Unsolved mystery of carb bowl drain (1 Viewer)

Joined
Jul 25, 2019
Messages
21
Location
Sumter, SC
OK...I've got an 81 FJ40 that is bone stock that I have owned since 1988...still has emissions junk. Over the last year or so I've started to experience dreaded carb bowl drain when it sits for a day or two. So I started doing some research. In fact, before somebody calls me out for not searching the forum for previous threads let me assure you I have read every one. But from what I can tell a cause/fix has not been determined. There are plenty on theories though. Which I have investigated and have come to the conclusion that nobody really knows what is going on even though the most respected LC gurus have weighed in. For those who don't have time to read all the posts...let me summarize some of the ideas:
1. Evaporation - doubt it. Carb fan works fine and it's 30 outside. Plus lived in Phoenix for 3 years and drove it everyday without a problem
2. Ethanol fuel - doubt it. Been running it for 30 years no problem...tried non-ethanol with same results.
3. Fuel bowl "O" ring - Not sure exactly what the result would be...would fuel leak inside carb or outside...no evidence any where on carb of fuel leak
4. Plugged air vents - maybe, but once fuel is pumped back up to carb starts and runs fine
5. Siphoning into intake due to vacuum produced by differential pressures as the engine cools. Sounds very scientific, but if true why did it not drain for the first 30 years I owned it?
6. Kind of "wicking" action due to surface tension causing leak past idle solenoid. Maybe. But once again 30 years worked fine.
7. Fuel pump/filter - both new.

Conclusion: Like I said unsolved mystery. However, seems like a very common problem not just with 2F, but 22RE engine in Toyo trucks, etc. Since mine ran fine for 30 years, I believe it has to be related to internal wear or contamination. What I don't understand about that is even after rebuilds it still happens sometimes. Seems like mine loses fuel all the way down to fuel pump since I have to crank it over for about the same amount of time as after a pump change. Maybe somebody will solve it, until then I guess I'll live with it. I wouldn't be surprised if someone figured it out...I've got so many fixes from really smart guys on here already.
 

ceylonfj40nut

Waiting for Barn Time
SILVER Star
Joined
Jan 11, 2015
Messages
3,222
Location
TX
Heard ethanol enhance fuel does cause “wicking”. Have you tried Ethanol free gas to rule this out?
 
Joined
Mar 10, 2005
Messages
1,186
Location
CT
Theory - try this, next time you shut it off for the night or several days, unscrew / open the gas cap.

Guessing, vent hose plugged or soft, results in a vacuum in the gas tank, which sucks the fuel back.

Fuel pump is strong enough to overcome the vacuum.

Between fuel and other leaks, vacuum is eventually reduced.

I had the problem with my 79 FJ40. Got tired of pouring little gas down the carb to get it running.

Blew out all the lines and replaced most of the soft lines.

So far, it starts even after sitting for weeks.
 
Joined
Feb 6, 2002
Messages
15,222
Location
OC, CA
In my case it was due to surface tension and wicking or creeping out of the idle circuit. It didn’t do it when I filled it with non oxygenated fuel. I added a fluorescent dye to the bowl and found that it was dripping out of the idle circuit so that was clear from the get go. Try polishing the idle fuel hole in the bowl so it is smooth and bright. It will creep more easily over the rough oxidized or corroded surface of the casting. If I had more time to fool with it, I would try drilling out the top 1/4 inch and installing a teflon tube liner. Also make sure that the O ring on the idle fuel cut off solenoid is fine and sealing.
 

Dizzy

GOLD Star
SILVER Star
Joined
Feb 5, 2018
Messages
1,899
Location
Santa Fe, New Mexico
?

2F sectional.jpg
 
Joined
Aug 4, 2009
Messages
7,467
Location
New Hampshire
Theory - try this, next time you shut it off for the night or several days, unscrew / open the gas cap.

Guessing, vent hose plugged or soft, results in a vacuum in the gas tank, which sucks the fuel back.

Fuel pump is strong enough to overcome the vacuum.

Between fuel and other leaks, vacuum is eventually reduced.

I had the problem with my 79 FJ40. Got tired of pouring little gas down the carb to get it running.

Blew out all the lines and replaced most of the soft lines.

