Autolite 4100 Evaporative Emissions Experiment

[LARIAT 85]

I have a completely stock 1965 Autolite 4100 carburetor on my truck that has run flawlessly for about 10 years. The Autolite 4100 is basically a pre-emissions 4-barrel version of the Motorcraft 2150 the Bullnose trucks used. One difference - and what this post is about - is the Autolite 4100 doesn't connect the fuel bowl to a charcoal canister to capture fuel vapor like the Motorcraft 2150 did. Instead, it has a very small external bowl vent hole in each fuel bowl that is open to the atmosphere all the time. In an effort to make it look a little more period correct - and hopefully save a little fuel - I decided to try to hook the Autolite 4100 to the charcoal canister. The results were not what I expected, and frankly, I don't really understand what is happening.

I purchased a scrap Autolite 4100 lid, and modified it by filling in the two small vent holes in each fuel bowl with epoxy. Then I drilled a hole in each fuel bowl to accept a screw-in 90 degree hose barb. The hole size and barb fitting is 3/8", just like the original Motorcraft 2150 carburetors used:

By doing this, I was able to easily duplicate the stock configuration by connecting a 3/8" rubber hose to each fuel bowl fitting and running it down to the charcoal canister. I installed a stock thermostatic bowl vent valve in each hose (this valve closes when the engine is started to block off the hose and route all fuel vapors generated from the fuel bowl into the air cleaner and is open when the engine is at rest to vent the fuel bowl and/or gas tank) and connected it to the bowl vent connection on each purge valve, which connects to two charcoal canisters. I duplicated what Ford did with the 4-barrel carburetors they used on some Bullnose trucks. The Mustang 5.0 used this same configuration:

But something isn't right. In order to achieve the best idle and vacuum reading, I had to richen the mixture screws by about a 1/2 turn to get my engine back to my normal 20.5" of vacuum. That's odd, because I expected it to be *richer* considering the fuel bowl vents were now closed to the open atmosphere when the engine was running. After I got everything set and the engine was good and hot, I turned the vehicle off and restarted it immediately; the engine started right back up without any problems. Then, I shut the engine off and let it heat soak for about 20 minutes and then restarted. Again, it started right back up without any problems. The only difference was, the curb idle speed was down from the usual 750RPM on PARK to 500 RPM in PARK. In order to get it back to where it was, I also had to turn the curb idle speed screw in about a 1/2 turn.

But the biggest surprise came the next day when I started the engine cold. I did my usual routine and pushed the gas pedal down once to set the choke and turned the key. I expected the engine to start right up, but instead it would not fire AT ALL. Not even close. I cranked it over 3 different times in 10 second intervals and it simply would not start. It never even caught. It was as if the ignition module had completely failed. I checked the choke plate and it was completely closed. I pulled the hoses from the fuel bowls and they were dry but smelled strongly of gasoline. My guess is the gas tank vented through the carburetor vents in the air cleaner and made it too rich?

I thought about it some more and decided to try it again but added a stock thermal vent valve in the hose line. Even though *my* truck never had this, some of the older trucks did. This valve closes and blocks the carburetor bowl vent hose at temperatures below 90 degrees to prevent the gas vapors from from the gas tank from venting to the carburetor. (Some of the older trucks from the late 1970s and early 1980s had only this valve, later vehicles got both this valve AND the bowl vent valve, and later vehicles like mine only got the bowl vent valve.) But I got the exact same result the next day when I started the engine for the day - a no-start condition.

So I put the original lid back on the Autolite 4100 and put everything back the way it was. The next day, the engine started right up and immediately the way it always did - with only one single push of the gas pedal to set the choke. Everything is completely back to normal before I tried this whole experiment.

I don't really understand what is happening here? I had to tune the carburetor richer at idle to make it work as well as it did, yet it was much too rich to start when the engine was cold? Something doesn't add up. The only thing I can think of is there is must be *something* internally different between the older Autolite 2100/4100 carburetors and the later Motorcraft 2150 carburetors that were set up for evaporative emissions?

But what could it be?

[85lebaront2]

Ok, the older Ford carburetors, 2100, 4100 had always open bowl vents. Normally this type system can lead to a mixture that gets progressively richer as the air filter element gets dirty. I never recall seeing this problem on these carburetors, in fact had one in the shop that the carburetor was mostly a dirty lump, but once the ignition tune-up was done, moving the mixture screws in either direction caused the rpm to drop. Apparently Ford had these really well set up as built. One item, if you look at the later 2100s and 2150s, the bowl vents into the air filter are much bigger than the 1/4" tubes on the 2100/4100 air horn.

If you could put a very sensitive manometer on the bowl vent hose you ran and run the engine with it plumbed up as an evaporative system would be, and see if there is a slight vacuum on the hoses, then when it does not want to start, just pull the hoses off the evaporative canisters for test purposes and see if it starts.

You might have to come up with a way to open the bowl vent system starting with a cold engine or make sure the system isn't trying to pressurize the bowls on a cold start.

[Gary Lewis]

Ok, the older Ford carburetors, 2100, 4100 had always open bowl vents. Normally this type system can lead to a mixture that gets progressively richer as the air filter element gets dirty. I never recall seeing this problem on these carburetors, in fact had one in the shop that the carburetor was mostly a dirty lump, but once the ignition tune-up was done, moving the mixture screws in either direction caused the rpm to drop. Apparently Ford had these really well set up as built. One item, if you look at the later 2100s and 2150s, the bowl vents into the air filter are much bigger than the 1/4" tubes on the 2100/4100 air horn.

