Autolite 4100 Evaporative Emissions Experiment

[Gary Lewis]

Interesting links and observations. Thank you, gentlemen.

"Manipulation or changing the length of the fuel bowl port can have dramatic affects on the fuel curve of a carburetor" is right. My truck went from starting and running stupid easy and flawlessly to a complete bear to start and struggling to run.

What else is different? Like 85lebaront2 said, the later Motorcraft 2150 fuel bowl vent ports in the air cleaner are much bigger than the tubes found on the earlier Autolite 2100/4100 airhorn:

Perhaps the larger, shorter fuel bowl vent ports somehow work in conjunction with the larger external bowl vents and evaporative emissions? And/or the internal calibration is different as a result? If you look at an early Autolite 2100 or 4100, the fuel bowl vent ports are much taller, and slimmer. And the external vent hole is also much smaller:

The internal and external venting of a carburetor is apparently more important and affects much more than I thought. It seems there is much more going on with the inner workings of the stock carburetors than you might would think. I thought it would be a simple upgrade, but apparently I was wrong. Trying to upgrade the earlier non-emissions carburetor this way only throws off the balance of the carburetor.

I agree, whole heartedly. There are complicated things going on that I don't understand, for sure. Like, are the diameters of the vents, or their heights significant? Do the vents just end straight down, or do they turn? Is there an orifice at the end of the vent and how big is it? Why is the external vent tiny?

The carbs appear to be tuned for the answers to those questions, and many more besides. And you just happened to change something that makes a noticeable difference.

[81-F150-Explorer]

I agree, whole heartedly. There are complicated things going on that I don't understand, for sure. Like, are the diameters of the vents, or their heights significant? Do the vents just end straight down, or do they turn? Is there an orifice at the end of the vent and how big is it? Why is the external vent tiny?

The carbs appear to be tuned for the answers to those questions, and many more besides. And you just happened to change something that makes a noticeable difference.

Rick has asked me to look into this thread for any insights.

Four things come to mind with what you are trying to do.

1: The Carburetor needs to vent from atmosphere into the bowl when the engine is running. Is there a stipulation for that? It also needs this vent to be one way, into the bowl. It can't be both ways etc. The evaporative carbs have a internal check valve to keep fuel vapor from escaping the bowl in any other way than the bowl vent, unless extreme pressure buildup happens.

2: The Bowl Vent hose also needs a one-way thermal check valve. D8TZ-9E589-B Motorcraft CX-233.

This also needs to be hooked up inline with the word "CARB" toward the carb bowl vent.

3: There needs to be a purge control valve to control when the canister purge operation happens. D9AZ 9B963-B Motorcraft CX-330.

4: The old carb configuration lets fuel vapor vent into the air cleaner. When the vehicle is started in the mornings this air in the cleaner is rich. This might help with starting in your original configuration.

So for evaporative emissions to work correctly, you will need:

1: An air vent into the carb bowl from the outside, that only allows air to enter the bowl, and not to escape. The biggest problem is your carb isn't set up for evaporative emissions so does not have these provisions.

2: From the bowl vent hose/s the one-way thermal check valve D8TZ-9E589-B with the "CARB" toward the bowl vent. This valve needs to open at the right temp, and only allow vapor through one-way.

3: From the Check Valve to the Canister Purge "T" fitting between the Canister and the PCV valve.

4: Between the "T" fitting and the PCV valve the canister control valve D9AZ 9B963-B Motorcraft CX-330. hooked up to an appropriate vacuum source for proper purge operation.

5: The Charcoal Canister itself must be able to vent clean air into itself, when in purge mode.

6: The fuel tanks fuel caps must be able to vent clean air, one way into the tank.

Any one of these can cause a fault or drivability issue if not working properly.