Autolite 4100 Evaporative Emissions Experiment

[LARIAT 85]

I'm wondering if the gas in the tank got hot and pressurized the carb and pushed fuel into the intake, pretty much as you were saying, Rick. Is there supposed to be a check valve in the system to prevent that?

Ok lets think how this could happen?

If I understand this the vents that were added are open to the canisters when the motor is off.

The stock bowl vents are pretty tall so thinking gas would not be pushed up them to the intake.

So the fuel would go to the canisters before the intake no?

Now being it did not even try to start maybe it would have taken more than the push to close the choke to get it to light off, read too lean and may need 2 full pumps?

I also don't think you needed to swap the non-vented lid when all that would need to be done is pull the hoses and cap the barbs no? Or did the other lid have that small pin hole vent in it?

Think he needs to do more testing and report back to us LOL

Dave ----

edit: Just thinking all this stuff was made for a different gas blend than we have today could that be a cause of your out come?

"If I understand this the vents that were added are open to the canisters when the motor is off. "

That's correct. The vents that I added to the Autolite 4100 would provide a direct path to the charcoal canisters when the vehicle is not running.

"Now being it did not even try to start maybe it would have taken more than the push to close the choke to get it to light off, read too lean and may need 2 full pumps?"

After the first push of the gas pedal and the engine failed to start after a few seconds, I went out and verified the choke plate was closed. Then I tried two full pumps, three full pumps, and four-five full pumps. I finally got it to start after excessive cranking and pumping the gas. That's quite a change from one single pump and the engine immediately firing up.

"I also don't think you needed to swap the non-vented lid when all that would need to be done is pull the hoses and cap the barbs no? Or did the other lid have that small pin hole vent in it? "

The original, unmodified Autolite 4100 lid has a small vent hole in each fuel bowl that remains open all the time to vent the fuel bowl to atmosphere.

"Other than the large Evap hole used with the Evap fittings over the small factory holes the lid has no other changes between the 2 right?"

That's right. The lid is exactly the same otherwise.

"What happens when you cap the 2 Evap vents, yes I know the bowls would not be vented other than thru the tubes but think it would be ok for the drive way testing."

I didn't see any reason to try that because even if it solves the cold start problem, it would cause problems when the engine got hot and the only place the excessive fuel vapors could vent to would be inside the air cleaner. That would cause a hot re-start problem, which is [one reason] why the fixed vent holes are there on the Autolite 4100 lid and why the hose is open to the charcoal canisters when the evaporative emissions system is hooked up on the Motorcraft 2150.

"Wait the Evap path from carb bowls to canisters is closed when running right? If so forget what I posted above and running lean unless it is not closing off then the bowls would build psi when running as the air down the tubes has no where to go."

That's right. When the engine is running, manifold vacuum closes the bowl vent valve, which effectively blocks off the path to the charcoal canisters. Then the only way to vent the fuel bowl is through the internal vents inside the air cleaner. When the engine is turned off, the bowl vent valve is open again to vent the fuel bowls to the charcoal canisters.

Both valves are new and work as they should.

"Sorry for running on & on

Dave ---- "

Don't be. Thanks for trying to help me understand this!

[Gary Lewis]

"If I understand this the vents that were added are open to the canisters when the motor is off. "

That's correct. The vents that I added to the Autolite 4100 would provide a direct path to the charcoal canisters when the vehicle is not running.

"Now being it did not even try to start maybe it would have taken more than the push to close the choke to get it to light off, read too lean and may need 2 full pumps?"

After the first push of the gas pedal and the engine failed to start after a few seconds, I went out and verified the choke plate was closed. Then I tried two full pumps, three full pumps, and four-five full pumps. I finally got it to start after excessive cranking and pumping the gas. That's quite a change from one single pump and the engine immediately firing up.

"I also don't think you needed to swap the non-vented lid when all that would need to be done is pull the hoses and cap the barbs no? Or did the other lid have that small pin hole vent in it? "

The original, unmodified Autolite 4100 lid has a small vent hole in each fuel bowl that remains open all the time to vent the fuel bowl to atmosphere.

"Other than the large Evap hole used with the Evap fittings over the small factory holes the lid has no other changes between the 2 right?"

That's right. The lid is exactly the same otherwise.

"What happens when you cap the 2 Evap vents, yes I know the bowls would not be vented other than thru the tubes but think it would be ok for the drive way testing."

I didn't see any reason to try that because even if it solves the cold start problem, it would cause problems when the engine got hot and the only place the excessive fuel vapors could vent to would be inside the air cleaner. That would cause a hot re-start problem, which is [one reason] why the fixed vent holes are there on the Autolite 4100 lid and why the hose is open to the charcoal canisters when the evaporative emissions system is hooked up on the Motorcraft 2150.