If you could put a very sensitive manometer on the bowl vent hose you ran and run the engine with it plumbed up as an evaporative system would be, and see if there is a slight vacuum on the hoses, then when it does not want to start, just pull the hoses off the evaporative canisters for test purposes and see if it starts.

You might have to come up with a way to open the bowl vent system starting with a cold engine or make sure the system isn't trying to pressurize the bowls on a cold start.

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?

[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?

"One item, if you look at the later 2100s and 2150s, the bowl vents into the air filter are much bigger than the 1/4" tubes on the 2100/4100 air horn. "

Yes, I noticed that. Perhaps there is a reason for this, and is causing what I am experiencing?

"You might have to come up with a way to open the bowl vent system starting with a cold engine or make sure the system isn't trying to pressurize the bowls on a cold start."

But the bowl vent system *is* open (to the charcoal canister) when I only used the bowl vent vacuum valve in the hose. It only closes when the engine is started. That is how my truck originally had it when I still had my stock Motorcraft 2150. And that is also how the 1985 Mustang 5.0 was set up.

When I later added the thermal vent valve (pre-1981 trucks only had this one valve in the hose), the bowl vent system was *closed* to the charcoal canister when the engine is cold. It didn't make any difference.

"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? "

That's a good question. It is set up exactly like the diagram. Do you remember that assembly I purchased from you a few years ago?

Unless - there is some sort of check valve between the gas tank and the charcoal canister that is responsible? From what I read, the gas tank is supposed to vent gas fumes to the charcoal canister. The fumes are captured within the charcoal, where they are later pulled out by the purge valve and into the PCV valve where they are burned by the engine under certain conditions.

[Gary Lewis]

"One item, if you look at the later 2100s and 2150s, the bowl vents into the air filter are much bigger than the 1/4" tubes on the 2100/4100 air horn. "

Yes, I noticed that. Perhaps there is a reason for this, and is causing what I am experiencing?

"You might have to come up with a way to open the bowl vent system starting with a cold engine or make sure the system isn't trying to pressurize the bowls on a cold start."

But the bowl vent system *is* open (to the charcoal canister) when I only used the bowl vent vacuum valve in the hose. It only closes when the engine is started. That is how my truck originally had it when I still had my stock Motorcraft 2150. And that is also how the 1985 Mustang 5.0 was set up.

When I later added the thermal vent valve (pre-1981 trucks only had this one valve in the hose), the bowl vent system was *closed* to the charcoal canister when the engine is cold. It didn't make any difference.

"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? "

That's a good question. It is set up exactly like the diagram. Do you remember that assembly I purchased from you a few years ago?

Unless - there is some sort of check valve between the gas tank and the charcoal canister that is responsible? From what I read, the gas tank is supposed to vent gas fumes to the charcoal canister. The fumes are captured within the charcoal, where they are later pulled out by the purge valve and into the PCV valve where they are burned by the engine under certain conditions.

I do remember that setup. And that pic.

Looking at the illustration I don't see anything beyond what you surely have, meaning the vent line going into the charcoal canister. So I'm baffled.

[85lebaront2]

I do remember that setup. And that pic.

Looking at the illustration I don't see anything beyond what you surely have, meaning the vent line going into the charcoal canister. So I'm baffled.

It could be just that a 1965 or so carburetor was never designed for evaporative emission control and the internal components of the metering systems are designed with the understanding that the bowls are always open to atmosphere with no restrictions.

You could have to deal with this:

[LARIAT 85]

It could be just that a 1965 or so carburetor was never designed for evaporative emission control and the internal components of the metering systems are designed with the understanding that the bowls are always open to atmosphere with no restrictions.

You could have to deal with this:

I think you are right.

It's guess it is not as easy as simply adding the later emissions plumbing to the early non-emissions carburetor like I thought. I expected some minor re-tuning, but it went from firing up immediately on the first try to not starting after 3 cycles of 10 second intervals after only sitting overnight! Although they look very much alike from the outside, apparently there are important differences internally between a mid-1960s non-emissions carburetor and a mid -1980s emissions carburetor.

Other things *did* work, however. I added the Motorcraft 2150 electric assist choke cap and the dashpot solenoid (solepot) to the Autolite 4100 about 5 years ago and they both work great.

Carburetors, while simple devices, can also be very complex when you take a closer look and realize how much trouble Ford engineers went through to precisely calibrate them for each specific application and/or emission regulations.

 

[FuzzFace2]

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?

[Gary Lewis]

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?

I wasn't thinking the pressure pushed the gas up the vents. Instead that it raised the pressure in the bowl and pushed the gas through the venturi orifaces and, therefore, into the intake.

[FuzzFace2]

I wasn't thinking the pressure pushed the gas up the vents. Instead that it raised the pressure in the bowl and pushed the gas through the venturi orifaces and, therefore, into the intake.

[Thinking out loud]If he had the Evap vents added or the factory vent holes you also have the vent tubes with BIG openings so how would pressure build in the bowl to push fuel any where let alone out the ventures? If the bowl was sealed with motor off then I might see that happening.[/thinking out loud]

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?

How can just a lid change cause the carb to run lean?

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.

It would have been nice to test that lid before the Evap vents were added to see if it was a lid thing or a vent thing.

Now running lean with my next thinking and what Gary saying of the bowl pressure:

When running and air rushing into the opening of the carb where the bowl vent tubes are air could air be forced down the tubes and if no place to exit, build psi in the bowl?

Now we have a small vet hole not in the air flow so could this balance out the psi coming down the tubes?

You now changed this opening size and made it larger so more air out than in so where ever that psi was pushing the gas is no longer there and runs lean.

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.

Sorry for running on & on

Dave ----