Apparently my truck now has gravity feed fuel tank. It's the only answer I can arrive at for the condition it has now developed.

It appears that when I fill the fuel tank completely (the tank is still located in the cab) the fuel level goes higher than the carburetor and because the Holley floats shut off at 6psi, gravity over pressures the floats and fuel pisses out all over the top of my engine. I think the engine sits a little low in the frame and that is why it happens. It is called siphon action.

I am going to try a fuel pressure regulator between the mechanical pump and the carb and set it at 5psi and see if that stops the flow.

 

Make sure that the cap is venting - work down from 5, 3 or 4 might be better
I just put in a new elec pump with a 5 psi rating and an adjustable regulator set at 3 psi along with a new carb that wasn't warped - It's a flat 6 and it was peeing gas on the manifold

 

For every ~3 ft of fuel height above the carb, you get one psi. To be overpressuring you'd have to have 15 ft, not likely, so Dick has likely identified the culprit.

But is this one of those Holley 4-bbls with a single fuel inlet and a feed tube connecting the two metering blocks? The O-ring seals on those go bad all the time.

 

It is a freshly rebuilt Holley 1850 600 cfm like you described. I have a vented fuel cap and the only time it does it is when the tank is full so it must be gravity.

I know when I disconnected the fuel line from the carb I had to use a rubber plug and raise it up over the fender height to stop it from flowing even after making sure the gas cap was off.

The guy who rebuilt it has a test bed and the carb holds 6 psi pressure all day long there. Put it on the truck and it flows like no tomorrow.

 

Check your manifold for level - If the carb holds on a test bed but leaks on the truck it sounds as if something is warping

 

Freshly filled fuel tanks can, depending on the season, be filled with cool gasoline that will expand upon warming up. Not likely in Winter unless you're cranking the heater. Then an unvented tank's pressure could climb freaky high. Bad juju.

 

OK, so the manifold mounting surface is flat and level. The gas cap is vented. The temperature today was 67° F. The carburetor is about 8" below the level of the top of the fuel tank. The carb leaked when I released the vise grips that were clamping off the fuel line.

I installed an adjustable pressure regulator between the pump and carburetor and low and behold, it's been 4 hours since I filled the tank again and the manifold is bone dry. Pressure is set at 3 out of a range of 1 to 5.

I think I am good to go now.

 

I'm guessing that your vent/cap may not be so well vented. To test this, you'd need to go back to having no regulator and pull the cap off to see if she still pukes. That's no fun, so, it sounds like you have found a solution.

 

Did you read my previous post about fuel flowing even with the cap off?

The manifold is still dry this morning and there was no hiss when I removed the cap to see if it had built pressure.

 

Ok, I missed it. Hmm, now you're making me think. Maybe your float is just starting to lose buoyancy. I dunno.

Might want to keep an eye on it for a while.

 

Are you using a Spectre regulator that is round and you turn the round cap to adjust? You may be trading one problem for another. They are awful.

I don't see why a regulator would help anyway, it should be wide open at low pressures. Do you have an electric pump besides the mechanical?

 

No electric pump, only the mechanical one. The carb re-builder checked the floats for pinholes and they are good, there are no issues with the floats. The regulator was suggested as a possible cure. It sounded logical to me that if gravity was causing the fuel to force the floats to open due to over pressurization then the regulator would stop that issue.


When I called Holley Tech support they told me the problem was the fuel level being higher than the carb and that I needed to fix that problem. They would not talk to me further about anything with the carburetor.

This is not a new configuration. The truck has been like this since before I bought it 25 years ago. All I did was freshen up the engine and carb. I had this problem once before shortly after I bought the truck and a friend of mine adjusted something on the carb and the issue never happened again until the carb got rebuilt. I wish I could remember what he did to the carb then so I could do it again.


Back to the drawing board. The carb is leaking again this morning with no gas cap on and the regulator in place. It is probably not pressure related then.

My carb guy thinks he knows what the issue is and is coming by tomorrow to try to fix it. He thinks it's something that warps easily in the rear float bowl and is going to try doubling up the gasket to fix it. We'll see. I'll post the results.

 

Sounds like the metering block. Do those carbs still use power valves?

If you run the fuel line up high on the firewall before it drops down to the carb it sould prevent any siphoning.

 

Here's a link you may want to read.

Holley CarburetorNeedle and Seat Flooding Repair Page 1