hot so can fire off with choke open while throttle is closed off vacuum pulls from ports of idle circuits of idle mixture ratio, not RICH, just normal.

I have stated this before few times. How can that be diffcult?

Cheers, Wizard

http://www.youtube.com/watch?v=ZiyyLBm4J7I&NR=1

Now you know why we had choke on carbed engines, except weber DOCE used valves to open another set of starting/choke jets for starting up/afterstart after the throttle butterfly instead of relying on choke butterfly.

Same thing with injection stuff, sprays extra fuel when engine is cold.

Cheers, Wizard

http://www.youtube.com/watch?v=ZiyyLBm4J7I&NR=1

Now you know why we had choke on carbed engines, except weber DOCE used valves to open another set of starting/choke jets for starting up/afterstart after the throttle butterfly instead of relying on choke butterfly.

Cheers, Wizard

There were many reasons it was difficult, but just one reason is: you very kindly, generously, thoughtfully, and helpfully stated the above , (in red), and about 90 other things, including lots of stuff about EFI, which, while interesting, kind of crowded the page and my old brain a bit. . Thanks for identifying the specific thing that I was hoping to see(in red above). I know it is hard to be a mind reader sometimes.

And sorry I missed the bit about the fuel vapor hanging out in the manifold for awhile. I was writing the last post while my beautiful and really cute (did I mention brilliant?) 2 year old grand daughter was running around more or less demanding I quit playing with the computer and come play with her.

So, if a warm carb can pull fuel up through the idle circuit with the vacuum created by crank, that would be less fuel than the engine needs at actual idle, since the rpm is probably about 3-4 times as fast at idle as at crank, and very little vacuum is present at crank. I know that for a fact, as I have watched the MAP gauge as I cranked the engine.

Since little if any residual fuel is hanging around in the manifold with a port injected engine, it is up to the injectors to supply just the right new fuel to get things to light off again. That gives me something to chew on for awhile. Just trying to get better here. Thanks for hanging in there.

Sam

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I know that cranking with a hot engine is less fuel than idle but remember, hot engine only need little to light off, once it takes off, VROOM, then MAP shoots up.

But the COLD engine is tough to light off, hence the choke because manifold is condensing the mist and dripping down the intake becoming too lean by then so choke brings lot more fuel than usual, hopefully to pull enough in during crank (vacuum). But cold engine just fired up is still cranky and cold so choke holds the throttle on purpose to keep mixture velocity high and advanced timing (byproduct of high rpm) for several minutes till some heat built up in the plenum and heads & pistons crown.

Cheers, Wizard

If your engine is warmed up and you turn the key WITHOUT touching the accelerator. It is getting gas by vaccum or the manifold was left wet.

If you press the accelerator you dump a stream of gas into the manifold from the accelerator pump. Remove the air filter lid look down the carb, work the throttle and you will see the gas goimg into the manifold. The little bit of gas from the accelerator pump starts it. If the accelerator pump is clogged or the plunger has failed it won't start. If you give it starter fluid or pour a tiny bit of gas in the carb it will run fine till you turn it off, it will not restart indicating you have an accelerator pump problem.

Hope that makes sense.

George

I don't tap the gas pedal on a warm or hot engine to start, just kick the start and away it goes.

That is why it is not good idea to wildly PUMP that pedal
for ANY REASON, that is FLOODING for sure on carbed engines because the accelerator pump pumps the fuel every time throttle is opening. Injected engine doesn't squirt every time one play with pedal when engine is not running. In fact, coded to have flood-clear mode by holding the throttle WOT and turn engine over several revolutions to clear flood then stop, release pedal, turn key. Vroom.
Also works on carbed engines too, just press down all the way and hold (this pushes the choke slightly open) and turn key to clear out flood for few seconds then, let go of throttle, this will also close the choke as well. Turn key to start. Vroom.

Cold engines and tapping the pedal releases the trapped choke to close but that causes the accelerator pump to pump once just a bit as a conquences of this action. Still, the choke bi-metal is insulated enough especially one with electronic choke heater that cools off too quickly and engine is still warm enough that choke is not needed that this hurts the MPG for few minutes till choke opens up once again and very un-necessary.

Cheers, Wizard

1. Never said to wildly pump accelerator. one pump will due.

2. We were talking carb. not fuel injection.

3. Just went through a no start when warm problem. No gas due to broken accelerator pump. No gas when warm and open choke. Rebuilt carb. Gives gas after its warm with a push of the accelerator. Then starts.


4. Never flood unless you want it to.