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- #21
BTW....drive it more is leading at this point.
Unsolved mystery of carb bowl drain (1 Viewer)
- Thread starter F16Roo
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rkymtnflyfisher
Trout Bum
It's the best answer for almost every problem in life.
I can sometimes watch the fuel boiling in the float after I park. I do not have the heat riser valve anymore but rather a steel plate instead. The intake definitely gets thermally soaked.
The problem persists because the surface of the castings is oxidized or etched by water and its associated corrosive compounds in the fuel, creating a surface that can wick the fuel up and over the top cover. Rebuilding doesn’t change that. That is why I think that polishing the fuel passsages or installing a short piece of teflon tube in the cover fuel passage might fix the problem.
The first step is finding where the fuel is leaking from. Put a drop of fluorescent tracer in the bowl through the vent tubes and see where it ends up. Mine ended up on the bottom of the intake manifold after dripping out of the idle port.
Heat had nothing to do with it. It would drip out even when filled without the engine ever starting.
The first step is finding where the fuel is leaking from. Put a drop of fluorescent tracer in the bowl through the vent tubes and see where it ends up. Mine ended up on the bottom of the intake manifold after dripping out of the idle port.
Heat had nothing to do with it. It would drip out even when filled without the engine ever starting.
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- #25
OK here's follow up...put tracer in bowl and checked with UV light. I couldn't see where fuel was leaking. Anyway while I had it opened up decided to try "Oklahoma rebuild." Well that didn't go very well at all. At first would idle OK but stumble coming off idle during acceleration. That problem went away and now can't get it to idle...starts fine with choke, runs fine at higher RPM, but when goes to idle misfires and then dies. Pull the choke out just a little and it runs fine again. Idle circuit maybe? Anyway, to clean it without removing carb?
A complete and proper rebuild is not difficult. Make sure you get a quality kit like the Keyster. I used a Hygrade kit that included the diaphragms. There is a guy on you tube that walks you through he process if you don't have a manual. I can't remember his name, I'm sure someone else does.
The most important thing is to completely disassemble the carb and blow compressed air through every passage. Then an overnight soak in chem dip, followed by even more compressed air. An ultrasonic cleaner can be a huge help too.
My carb leaks off, although it runs quite well. I'm not worried about the leak. It should stop when I put the Holley on.
4Cruisers
SILVER Star
Did you check for fuel at the bottom of the intake under the carb? You need to open the throttle plates to see down there.
The off idle stumble and idle problem are due to either lack of idle fuel or a vacuum leak. It is difficult to distinguish the two possibilities, so you just check for vacuum leaks everywhere the manifold can leak (including cracks at the bottom of the manifold under the carb) and if there is no evidence of a vacuum leak, then it is more likely a idle fuel deliver problem. If you disconnect the idle fuel cut off solenoid and there is no change in the idle, this may indicate that there is an obstruction before the solenoid, like the slow jet.
You can also try removing the solenoid and the idle mix screw and then spraying carb cleaner and compressed air in and see if you can clear it. Sometimes the solenoid o ring fails and clogs the idle circuit down stream between the solenoid and screw.
The off idle stumble and idle problem are due to either lack of idle fuel or a vacuum leak. It is difficult to distinguish the two possibilities, so you just check for vacuum leaks everywhere the manifold can leak (including cracks at the bottom of the manifold under the carb) and if there is no evidence of a vacuum leak, then it is more likely a idle fuel deliver problem. If you disconnect the idle fuel cut off solenoid and there is no change in the idle, this may indicate that there is an obstruction before the solenoid, like the slow jet.
You can also try removing the solenoid and the idle mix screw and then spraying carb cleaner and compressed air in and see if you can clear it. Sometimes the solenoid o ring fails and clogs the idle circuit down stream between the solenoid and screw.
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- #29
Thanks appreciate the expert advice. Really don't think it is vacuum problem since it started just after I was jacking around with carburetor. Was thinking I might have dislodged some "gunk" when I did the OK rebuild. However, I do think the brake booster could be leaking some...I hear a bit of a hissing sound sometime from around brake pedal...never seemed to affect idle though. One other question...after taking out idle mix screw what is best way to set idle. Thinking I might should get carb rebuilt...never been done to my knowledge and I've owned it over 33 years. Thanks again.
- Thread starter
- #30
Mr. Pinhead after your analysis decided to start over. Idle solenoid first...as I said I had it direct wired to radio circuit so it would be open when switch was on. So, no 12v there. Aha. Traced it back to fuse for radio, 5A blown. Replaced fuse idle back to normal. Lessons learned...don't jump to conclusions, start with simplest possible causes first. Not sure why fuse blew...maybe when I was messing with carb I shorted wires on solenoid? Anyway, back to previous normal. Still working on carb drain. Thanks.
