Unsolved mystery of carb bowl drain

[Wakanai]

Hi guys,
I think I may have some progress.
Most reports I see is when people rebuild the carb with a new gasket set. So did I.

I noticed the drain does not only happen after a test drive when I shut down the engine, but also during driving actually. During a test drive, I stopped and checked the (closed) secondary throttle and saw fuel collecting there while the engine was still running idle.
As said, I have a dual carb and had fuel draining in both. So I removed the cover of the 'Idle Compensator' and replaced the gasket by the original one, and made sure the screws are really, really tight.
And that actually did it for me, in both carbs.
I did several testdrives, while stopping and checking, and also after shutting the down the engine, and I don't see fuel anymore on the secondary throttle and the fuel level stays high.

It would be helpful if someone could confirm this:
- when fuel drains from the bowl, you see it drip onto the secondary throttle valve
- you carb has an Idle Compensator at the back
- the gasket for the Idle Compensator was replaced by new from the gasket kit.
- tightening the screws actually helps
Thanks!

 

[Wakanai]

20 minutes testdrive today, afterwards the fuel drained empty in the secondary of one carb, the other one stays good.
The search continues...

 

[Pin_Head]

Check the air vent in the top cover for the secondary slow jet circuit. It is supposed to prevent fuel siphoning as long a it is not obstructed. Mine would leak out of the slow jet even though the vent hole was clear, so the only way that could happen is by surface tension and wicking up the rough surface of the slow jet circuit in the top cover after cleaning it in carb dip. If I still had one of these carbs, I would try applying a thin film of gasoline resistant epoxy in the bottom part of the hole to see if this prevents wicking. Either that or a short piece of teflon tube. Gas can’t climb a hydrophobic surface.

 

[Wakanai]

Check the air vent in the top cover for the secondary slow jet circuit.

Actually my model does not have a second slow yet, there's only one.
I see some models have also 2 cut-off solenoids, mine has only one.
So I still need to find a diagram that matches my model.

 

[Wakanai]

As I noticed the fuel level was too high in both carbs, I opened the top covers and adjusted the float.
(Actually the level is too low now, so I have to do it again).
But now the left carb starts leaking fuel again on the secondary throttle valve after a test drive.
And I cannot see from which hole the fuel is coming...
And it goes fast, the whole barrel empties in less than a minute.

 

[Wakanai]

Actually my model does not have a second slow yet, there's only one.

Correction, it does. I found it together with the second Main Jet in the second venturi. That one has 2 brass pipes.
This second slow jet is connected to an opening in the housing that runs down just on top of the second throttle valve.
I check by spraying WD40 oil in it and they're all clear, not blocked.
So how can the fuel run through the second slow jet and down the throttle instead of vaporising through the second slow jet?
If the second venturi gasket is not good, I guess fuel could just overflow like that...

 

[Pin_Head]

It flows by surface tension. Fill a glass with water and put a strip of paper in the glass draped over the side to the bottom and see what happens. Reformulated gasoline has a higher surface tension than non-oxygenated gas. That is why it creeps over the top of the slow jet and drips from the slot above the throttle plate.

 

[Steamer]

Another term, "Capillary Action"

 

[Wakanai]

Update of the day: I'm now using 2 gaskets under the second venturi, and now the bleeding has stopped.
After the testdrive, the fuel does not run onto the second throttle valve anymore.
And fuel level in the bowl/barrel stays high.
Problem solved, I guess. And cause: siphoning as Pin_Head mentioned a few times.

 

[456z]

F16Roo Some good ideas here but wanted to add my 2c.
I believe it being a new problem is a clue.
As pinhead has pointed out make sure your solenoid O ring is good but wanting to add the best o ring will not help if your solenoid's seat/plunger is shot and leaking.

 

[sandcruiser90]

This is my situation. I put fluorescent dye in the carb. I see the leaks. Can someone explain the problem? I know little about carbs.

 

[sandcruiser90]

Other side

 

[Pin_Head]

It appears to be seeping out of your secondary around the throttle shaft, so it is probably either seeping out of the slow jet circuit or perhaps event he secondary main nozzle. Get your fluorescent light in there and see where it is dripping from inside.

 

[sandcruiser90]

I poured some dye in the carb and let it sit overnight. Did not start engine. This is it this morning.
@Pin_Head How do I see inside to see where leaks are coming from. What do I be look for when looking in from top?

 

[Pin_Head]

Remove air cleaner, open choke, and look inside. Look for gas drips on the throttle butterflies, then open the throttle and look for gas in the bottom of the manifold. If it is leaking from the main jet, you will see a drip trail on the center of the butterfly. If it drips from the slow jet transition slot, it is harder to see but you will often see gas on the bottom of the manifold.

 

[Wakanai]

As pinhead has pointed out make sure your solenoid O ring is good

Anybody has any idea what the correct dimentions would be?

 

[nachohouse]

Having a similar issue as post #54. Throttle shaft hinge on the primary side is leaky. Stock 2 bbl Aisan on my F145. Fuel bowl will empty completely after sitting for a few days.

If the primary throttle shaft looks wet, what’s the source of the leak or how can I break the wicking/siphon effect?

 

[Aloha Jen]

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.

I know this is an old post, but did you ever get to the bottom of this? Your original post and problem is what I’m going through on my ‘75 with a rebuilt ‘78 Aisan carb. Just wondering if you ever solved it on your 40?

I poured some dye in the carb and let it sit overnight. Did not start engine. This is it this morning.
@Pin_Head How do I see inside to see where leaks are coming from. What do I be look for when looking in from top?

View attachment 2605980

View attachment 2605981

Did you ever figure out the main source of your leak? If so, how did you solve it?

Having a similar issue as post #54. Throttle shaft hinge on the primary side is leaky. Stock 2 bbl Aisan on my F145. Fuel bowl will empty completely after sitting for a few days.

If the primary throttle shaft looks wet, what’s the source of the leak or how can I break the wicking/siphon effect?

View attachment 3521107

Were you ever able to fix the leak coming from the end of the shaft hinge?

 

[sandcruiser90]

No. I did have carb rebuilt by Marshall. After sitting for days it takes a long time to start.
I’ve cut differed putting in an electric fuel pump to prime it.

 

[F16Roo]

No haven't figured it out...bowl doesn't drain, no obvious signs of leaks, don't see any fuel in carb/manifold. I'm thinking somehow the fuel pump just loses prime and it takes about 30-45 seconds of cranking to re-prime and then runs fine. I'm just living with it until I can figure it out.