Carb bowl draining overnight

[ducktapeguy]

After sitting overnight, the fuel level on my carb drops to the bottom of the sight glass. I've taken it apart twice in the last two days, but can't pinpoint the problem. The engine runs fine, starts up easily, idles great, so drivability isn't an issue, but it just bothers me to have fuel dripping down my engine all the time.

It's a 1975 stock 2F engine with stock carb. It's not desmogged, but I did take the EGR cooler and EGR valve off because they were plugged up by the PO anyway. So right now the carb is capped off at the EGR insulator, I don't know if that makes any difference. And I did find out that I'm missing the o-ring on my idle solenoid, but can't see how that would affect anything.

I disconnected the fuel pump lines thinking maybe it was siphoning back torwards the fuel pump, that wasn't it. No visible leaks on the outside either. First time it was happening, it was leaking from EGR valve, that's why I took it off and capped off the tube coming from the EGR insulator. It's not doing that since I took it apart again yesterday (I uncapped to check), but the fuel level is still going down. One thing I do know, it's definitely draining into the intake manifold, I saw gas at the bottom of the manifold last time I took the carb out. Nothing dripping from the venturis when parked, I can't see any fuel dripping down the walls anywhere, and I inspected the casting carefully and no cracks, so I have no idea where the leak is coming from. The only possibility I can think of is the power valve? Thats' the only thing below the fuel level aside from the jets. Is there supposed to be a check ball or anything in there? When I took it apart, there's just the spring plunger and end cap orifice, I don't know whether there's anything else missing inside. This problem is starting to drive me crazy!

 

[Degnol]

It is probably siphoning into the carb. If you pull the aircleaner and look down the air horn, you'll probably see wet fuel.
I think PinHead's "Oklahoma Rebuild" might cure it, at least short-term.

GL

Ed

 

[Pin_Head]

The only possible ways for fuel to siphon out of the bowl would be from fuel outlets that are below the level of fuel in the bowl. These would be the idle/transition slot in the primary bore and the transition slot of the secondary. Both of these outlets are in the cast iron throttle body section. There are a couple of air vents that prevent fuel from siphoning; one in the slow jet and the other at the top of the primary and secondary bores of the bowl casting. To siphon, both of these air vents must be plugged and in the case of the primary bore, the fuel shut off solenoid must also be leaky.

It is possible that the air vents could be plugged, so you could clean them out either by trying the Oklahoma rebuild or if that fails, taking off the top cover and running a wire through them.

I had a '76 carb that leaked overnight into the intake and it wasn't due to siphoning and plugged air vents. I through in the towl and got a new core carb to rebuild and that one doesn't have the problem. I suspect that the casting may be cracked, but I didn't bother finding out.

 

[Nomis]

The stock carb gets mighty toasty on shutdown. Most folks aren't running a carb fan - couldn't the heat cause some of the fuel to evaporate over time?

 

[ducktapeguy]

Update - Took the carb apart again tonight (I'm getting really good it) and just noticed what might be causing the problem. Is there supposed to be a ball bearing underneath discharge weight? It's that aluminum golf tee like thing right before the accelerator pump nozzle. Seems like there should be one there. I don't know how that might drain the carb bowl, but I notice that the fuel only drops to the bottom of the sight glass, which just happens to be the same level as the hole leading to the accelerator pump chamber.

 

[Degnol]

Update - Took the carb apart again tonight (I'm getting really good it) and just noticed what might be causing the problem. Is there supposed to be a ball bearing underneath discharge weight? It's that aluminum golf tee like thing right before the accelerator pump nozzle. Seems like there should be one there. I don't know how that might drain the carb bowl, but I notice that the fuel only drops to the bottom of the sight glass, which just happens to be the same level as the hole leading to the accelerator pump chamber.

There is one in mine.

Ed

 

[Pin_Head]

It is not going to siphon from the accelerator pump circuit because the outlet jet is above the level of fuel. If there was a crack between the pump circuit holes and the inside of the bores, then that might explain it.