"Wait the Evap path from carb bowls to canisters is closed when running right? If so forget what I posted above and running lean unless it is not closing off then the bowls would build psi when running as the air down the tubes has no where to go."

That's right. When the engine is running, manifold vacuum closes the bowl vent valve, which effectively blocks off the path to the charcoal canisters. Then the only way to vent the fuel bowl is through the internal vents inside the air cleaner. When the engine is turned off, the bowl vent valve is open again to vent the fuel bowls to the charcoal canisters.

Both valves are new and work as they should.

"Sorry for running on & on

Dave ---- "

Don't be. Thanks for trying to help me understand this!

How 'bout pulling the hoses off of the barbs to verify that the carb works ok with that lid?

As for the hard starting, I'm sure you are thinking like I am - it is flooded. So can you verify that by instead of pumping the gas pedal just flooring it? I assume the 4100 has a choke kicker at WOT, right?

With those two data points we'd know a bit more.

[LARIAT 85]

How 'bout pulling the hoses off of the barbs to verify that the carb works ok with that lid?

As for the hard starting, I'm sure you are thinking like I am - it is flooded. So can you verify that by instead of pumping the gas pedal just flooring it? I assume the 4100 has a choke kicker at WOT, right?

With those two data points we'd know a bit more.

"How 'bout pulling the hoses off of the barbs to verify that the carb works ok with that lid?"

I don't follow.

Other than the choke rod that moves to close the choke plate, the lid has no other moving parts and does nothing. Again, it is exactly like my original Autolite 4100 lid. I verified the choke plate closed with the new lid, and it did. When I finally got the engine to start, it opened up like it should and the choke operated correctly.

"As for the hard starting, I'm sure you are thinking like I am - it is flooded. So can you verify that by instead of pumping the gas pedal just flooring it? I assume the 4100 has a choke kicker at WOT, right?"

I tried holding the gas pedal down to the floor while cranking it, and it didn't seem to help. I don't know if it has a choke kicker or not? Other than with this experiment, I have never been in the position of having to need one.

Also, I checked the fuel bowls, and the fuel level was normal.

 

[Gary Lewis]

"How 'bout pulling the hoses off of the barbs to verify that the carb works ok with that lid?"

I don't follow.

Other than the choke rod that moves to close the choke plate, the lid has no other moving parts and does nothing. Again, it is exactly like my original Autolite 4100 lid. I verified the choke plate closed with the new lid, and it did. When I finally got the engine to start, it opened up like it should and the choke operated correctly.

"As for the hard starting, I'm sure you are thinking like I am - it is flooded. So can you verify that by instead of pumping the gas pedal just flooring it? I assume the 4100 has a choke kicker at WOT, right?"

I tried holding the gas pedal down to the floor while cranking it, and it didn't seem to help. I don't know if it has a choke kicker or not? Other than with this experiment, I have never been in the position of having to need one.

Also, I checked the fuel bowls, and the fuel level was normal.

It isn't moving parts I'm wondering about. But if the lids are truly identical. Just thinking that maybe that test would tell us something.

As for the choke kicker, you can verify that with the engine cold by pulling the air cleaner and opening the throttle by hand.

And by the time you tried to open the choke that way you'd pumped it several times, so the engine really was flooded.

Another possibility: What if the carb has a vacuum on it from the canister? I'm just thinking, and pulling the hoses might tell us something.

[LARIAT 85]

It isn't moving parts I'm wondering about. But if the lids are truly identical. Just thinking that maybe that test would tell us something.

As for the choke kicker, you can verify that with the engine cold by pulling the air cleaner and opening the throttle by hand.

And by the time you tried to open the choke that way you'd pumped it several times, so the engine really was flooded.

Another possibility: What if the carb has a vacuum on it from the canister? I'm just thinking, and pulling the hoses might tell us something.

"It isn't moving parts I'm wondering about. But if the lids are truly identical. Just thinking that maybe that test would tell us something."

I see. I had them both in my hands and checked them very carefully. I can assure you they are the same.

"Another possibility: What if the carb has a vacuum on it from the canister? I'm just thinking, and pulling the hoses might tell us something."

Okay, but why or how could there be a vacuum on the carburetor from the canister? And what exactly would a vacuum do to the carburetor?

And if there is - and that is the problem - why wasn't it a problem on the emissions carburetors that used the evaporative emissions system?

[LARIAT 85]

It isn't moving parts I'm wondering about. But if the lids are truly identical. Just thinking that maybe that test would tell us something.

As for the choke kicker, you can verify that with the engine cold by pulling the air cleaner and opening the throttle by hand.