Hello everybody,
I found this thread because I was searching for a solution for the problem I have, namely carburetor draining almost empty after driving.
I don't have the same car, but a 75 Celica that also has dual Aisan carbs. I have acquired several of them, tried to rebuild them, not always sucessfully.
Anyway, I now have a good set that I managed to adjust and synchronise perfectly.
The only problem is, after a test drive the bowl runs empty.
I'm not using airfilters at the moment, which allows me to look in the carbs from the air-inlet.
What I noticed, and what nobody mentioned yet, is that the fuel runs out of the bowl into the second inlet, on top of the closed secundary throttle valve.
And it stays there, while slowly leaking down into the inlet manifold (I can see small air bubbles).
Result is of course that the car starts well when cold (there's always some gasoline in the bowl, it's not completely empty) but because of all the gasoline in the inlet manifold it takes a long time of cranking before it starts after driving it first.
I looked at the left part of the drawing on the previous page, could it be that gasoline runs through the secondary slow yet and than down the dotted channel onto the secondary valve?
I have not found a reason for this yet, but from reading above explanations it could be the O-ring of the solenoid that I replaced.
Does that make sense? Can anybody with the same problem confirm that the fuel actually runs empty in the secondary?
Thanks!
I found this thread because I was searching for a solution for the problem I have, namely carburetor draining almost empty after driving.
I don't have the same car, but a 75 Celica that also has dual Aisan carbs. I have acquired several of them, tried to rebuild them, not always sucessfully.
Anyway, I now have a good set that I managed to adjust and synchronise perfectly.
The only problem is, after a test drive the bowl runs empty.
I'm not using airfilters at the moment, which allows me to look in the carbs from the air-inlet.
What I noticed, and what nobody mentioned yet, is that the fuel runs out of the bowl into the second inlet, on top of the closed secundary throttle valve.
And it stays there, while slowly leaking down into the inlet manifold (I can see small air bubbles).
Result is of course that the car starts well when cold (there's always some gasoline in the bowl, it's not completely empty) but because of all the gasoline in the inlet manifold it takes a long time of cranking before it starts after driving it first.
I looked at the left part of the drawing on the previous page, could it be that gasoline runs through the secondary slow yet and than down the dotted channel onto the secondary valve?
I have not found a reason for this yet, but from reading above explanations it could be the O-ring of the solenoid that I replaced.
Does that make sense? Can anybody with the same problem confirm that the fuel actually runs empty in the secondary?
Thanks!
The solution is between the carburetor and the tank, I'm guessing the seal in the fuel line is broken, allowing back draining to pump/tank circuit. It may be sealing under pressure, becoming a vent when pressure is taken off.
So, how does the float bowl get emptied in your scenario?
I know a lot of my fuel disappearance occurs as a result of forced evaporation from heat transfer. once the bowl is pressurized it can go anywhere it has a place to escape. No worries, the fuel injection will solve this issue.
Maybe this does occur but I haven't observed any liquid fuel in the intake in my case.
Wouldn't the reason for the vent be to allow the pressure difference to become equalized? To say that the bowl does not see vapor pressure at all is to say that a vent is not necessary. There is always some sort of pressure difference, this is what drives flow.
I say this because I can see my fuel changing state through the sight glass. I'm 99.999% sure it (the vapor) doesn't remain in the bowl until the enthalpy is reduced enough for it to condense again. In fact it cant due to the volumetric change that occurs between the states.
I'm going to conservatively assume the float bowl will hold 2 cups of fuel at full level, this is approx. .47 L. This fuel would only need to absorb 123kJ of energy to reach its enthalpy of vaporization (@ 25 *C). 123kJ is only 30 food calories worth of energy. For a good comparison, a box of raisins contains between 100-130 calories.
With the exhaust and intake manifolds being bolted together and the carb sitting directly above the exhaust, there is magnitudes more than 123kJ of energy moving about the various components. I think it is reasonable to assume that there is a significant amount of float bowl fuel loss due to thermodynamic processes. I think this partly describes why when one gets around an older vehicle that is carbureted, you can smell fuel vapors.
As far as the fluid mechanics involved with the wicking of fuel through the jets, this would be a rather complicated study. I'm not sure any amount of assumptions could arrive us at any reasonable conclusions. Anyone here study fluids and want to chime in?
My intake manifold is basically vacuum-tight, but the crack allows carb-drool to find its way thru, to the headers before the next day's startup. I've only seen drip from the (solenoid-closed) primary when I unbolt the Aisan from the manifold.
I have the same problem. Starts right up if used everyday. If not used for a week then several minutes to turn over. Not sure if better starting with choke in or out.
