Float bowl leaking??

[haystax]

The float bowl on my '76 Aisin carb is empty every time I try to start my 40 after only sitting a few days in between start-ups. I know this is a potentially bad problem with the cylinder walls being washed down and such - i think the leak is internal, cannot tell that there is gas on the outside of the the carb or anywhere. What do I need to look at to fix this problem?

Thanks

 

[65swb45]

If it's taking a few days to happen, in the middle of a hot summer, in high desert country, how can you be certain it is not just evaporating?

 

[Big B.]

My 72' 40 does the same thing. no biggie

 

[haystax]

If it's taking a few days to happen, in the middle of a hot summer, in high desert country, how can you be certain it is not just evaporating?

Fair enough, that could be it and I haven't noticed a gain in engine oil or a gasoline smell to the oil. I will keep a closer eye on things and report back. Thanks Mark.

 

[Nagalfar]

If it is leaking into the intake, you should be able to see a dark irregular spotting in the intake right under the carb if it is happening pretty regular. If you take and open up the throttle all the way and use a flashlight it should be able to be seen. You might wanna check it a couple of hours after you park it to see if you can actually see any gas sitting in the manifold under the carb(parking on curb so that the manifold has a tilt to it to help puddle the gas away from the motor side and make it more easy to see). Does it always happen, or does the engine have to be hot for this to happen? it would be best to check it while the engine cold, for the simple fact there would not be much evaporation of any puddled gas due to heat adding to evaporation. Do you have your heat shield on?

 

[Pin_Head]

My 76 started leaking about a year ago. I took it apart, put a kit in it, reassembled it and adjusted it. Then I filled the bowl and let it sit on the bench over night. No leak.
I put it back on and drove around and parked it and let it sit over night. Next morning the bowl is empty and the fuel is in the bottom of the intake manifold.

What's up with that? No it's not boiling out. I can fill it back up without starting it and it will leak out overnight. I threw in the towel and I am looking for a rebuildable core. It drives OK and it passed smog, so I'm not in a rush.

 

[IDave]

Can these things siphon back out?

 

[65swb45]

Here's my theory.

Logic says that in order to have siphoning [actively disappearing gas], as opposed to evaporation [passive] there would have to be a sufficient difference in ambient air pressure between the bowl vents and the discharge nozzles to cause the nozzles to draw gas thru them when the vehicle is not running.

The differential in ambient pressure is easy when the engine is running: manifold vacuum. With the engine off, the only difference in pressure that I can think of would be that created by the residual heat of the engine. Whatever position the crank is in when the engine is shut off, at least one intake valve and one exhaust valve would be open. Sometimes two.

That means that ambient air coming back up a cooling exhaust pipe can also travel at some point all the way into the exhaust manifold, creating fluctuations in the cooling of the manifold between the outside surface of the manifold and the inside. More importantly for the discussion at hand, the intake manifold will also continue to recieve residual heat from which ever cylinder has the intake valve open.

I do not think that the cooling process of the engine occurs in a straight line from hot to cold; rather, I think it occurs in a series of small 'pulses'. When you shut off your engine, the water temp continues to climb for a while, often getting 30 to 40 degrees hotter than running temp, which is already 60 or 70 degrees above ambient temp. So now we are dealing with cylinder head temps over 100 degrees above ambient. Temp creates pressure.

The very hot air in the one cylinder which has the open intake valve builds pressure behind the closed butterfly valve in the carb until it 'burps' that hot air out. Now the cylinder has vacuum. Then the cylinder draws in a charge of cooler air, which happens to pass by the carb discharge nozzle, creating a 'draw' on the carb.

Also consider that even on days when the truck is not running, the oil, coolant, and engine block will store passive environmental heat and release it again at a different rate than the aluminum body of the carb in which the gas is stored. Granted, it's not a great temperature differential. But its still enough to create vacuum.

 

[Pin_Head]

I don't think that would explain my leak as I can fill the carb without starting it and it will leak out at the same rate as if it is hot. I'm leaning toward a hairline crack somewhere in the bowl that gets spread when I tighten the bolts to install it. Even that seems pretty far fetched.

They can siphon under very unusual circumstances. The only outlets that are below the bowl level are the primary idle/transition slots and the secondary transition slots. In order for fuel to siphon from them, all of the air vents at the top of these circuits would have to be plugged. In addition the idle solenoid valve on the primary would have to leak. I made sure that all of these air holes were open when I rebuilt it. I think that I will try replacing it before I have to go in for another smog check in two years.

 

[nicksfj-40]

How critical is it to get the measurments with the float correct? I mean what if you are off a couple of millimeters? I think that is what is causing mine to overflow into the intake.

 

[Pin_Head]

It has some effect on how rich or lean it will run, but I don't think a couple of mm will be a problem. Better a little high than low. When my fuel level was high (mabe 6 mm), I noticed that it would pull a little fuel out of the main nozzle at idle with the throttle all the way closed, which made it hard to set the idle mix so that I could pass smog.

 

[Nagalfar]

Try blocking your bowl vent and see what happens when the bowl is full.. that would eliminate it the fuel doing the evap out the top of the carb (boiling out).. that only leaves the other answer of it heading into the motor when parked, I think if you had a hairline crack you would see the area that the gas would be weeping into or onto on the outside the carb or the area close to it.

 

[haystax]

I plan on driving the 40 this afternoon, it has been sitting for about 10 days to two weeks and I am sure the float bowl will be completely dry. I'll look down into the manifold with a flashlight and see if there is a residue indicating a puddle at some time. I'll check it again tomorrow night and see if the float bowl is still a little full, if there is a puddle or any other leak.

I have to admit I have never really gave much thought to the whole cylinder heat differential creating vaccum theory but it does make sense, what doesn't make sense is the fact that all our other gas pickups that get used only once every couple weeks seem to fire right up, or at least none exhibit the exact behavior as the Toyota does...

I have no reason to believe the carb is in any less than perfect condition after a recent rebuild/mod by a reputable Cruiser mechanic, thanks again Mark...and thanks everyone for the input.