And by the time you tried to open the choke that way you'd pumped it several times, so the engine really was flooded.

Another possibility: What if the carb has a vacuum on it from the canister? I'm just thinking, and pulling the hoses might tell us something.

Perhaps this article could explain carburetor venting better than I can:

https://carbkitsource.com/tech/articles/Carburetion/Carburetion6.html

Bowl Vents:

Some float systems will employ a fixed external vent as well as an internal vent. The external vent, located in the air horn section above the float chamber, provides a means for venting to the atmosphere any fuel vapors or pressure that might accumulate or build up inside the float bowl chamber due to high under-hood temperatures. If these vapor pressures are not disposed of, then the pressure inside the bowl pushing down on top of the level of the fuel will be greater than the calibration of the carburetor intended it to be. This will result in a rich mixture due to excessive fuel being forced through the jets into the carburetor and on into the intake manifold.

The Autolite 4100 carburetor I have has an external vent in each fuel bowl that is open to the atmosphere at all times.

Idle Vent Valve:

Most carburetors today will use a movable atmospheric idle vent valve in place of the fixed external vent system. This movable valve will be held in an open position only during idle and park positions by carburetor linkage contacting the idle vent arm. Any tendency for fuel vapors to collect inside the float bowl will be vented to atmosphere through this opening at idle and park positions. As car speed increases and throttle linkage is progressively opened, the actuating lever from the linkage will no longer contact the idle vent arm thereby allowing the idle vent valve to close. This action will now return the carburetor to the internal vent system thereby once again maintaining the same pressure on top of the fuel as entering the carburetor through the air cleaner.

It is important that the idle vent valve be closed during all periods of operation except at idle, otherwise excessive richness can be caused by the higher atmospheric pressure acting upon the fuel in the float bowl.

The emissions-style carburetors most Bullnose trucks came with used a bowl vent solenoid to accomplish this by blocking the hose to the charcoal canister whenever the engine was on. This solenoid was wired into the ignition system. Later models used a bowl vent valve to accomplish the same thing, but it closed using manifold vacuum instead.

I just had a thought:

If your theory is correct in that carb *might* have a vacuum on it from the canister, do you think the earlier electrical bowl vent solenoid would help? Wouldn't it block the hose earlier from the the key being in the ignition, compared to the later bowl vent valve (which is what I am using) that doesn't block the hose until the engine is actually started?

 

[Gary Lewis]

I just had a thought:

If your theory is correct in that carb *might* have a vacuum on it from the canister, do you think the earlier electrical bowl vent solenoid would help? Wouldn't it block the hose earlier from the the key being in the ignition, compared to the later bowl vent valve (which is what I am using) that doesn't block the hose until the engine is actually started?

That's exactly what I was thinking. (Before I went out to eat weeds and mow the yard. Yeah, it seemed like I was eating weeds as I had them literally ALL over me.) Which is kinda my rationale for pulling the hoses. If it does it with the hoses on but not with them off, then there's something going on while the truck is shut off. Vacuum, pressure, something.

[LARIAT 85]

I just had a thought:

If your theory is correct in that carb *might* have a vacuum on it from the canister, do you think the earlier electrical bowl vent solenoid would help? Wouldn't it block the hose earlier from the the key being in the ignition, compared to the later bowl vent valve (which is what I am using) that doesn't block the hose until the engine is actually started?

That's exactly what I was thinking. (Before I went out to eat weeds and mow the yard. Yeah, it seemed like I was eating weeds as I had them literally ALL over me.) Which is kinda my rationale for pulling the hoses. If it does it with the hoses on but not with them off, then there's something going on while the truck is shut off. Vacuum, pressure, something.

Or maybe not.

I just remembered that for my second try, I added the thermal vent valve to the hose and nothing changed. This valve also blocks the hose whenever temperatures are 90 degrees or less.

[Gary Lewis]

Or maybe not.

I just remembered that for my second try, I added the thermal vent valve to the hose and nothing changed. This valve also blocks the hose whenever temperatures are 90 degrees or less.

Then I don't know. Lost. I understand what you've done, I just don't know why it is doing that.

[FuzzFace2]

Then I don't know. Lost. I understand what you've done, I just don't know why it is doing that.

X2

Now you said no vacuum when motor is off to pull vapors from the bowls to the canisters but ....

The charcoal mush have some action to pull vapors from the tank(s) and bowl(s) with the motor off other wise how does it work and why use them?

I think the charcoal is pulling in anything from around it and if plumbed to the tank(s) & bowl(s) it will pull vapor.

Still don't know why with the testing you did it run leaner and was harder to start?

Dave ----