I have already posted this in the Y-Block section, butI know a lot of you have extensive experience with the Y-block also.

I have a 1955 Y-block 272 with a stock 2-barrel Holly carb. It apparently has a fuel leak that I cannot find. When I drive it somewhere, I can smell gas when I get out. I look under the hood, and there is a black gummy residue on the passenger side of the intake manifold just at the base of the carb. I can wipe it clean and start the engine and no gas leak is present. But if I drive somewhere, it happens again. Is it possible that the float is sticking intermitantly? I have already checked the screws and nuts to make sure they are tight. Have any of you had this problem with your trucks? Thanks, Jag

 

They make a powder (I forget what it's called) that turns bright flourescent pink when it comes in contact with gas. We use almost the same stuff on our jets to track down fuel leaks. It has the same consistency as baby powder, really fine. Just wipe down the area really good, squirt that orange powder on, and the second it touches even a tiny drop of fuel, it changes color. It makes it really obvious where your leak is.

 

Thanks, that sounds easy enough even for me. I'll check with Napa to see if they have it. Jag

 

My Holley 94 is the same way. I've come to believe it is coming out the vent, but I've never figured out how. I was sure it was from the lower body/bowl gasket (near power valve) but I filled it and set it on a block of wood, and nary a drop came out (the wood holds the gas a long time)

 

I did an internet search for "fuel leak detection powder" and all I could find was the stuff used for jet fuel. I'm sure I've used something similar on an old T-Bird I used to drive, and that was before I ever even touched a jet. WTF???

 

I haven't had a chance to get to napa yet but tonight might be possible. If they don't have it, they'll prob suggest using a match!! Jag

 

i am not sure what it is called, but motorcycle dealerships use this stuff all the time..

 

If the carb leaked extra fuel internally, it would not idle.

If it is hot under the hood, the culprit is the crappy gas we are forced to buy today that has had its evaporation point altered by smog rules, particularly so in CA. This is called "vapor pressure". So what happens is: you shut down, the heat under the hood transfers to the carburetor, rapidly evaporates the fuel through the vent tube, where it moves upward, being lighter than air and goes up to the air cleaner top, where it stays until the air cleaner cools, the gas condenses and falls down into the carburetor against the throttle blades, and if your shaft is worn, leaks a little through the shaft to the outside, and then drips down into the intake manifold where it stays. This is why a hot engine is so hard to start, and why it helps to floor the throttle when starting hot. Fuel injected cars do not have his problem, and we did not have it so bad when gas was not screwed with, and when lead was in it. (as God intended) This is why Holleys now come with smaller fuel bowls and why there are bowl stuffers to reduce fuel bowl capacity.

Regards,

Alanco

 

I had the same problem with my carb. It turned out to be the top of the carb was slightly warped and would allow gas to leak from the gasket.

 

After years of rebuilding and over tightening the ears were the screws hold the carb top on warp a little. Its easy to fix just plane the gasket surface with fine sand paper by fastening the sandpaper to a super flat surface and running the gasket surface of the carb on the paper.

 

I've always had success detecting fuel/vacuum leaks by using a length of hose (5/16th i.d. works fine) and probing around the carb with one end of the hose in my ear, stethoscope-like, and listening for that familiar hissing sound. Pay close attention to the throttle shaft openings and carb base. Also, as recommended above, when rebuilding the carb I always surface mating carb pieces on a flat surface, using 220 grit wet-dry sandpaper and a little lite (cutting) oil.

 

If it is a top plate leak, you will see gas residue on the sides below the leak. All the junk that is in gas will leave a residue. If the top is not leaking, then test for heat soaking by removing the air cleaner.

Regards

Alanco

 

The evaporation point as you called it may not be the culprit.

This is known as the Reid Vapor Pressure. This is the pressure produced by a sample of gasoline in a closed container where the temp is held at 100 degrees F

If you view this link: http://www.arb.ca.gov/fuels/gasoline/rvp/rvp.htm you'll see that the Reid Vapor Pressure of gasoline has been falling for many years. The lower the vapor pressure the LESS likely gasoline is to evaporate. The EPA wants this since low RVP results in less evaporation and less smog due to evaporation.

Californias gasoline now is at 7.0 PSI RVP which makes it the same as Aviation gasoline which has always been at 7.0 PSI. You need low vapor pressure gasoline at high altitude to prevent vapor lock..... 14 psi RVP gasoline will boil in an open container on a hot summer day

A gasoline with a high RVP would be MORE likely to evaporate (vapor lock is caused by high RVP and or high temp). Gasolines today are much lower than they have ever been making vaporization more difficult in a carburetor. (direct fuel injection engines are not as affected by low RVP since vaporization is accomplished by pumping the fuel thru an injector at high pressure instead of being vaporized in a venturi.


If you have a 2 bbl carb like the Fomoco model on my 292 you may have a float valve that's either leaking or the float is just set too high. There's a small vent hole just above the float chamber that appears to go directly into the float chamber. If the float was a little high or the valve was leaking a little then gasoline could overflow out of that hole and run onto the manifold. If the engine is hot then the gasoline will evaporate and fill the engine compartment with vapor. Not a very safe prospect.

(By the way gasoline vapor is quite a bit heavier than air.....from the MSDS for gasoline:
Vapor Density (Air=1): 3.0-4.0.....this means that gasoline vapor has 3-4 times the density of air. This is why gasoline vapor will hang on the floor if you spill it.

see : http://www.brownoil.com/msdsgasoline.htm



Regards,

Rick

 

I realize that lowering the Reid Vapor Pressure is supposed to lessen the evaporation of gasoline, but the opposite is occuring in the real world. I suspect that the vapor pressure of the higher octane stock required to produce an unleaded fuel is the reason. When tetraethyl lead was added to raise octane, a lower octane stock was used and it apparently did not evaporate as easily. I do know that in living at 6200' elevation, there are a tremendous number of cars that have heat soak problems and vapor lock problems. The evaporation from heat soaking is so bad in hot weather, that many cars have to use electric fuel pumps to refill the carburetor to avoid excessive cranking, and they get a lot of fuel dilution in the oil. I would trade all the unleaded gas in the world for some real leaded gas.......
Don't forget that heated fuel vapor will rise as it displaces the cooler air with pressure. This fuel vapor will also smell as it pushes through the air cleaner.

From what I have read, gasoline is made up from a lot of hydrocarbon types, pentane, octane, etc. and the "cracking" of the crude requires about 10% more stock for unleaded than it did for leaded fuel. In my own experience, gasoline goes bad more easily than it used to, and heat soaking is much worse. I suspect that the mixture that gasoline is is less stable than it used to be and some of the components evaporate more easily than others.

Thanks for your technical description, because it's what you learn after you know it all that counts!

Regards,

Alanco

 

Good and bad news. I readjusted the float and it seems to have fixed that problem. The book gave me a range of measurements. The float was technically correct as it was at the minimum setting. I readjusted it to the max setting which results in a lower fuel level in the bowl. However, I have found that I am also having some spillage from the top of the tank where the sender mounts. This all started just after I filled the tank. So now I get to take the tank out and work on making it seal. Thanks, Jag