The Stovebolt Site Forums The Shop Area Engine Shop Carburetors leaking down after running

A question in a different thread prompted this thread.

A carburetor "leaking down" is generally a misconception by the enthusiast. Can the inside of the manifold under the carburetor be wet? Certainly. Can the fuel bowl be low? Also certainly.

So if "leaking down" is a misconception, what causes the above?

The answer is generally the volatility of modern fuel. Volatility of any liquid is measured on a scale called the Reid Vapor Pressure Scale (probably because a gentleman by the name of Reid devised it). The scale refers to the propensity of a liquid to vaporize at standard conditions. The higher the number on the scale the more "volatile" the liquid, or, greater ease in converting the liquid from the liquid state to the gaseous state. The scale in scaler in nature, rather than exponential.

Gasoline volatility in the 1940's, 1950's, and 1960's could vary from maybe 5 to 6 1/2, depending on the season (winter fuel is designed to be more volatile than summer fuel) and the locality.

Today, other than gasoline especially formulated for auto racing, or for aircraft use; I doubt seriously that one can find fuel with a Reid VP of less than 10. So, apologies to the moderators, this truly is a "political" issue; where various government agencies have regulated the fuel companies.

So what actually happens?

When the engine has been running, and up to normal operating temperature and then turned off:

(1) the inlet check valve in the fuel pump closes, preventing fuel from draining back to the tank.
(2) the residual heat in the engine heats the fuel in the fuel line, and, due to the volatility of the fuel, creates a pressure which the carburetor fuel valve/float combination cannot withstand.
(3) the carburetor fuel valve opens, allowing the pressure in the line to be bled off, in the form of fuel flowing into the carburetor. Generally, from 1 teaspoon to maybe 3 or 4 tablespoons will flow; this will vary with the diameter of the fuel line, the length of the fuel line from pump to carburetor, and the proximity of the fuel line to the engine.
(4) the level of fuel in the bowl is raised ABOVE the main discharge nozzle(s).
(5) fuel then flows through the main discharge nozzle(s) and drops down onto the throttle plate(s). If the throttle plate(s) is/are closed, then fuel runs out of the throttle body beside the throttle shaft, and drips onto the exterior of the intake manifold. If the throttle valve(s) is/are cracked open, then the fuel runs down into the intake manifold beneath the carburetor.

But why is the fuel bowl low when you just said it was too full?

When the equilibrium point (where the pressure from the line can be stopped by the buoyancy of the float and fuel valve) is reached, additional fuel stops flowing into the carburetor. But the engine is still hot, and fuel from the carburetor bowl begins to vaporize, and escapes the bowl through the bowl vent into the air cleaner. As the pressure in the line has subsided, the level in the bowl is lowered.

Well, can a carburetor leak down?

Most cannot, unless the vehicle is parked on its top (upside down). Why? Most carburetors have fuel circuits that require the fuel to rise above the fuel level in the bowl to flow into the throttle area.

Are there exceptions? Yes. The most common exception is the Holley model AA-1 (a.k.a. 94). This carburetor utilizes a diaphragm type power valve (a.k.a. economiser valve) that has a direct passage below the valve into the intake manifold. If the power valve blows, fuel drains by gravity directly through this passage to the intake manifold. The Holley AA-1 was introduced in 1939; shortly thereafter, the term "leaking down" was coined.

Another example is the Holley type 4000 4-barrel. There are bleed holes in the top of the bowl cover that have a dual function: (1) an air bleed for the secondary circuit, and (2) an "air brake" to prevent syphoning of fuel through the secondary once the secondary has been activated and then deactivated. Holley service literature instructs the enthusiast to check these holes periodically and make sure they are clean. Unfortunately, much literature printed in this period went the way of the Sears and Wards catalogues The service literature was better, as it was not "slick" paper but I am showing my age.

OK, so is there a "fix"?

Yes, there is a fix, at least to the leakage. The fix involves adding a return line back to the tank to relieve the pressure in the fuel line. The easy way is to acquire a three-line (inlet, outlet, return) fuel filter from your FLAPS, and install this filter right at the inlet to the carburetor. The return line will be smaller than the inlet and outlet lines; thus a 5/16 inch fuel line would have a 1/4 inch return line. You then MUST run a new line to run from the vapor port on the filter clear back to dump in the TOP of the fuel tank. Note that this fix will minimize or eliminate the leakage, it will not help the lowering of the fuel level in the fuel bowl.