So far, it starts even after sitting for weeks.

Impossible for the tank to suck fuel back from the bowl.
 
Joined
Mar 10, 2005
Messages
1,186
Location
CT
Why not? There are no check valves in the system. You can blow the fuel lines using compressed air back into the tank.

Fuel shutoff solenoid maybe will block but fuel return line from the pump could pull fuel.

The fuel is going somewhere. It's not leaking out, so it is within the system.
 

73FJ40

After another night of rust removal!
SILVER Star
Joined
Dec 28, 2006
Messages
1,477
Location
Western Massachusetts
Why not? There are no check valves in the system. You can blow the fuel lines using compressed air back into the tank.

Fuel shutoff solenoid maybe will block but fuel return line from the pump could pull fuel.

The fuel is going somewhere. It's not leaking out, so it is within the system.
The path from the float bowl back to the fuel tank is through the needle valve.

How does the fuel, half way filling the bowl, levitate past the needle?
 
Joined
Feb 15, 2016
Messages
324
Location
Somerset KY
Theory - try this, next time you shut it off for the night or several days, unscrew / open the gas cap.

Guessing, vent hose plugged or soft, results in a vacuum in the gas tank, which sucks the fuel back.

Fuel pump is strong enough to overcome the vacuum.

Between fuel and other leaks, vacuum is eventually reduced.

I had the problem with my 79 FJ40. Got tired of pouring little gas down the carb to get it running.

Blew out all the lines and replaced most of the soft lines.

So far, it starts even after sitting for weeks.
Its impossible to suck fuel back through the needle and seat as it would be closed when float bowl was full and is at a higher elevation the the fuel level.
 
Joined
May 24, 2016
Messages
345
Location
Bainbridge Island, Washington
Indeed- drive it every day! Mine is a 67 and fuel drained out of the bowl before and after rebuilding the carb, and still does with a NOS carb I installed last year. The rebuilt carb clearly had fuel around the base of the carb, while the rebuilt carb seems drier, though drains about as fast (two days and the sight glass is empty). Just installed an electric pump inline, hidden under the battery- run it for a few seconds and it starts right up. Nice to know if the mechanical goes out on the road, I've got backup.

IMG_1220.JPG
 
Joined
Oct 30, 2007
Messages
9,739
Location
Plano texas
I forgot I used to put electronic pumps back by the gas tank on the frame. You have to write down in your maintaince book you put it there so you will remember years later when they go out.Years ago wife said truck quit working,I was out of town. The mechanic called me because they put a new mechanical pump on and the truck still wasn’t getting fuel did I have any ideas... I remembered the extra pump in the back. They replaced it too...problem solved. Mike
 

Dizzy

GOLD Star
SILVER Star
Joined
Feb 5, 2018
Messages
1,899
Location
Santa Fe, New Mexico
I'm still thinking that it is something to do with the fuel cut solenoid. From a Keyster rebuild kit, a couple of '75 carbs, and an aftermarket solenoid valve, I came up with about six different combos for both o-ring sizes, and the diameter of the solenoid where the o-ring sits. If the solenoid can't seal when the engine is turned off, why can't fuel go up a capillary, and out a tube that exerts negative pressure due to siphoning, partly as a result of the very unusually tall dimension of the Aisan smogerator? I could see that happening on the secondary circuit as well, but I think that, currently, all of my bowl emptying is mostly due to evaporation, as the air cleaner assembly usually has some liquid that probably condensed on the inner surface.
 
Joined
Jul 25, 2019
Messages
21
Location
Sumter, SC
So still looks to be "unsolved" to me...some of the "fixes" sound plausible, but I don't understand why problem would persist after rebuilds. Multiple folks said that is the case. Anyway, seems like the easiest place to start is with bowl and solenoid "o" rings. BTW, I forgot to mention I have my fuel solenoid wired directly to switched circuit...opens when key is on. Wasn't getting 12v to open it from "computer" so I bypassed it. Don't think that would cause this problem, but thought I would mention it. On the other hand, I had it wired like that before fuel drain issue started. I know some intrepid individual will track this down and everybody's problem will be solved.:cheers:
 

Users who are viewing this thread

Top Bottom