Fuel return with Webers?

[acaskie]

This is an issue for me and at least one other person I know running a weber set-up: When the engine is shut down, the remaining pressurized fuel in the system finds a way to squeeze out, presumably along the butterfly shaft, and leak down onto the manifold. In my case, it isn't a lot of fuel, usually about a teaspoon full, but enough so that when it evaporates on the manifold it leaves a noticeable residue. Call me old-fashioned, but it doesn't seem like such a good idea to have any fuel running around loose in the engine compartment.

Even with only 3lbs of pressure showing on the gauge while running, when the engine is shut down, the pressure initially rises to about 5lbs before slowly bleeding off.

I'm not sure that this is enough of a fuel stream to consider a fuel return mechanism, but I am curious if this is a common problem and what others have done about it.

[Bob Putnam]

5 psi is probably a bit much for the Weber floats. Why not just run a return line with a restrictor back to the tank? All you need is a 1/16" hole to return excess pressure but not bleed off so much that you starve the engine.

Just as an experiment to check whether your standard float levels are causing this, I would run the car, shut it off, and then bleed off pressure at the pump. Then check if you're still getting the fuel drip.

[Tom Wells]

Hi Allen,

Consider this the blind leading the blind.

Could this post-op pressurization be a due to a defective fuel pressure regulator, or one that is inappropriately sized or chosen for this particular application?

I can't picture anything the carbs could do to pressurize the fuel line after the engine stops. They'd almost have to have tiny pumps in the float valve area, so I'd suspect something about the behavior of the regulator instead. Then again, with Webers anything might be possible...

If this makes sense, it'll cost you a doughnut!

Tom

[zimmy]

Allen:

I know you were referring to me. I have spoken to several guys with the same problem. One of them is the gold webers at the ERA booth in DVSF II. It seems to be common. I think that the floats close the intake valve and the heat in the engine area raises the line pressure. I talked to Inglese and Dan checked the pressure regulators and they do not have a check valve or anything that he felt would cause the pressure to not drain. The suggestion of putting a small (65 jet size, or Bob's suggestion) seems to be the accepted method. They run the line back to the tank. I'm going to do this to mine while I have the engine rebuilt.

Zimmy

[Tom Wells]

Hi Allen & Zimmy,

Here's a quote from the Holley site that you might want to consider:

QUESTION After I shut my car off and come back out the next morning there is gas on the throttle shaft and puddled on the intake. What causes this?

ANSWER This is usually caused by percolation. This is when the engine is shut off and the engine temperature rises it causes the fuel to boil in the bowl and leak out of the boosters. There are a couple of things you can do to cure this one is make sure the fuel level is not too high. You can also lower the fuel level about 1/8" below the sight plug hole and this will cure it sometimes. The heat from the engine will rise into the carburetr sometimes and will cause the fuel to boil. Installing a phenolic heat spacer between the carburetor and the intake or a heat shield can cure this. These parts will prevent heat from getting to the carburetor and boiling the fuel.

Tom

[acaskie]

Bob, Tom, Zimmy,

Thanks for your input.

It doesn't seem to be the float level, so I guess a fuel return is the answer.

Bob, pardon my ignorance, but what did you have in mind with respect to a restrictor?

[Bob Putnam]

A,

Jeeeez - now you're getting technical!

It's easy for me. I'd just drill a hole in a solid piece of round rod and push it up the hose where it connects to the fuel tank return barb.

Maybe that there's something commercial available for someone who doesn't have a lathe and lots of raw material...

[Cal Metal]

The phenolic spacer will help, as Tom indicated. Additionally, the OEM Weber needle/seats are not the best--one of the inherent weaknesses of the carb. There is an excellent factory replacement that prevents any leakage pass the float. Give me an email if you have any interest.

Let me ask the question. On all the puddling that is found on the throttle shafts, is it typically on the last carb; that is, the carb that accepts the end of the fuel line?

[acaskie]

CM,

There appear to be puddles emanating from the first and the last carbs in the fuel chain. More out of the first.

Thanks for the tip on the replacement pieces. I will e-mail you on that.

Allen

[Tom Wells]

Hi Allen,

I queried Holley's tech line and they came up with YAS (yet another suggestion):

QUOTE:

From: fuelpumps@support.holley.com

the pressure should not rise when the fuel pump is not running. is the pump still running when the engine is off? which pump is he running?

thank you

ENDQUOTE

Hope that helps

Tom

PS: Even if this is off-target you may want to e-mail them some further questions

[acaskie]

Tom,

I think with those outboard floats and being farther away from the cylinder heads, the Holleys don't have the same issues with boiling fuel that the Webers do (all the innards being so close to the heat source).

Since I have a mechanical pump, it's off when the engine is. I have watched the pressure rise slowly after everything is shut down, so it's pretty clear what is happening.

Spacers aren't the answer, since the clearance with the hood is already pretty tight. So I guess I'll have to put in the return line to the tank or get used to wiping down the base of the manifold with degreaser every few days.

[rlearp]

As a novice Weber user the part about "boiling" fuel is interesting to say the least.

All the Holley's and other 4 barrel carbs I've used sit in the center of the intake manifold, where it is damn hot. But, I'm reading that Weber's have a boiling fuel problem more than regular 4 bbl carbs? Why? Bad design? This shouldn't happen.

I've got a set of Webers for my GT40 but I've always wondered why other than cosmetic reasons - my experience (limited but growing) tells me the 4 bbl will do better in drivability and CAN do just as well in hp if one if willing to give up a little (as the Webers apparently do).

Ron

[acaskie]

Ron,

I'm pretty much a Weber novice as well. What I have been told is that, when properly tuned, Webers deliver a more balanced charge to each cylinder than is possible with a dual-plane or single plane manifold. That plus the short runners apparently provides a big torque boost over the four-barrel set-up.

However, because each of the Weber barrels is directly over the intake port and such a short distance away, engine heat is transferred more easily to the fuel remaining in the carb after shutdown. As it heats and expands, the pressure rises in the system (what the Weber folks apparently refer to as percolation) and the pressure is relieved when fuel finds a way out through the throttle shafts. This doesn't happen with Holleys as easily because of the difference in construction and position further above and away from the intake ports. --Or I could be completely misinformed, but it seems plausible.

[Tom Wells]

Hello Allen,

That's a "Roger" on the mechanical fuel pump.

Which brings up another question about the regulator/pump combo. Could the input pressure to the regulator be bleeding through the regulator after shutdown?

This would require a return line to negate....

To amplify: the pump produces more than three psi, a higher pressure than your regulator setting. Say the pump is actually producing six or seven psi. Could the regulator be allowing part or all of the seven psi to bleed through it after the engine stops? Not sure how you could tell whether this happens or not, though, unless you monitor the input pressure to the regulator too. It wouldn't take much volume to bleed the pressure down since gasoline is essentially incompressible; perhaps a drop or two. Unless there is a gasoline hose somewhere that expands under three or seven psi, then relaxes after shutdown.

To put it another way, could the regulator quit regulating after the engine stops?

Still a puzzle.

Tom

[acaskie]

Tom,

That is certainly a thought. I wouldn't rule it out at this point, but the return line would seem to solve the problem either way.