Howdy, so 1973 f100, with a mild fe390 here.
so my engine-fuel system is very stripped down - just a few necessary wires, and vac hoses. No emissions at all. Been having flooding issues such as- drive to store, engine off for more than 20 mins=flooded. But if it sits for 40+ mins not flooded. Anyhow, now that it's warming up it is flooded all day after the sun comes up. Could sit unused for days, but if the sun's on it, or it's hot out it's already flooded before I try to even start. Fuel system has an abandoned charcoal can that still has one line from tank going into it, but nothing from carb. Gas cap is NON vented, as it wooshes when opened. Is this vacuum that is being created causing gas to be pushed into intake while sitting, or mabey pressure from the rising temps evaporating creating this pressure to push gas? If so, can I just cut hose to evap and put a cheap little check valve, or can I just drill a tiny hole in gas cap until I remember to order a vented cap. Any thoughts?

 

You need a vented cap to start with to prevent pressure build up/down. After that check fuel level in the carb. You may be experiencing vapor lock too.

 

Are you sure the whooshing sound is going in, and not coming out?
A truck originally equipped with a charcoal canister would not have had a cap that could vent outward. But it should definitely have been able to vent inward.
And when you say flooded, is it simply hard to start? taking a lot of cranking? Or do you see gas in the carburetor Venturi, and also smell gas?
If it’s truly flooded, have you tried the usual trick of holding the throttle all the way open while you’re cranking the starter?

 

X2 vented gas cap 1st. Drilling it would be a easy fast fix. Without one is it sounds like heat is causing the gas tank to pressurize and push gas to the carb. And X2 on I bet you are suffering fuel boil or vapor lock. Might need to install a phenolic spacer for the carb.

 

Do a search for 'Hot start" and see how others have dealt with it but in short keep the carb as cool as you can and the phenolic spacer is the first step.

As for the cap why not only put it on 1 click or not at all?
This way you will know if a different cap will work with out spending money.

Also if the tank was building PSI I would think it would need to be pretty high to push past the fuel pump then over come the needle & seat and then push out the carb bowl vent(s) and down into the carb opening.
Easy check on that also. Pull the air filter off and before starting look down into the carb and see if fuel is dripping or coming out the bowl vents.

I would also check the float level and maybe run it a little lower but not much.
I think now that it is getting hot the gas in the carb bowl is boiling / vaporizing and pushing out the bowl vents and the vapor being heavier than air now goes down the carb opening flooding the motor.
This is the same thing that happens on hot restarts and why you need to keep the carb cool as you can.

On v8 motors the intake has a cross over that sends hot exh gases under the carb.
If you can block this off, yes you need to pull the intake to do this, it helps.
Some also have a heater hose along the side of the carb / choke or a plate under the carb bypass the plate and dont run the hose next to the carb.

It is todays fuel blend and why we have the problems we have when hot.
Dave ----

 

1. It is wishing out. The current cap vents in only, and verified by inhaling/exhaling through cap.
2. My crossovers are blocked, and have been for 6 years.
3. I have searched, and read A LOT, but truck does this even if I haven't ran it for a day. Never does in winter, but summer is Everytime, even at 7am if it's warm out.
4. For sure flooded. Have to hold pedal to the floor, and crank for 5-10 seconds only to be smoked with gas fumes when it catches.
5. Have since drilled a hole in the check ball in the middle so as to vent both ways, but made no difference in engine performance/starts
6. Tried 2 other motorcraft 2100's to the same effect as my Holley 4412.
7. Have a 2" spacer between intake and carb, along with a 1/4 neoprene gasket on top, and double paper gaskets on bottom.

It's always done this when driving in mountains for a while, then stopping for a little, but now it's just doing it always.
Mixture is set by vacuum like always, float fuel hight is just under the sight glass.
Could a swore it woulda been excessive positive pressure in tank pushing gas past stuff, but that wasn't it.

 

Once it starts, how does it run? Richer than it used to be maybe? Wonder if the power valve is leaking. If the power valve is loose, has a leaking gasket, or a ruptured diaphragm, it can leak gas right into the intake.