There is another fix, but if I mention it, the moderators would probably lock the thread and I didn't do all this typing to have the thread locked!

Jon.

Jon, you forgot one- - - -the early Quadrajets with the main jet passages at the bottom of the float chamber and soft plugs with the carburetor body material peened over the plugs. They were notorious for leaking the float bowl dry when the peening job wasn't done properly. The fix was a good cleanup job with a wire brush and a coat of very durable epoxy applied to the jet well area. I've done that to dozens, if not hundreds of Q-jets over the past 40 years!
Jerry

Jerry, you are, of course, correct. This was common on 1965~1967 Q-Jets. Rochester addressed the issue with the 1968 versions, by spinning the body over the plug rather than using the pressed-in plugs.

Cliff Ruggles, who wrote the definitive book on Q-Jets, has devised a threaded plug that is installed using Loctite, and solves the issue forever. The epoxy was generally good for about 5 years, which is longer than most will keep a vehicle. I differ here, as my 3 collector vehicles have been in my possession 39, 40, and 49 years respectively.

There are probably other examples, if we think about it; however, I was just wishing to give an example.

Jon.

Another candidate for a Tech Article!
My vote is that we make a whole tech article section just on carburetation (or however you spell it). (How to rebuild Rochester B's, Zeniths, Carters, and other useful articles like this)

I second the motion.

Jon, you are my latest hero. On my 37 Buick with a vacuum start switch Stromberg I had thrown in the towel on keeping fuel from bleeding into the intake after a run of as few as 20 miles. Run fuel line above the valve cover rather than past the T-stat housing in the front and above the exhaust manifold like the factory did, 2# regulator, float down till it started leaning out, a drag racers "Kool-Kan" which was just good for a short time till ice melted, a ball valve right at the carb inlet to cut off fuel till engine died, and jumping out and opening both hood sides upon stopping. The ones that helped at all were a PITA and more so in unfriendly weather. Not being able to beat it I finally brought out the big guns with a GM HEI, plugs set at 0.065 for a fat spark, and a special built 6v starter on 12v. Now it fogs out a big cloud of black smoke but it DOES start. Will definitely put in a return line and give it a test. When March comes and the Texas heat is on it's way it was for sure going to get the FiTech 2bbl EFI but I hate to give up my anti-theft push the foot feed down to start feature---even better than a stomp starter. Pic shows fuel line and HEI.

[img]https://i.postimg.cc/8f5W2Xd3/Dar-Dog-HEI-003.jpg[/img]

There's another, somewhat simpler fix that only involves a couple of T fittings and a short piece of tubing. Install a tee on each side of the fuel pump, one in the inlet line and another on the output to the carburetor, and join the two lines with a short piece of tubing. Here's the trick- - - -install a bushing resembling a carburetor main jet into the bypass line with a TINY hole in it, like maybe .010" or so. It won't pass enough fuel to interfere with normal running, but once the engine is shut down, it will prevent pressure buildup in the fuel line from the pump to the carburetor by bleeding the pressure back to the pump inlet.
Jerry

Am very interested in this other fix which would Jon deemed to be to politicly incorrect to share.

Maybe he will send you a Private Message.

That's what I like about this site, it is like the TV show "Masterminds" where the crook may be smarter than any ONE of the cops but not smarter than ALL the cops combined. Looks like y'all just aced FiTech out of 800 bucks and I get to keep my foot feed pedal start. Thanks Smarties.

I was working on a I-6 this morning and was chasing some gas leaks. I removed a fitting at the fuel inlet of the carburetor that the line from the fuel pump screws into. To my surprise I found a backwards needle and seat set up that apparently keeps fuel from emptying from the float bowl. Never saw one before and thought it was a good idea to solve the drain back problem. The fitting looked new compared to the 80 year old carburetor. I bet one of our expert searchers can find a source.🛠

Martin - many older carburetors (including the Carter type BB-1) have a fuel valve external to the carburetor. The valve seat screws into the carb from the outside, and the fuel line screws directly into the fuel valve seat. Virtually all single barrel Strombergs in the 1930's including the so-called Stromberg 97 (actually an EE-1) are so-equipped.

It minimizes volume inside the carburetor required for the float arm.

Jon.

Jon, would they work on the carburetors on our Stovebolts or too restrictive?

Martin - don't know, guess if one has a large enough hammer, something can be made to work.

But why bother??? Even the Rochester B's cannot leak back to the tank. As Jerry is so fond of saying "this is a solution looking for a problem"

Jon.

Thanks