 

A vented cap will freely vent going in. It will vent after a preset pressure level is reached, you'll likely hurt your ears trying to reach the release pressure level blowing through the cap. Some caps will release at lower pressure than others. It does that so you don't have fuel vapors gathering in your garage as the gas evaporates.

I have seen several articles that a none functioning gas cap can allow up to 30 gallons evaporate over a year. I don't know if true, but I have seen my truck have less gas after long periods of non use (that gas cap vented both ways pretty freely) and a new propper gas cap (freely vents inward, but takes effort to vent outwards) stopped it as well as eliminated the smell of fuel as I walk past.

The suggestion to just turn to the first click is good as a test. A hole drilled in a cap .... OK if already a junk cap, but a good operating cap is better.

As your "mild 390" sits, it is likely boiling float bowl fuel in Az.. I had a Chevelle would do that, act like it was flooded, I could see gas boil in the float vent tube ... was during transition from Spring into Summer, when gas blends are still in flux between winter and summer blends. Only did it when the engine was hot. I used snake oil of some kind in the tank, reduced it until next fill up.

 

Once started it runs tip top after that first 15 seconds of roughness burning off the excess gas. Tried running it without cap, and didn't help . Have since drilled the check ball in cap so it vents both ways. Not worried about fumes, as I have no garage. I am in AZ, but 7500 elevation in the mountains. A hot day here is mabey 85°f hottest has been about 95°f a few years back. Nights are running bout 35+45°f now, days are 65-70. Winter is around 0-35. Not like the desert at 120°f, I could see that doing this. Gonna drive it, get it hot, and check for fuel dribbling into engine.

 

Once more, if you don't have a functtioning vent you will either build up pressure in the tank and flood the carb or build vacuum and starve the carb.

 

Have both an open and clear return line to tank that used to be to the charcoal can, and a drilled gas cap venting both wAys. Had a coworker look at it ( towns public works fleet/equipment mechanic) and he was lost as well. Only thing left is mabey a blown power valve, but in all 3 good carbs? Also if that was the case, it should run like **** once going? Think it's just gonna have to stay this way

 

I missed that you once had a charcoal canister, which means you had evap emissions, which means the original fuel cap was not freely vented out ... so the tank was forced to vent out only through the canister. When you left the hose loose from the canister, you opened the gas tank to atmosphere. I don't understand how your tank can "whoosh" when the cap was opened then ... unless a vacuum operated valve was shut still.. It doesn't matter with a drilled cap though.

At 7,500 feet there is about 11.1 pounds of air above each square inch of space, but Holley sets their carbs up for sea level where there is 14.7 pounds of air over each square inch of space.
Fuel will boil sooner at 7,500 feet than down at sea level ... because there is less atmospheric pressure to stop the boil.
Air is thinner up there too, and so oxygen is harder to come by. Your carb's set up for sea level will run rich up there too, unless tuned (leaned) for altitude.

 

Instead of starting a whole new thread, thought I would keep it on the same track since it's all the same troubleshooting.
So I figured out what the flooding was from, but not the why,and hows yet.
So my Holley 4412 has the sight glass to see into the bowl. Running the engine my fuel level is right on, as long as it keeps running, BUT, as soon as I shut it down, it seems the same amount of fuel is still getting pumped to carb, but isn't getting used so just fills the bowl, until it gets to the top and starts dripping into intake. I can watch over the course of 30 seconds or so, the fuel rising up the window, and start dripping out. Stock mechanical fuel pump, to Holley reg, to inline filter, to carb. If I take the inlet hose off the carb right away after shut down, it sprays with some good force for a sec until that pressure in the line is relived-a few seconds. Pulled needle and seat- nothing visible, cleaned it up-still the same. I have a new fuel pressure gauge to put in today, as well as a new needle and seat. Floats floating, had a ripped accelerator diaphragm- changed that to no effect. What gives?

 

Once more, no vent you have pressure in the tank.

 

Did you ever just partly open the cap, like to safety step just as a test .... and if it's routinely parked level or pointed down a hill?

Also, have to ask .... with a Holley 2bbl, factory fuel pump ... why the fuel pressure regulator too?

Sounds like it ... but he wrote that he's drilled the cap.