Jims79

I'm working on a 66 coupe. It runs good but I noticed gas on the manifold under the carb on both sides. I cleaned the gas up and tighted down the four carb bolts. Started it up ran no leak. Checked it the next day after sitting in the garage overnight & gas is leak is back? I tighted the screws on the carb bowls and went as far to remove the carb but the gasket is fine?? Has anyone else had this problem.

66BB

I would at least replace the bowl gaskets with a new set. The Holley's are notorious for warping over time and a new set of gaskets may not solve the problem, but it is a good starting point. If the new set of gaskets does not do the trick, your in for a more serious rebuild.

wombvette

Sure the leak is not internal? Check for leakage on the rear throttle plates after shut down.

vref

Allcoupedup

Jim,
I have the same exact problem. I have never been able to set and stare long enough to watch where it comes from.

Brian

rgs

Replace the accellerator pump diaphram.

Jims79

Thanks you guys are great.

I have two dumb questions from a ok backyard mechanic.
If its the throttle plates how do I check them & does that mean a carb rebuild or should I replace it w/new. I have replaced carb but never rebuilt one.

2nd question where is the accellerator pump diafram located? Again does replacing it require rebuilding the carb?

67saginaw

Accelerator pump diaphragm is on bottom of front and/or rear fuel bowl - a small plate with a lever arm to the throttle linkage only the larger carbs have rear pumps. Get a Holley rebuild kit will have instructions and diagrams and all parts for most repairs. Check the bowl/metering block/metering plate mounting surfaces on the main boy with a steel straght edge for warpage - if OK put it together and be sure to torque screws to specs. Good luck.
Throttle plates just indicate show leakage from the bowls - gas on the top would be coming out of the front or rear bowls and/or accelerator pump from heat causing the gas to dump over onto the throttle bore or leaking thru the metering block gaskets.

66BB

Jim,

If your seeing leakage of any type, why not just rebuild the carb. I have rebuilt mine about every 10 years or so ±. When I have had a leak, it is usually after I take the car out of winter storage. I have always had leakage in the primary bowl that shows up as gas on the intake manifold.

The rebuild is not that bad or difficult. If you purchase a Holley rebuild kit, it will come with a good exploded diagram that will help you on reassembly. I also purchase a can of parts cleaning solution from NAPA to soak the parts that you remove from the carb.

There are also quite a few very good rebuild services, even those that will correct warped parts.

lars

The leakage you're seeing is very common, and has nothing to do with the carb gaskets or the throttle shafts/plates. You don't need a carb rebuild. Here's how it happens:

Modern fuels have much higher vapor pressure than the old fuels that existed when these cars (and carbs) were built. After hot shutdown, manifold heat heats up the fuel in the accelerator pump well through radiant heating. This heat in the accel pump increases the vapor pressure in the well. The check ball in the accel pump "locks" the pressure in the well until the pressure unseats the accel pump discharge needle. At this point, fuel will squirt out the accel pump discharge nozzle just as if someone were pumping the gas pedal. This fuel hits the throttle blades and the shafts. Since there is no airflow pulling the fuel into the engine (the engine is off), the fuel siphons out the throttle shafts and drips onto the intake manifold. Once the accel pump has relieved its pressure, the inlet checkball unseats, draws more fuel into the pump, and the cycle repeats until the engine cools down. I've seen a good hot engine empty nearly the entire float bowl in this fashion, completely flooding the engine, and pooling gas all over the intake manifold.

Best solution is to install a reflective heat shield between the carb and the manifold - one that is long enough to place a reflective barrier between the accel pump well and the manifold. Note that insulating gaskets under the carb will have no effect on this: You have to address the radiant heat hitting the accel pump well.

Matt Gruber

the lazy way out

lars

Matt!
You lazy dog!
...Actually, the heat that nails the accel pump well is radiant heating - not convection heating. So opening the hood actually doesn't help much... although it does help cool down the exterior surface of the carb a little, so it might gain a slight advantage. And it gives you an excuse to open the hood..

Matt Gruber

Have u tried it or are u BSing? It ALWAYS WORKS
here in FL. In fact, i use it as a diagnostic test to determine if something else is wrong.
(if a wide open hood doesn't help, look for leaking carb plugs, for example)

Jims79

OK now I'm more confused then before. Which doesn't take a whole hell of alot these days. I use to have all the right answers when my kids were young. Now I "don't know nothing anymore".

So back to the carb leak, should I have it rebuilt or not.
The car runs like a bat of out hell more ***** then my 79 which has been worked on. I was thinking if its not broke why fix it.

Why not just replace gaskets in the bowls and Acc pump.

TJefferson2020

I use a heat sheild on my LT1 although I've never had leakage. I did have some vapor lock issues but the shield works great.

Of course you could just toss a match under there - the gas will be gone in no time!

Viet Nam Vett

Or...By A Carter Carb.........

Matt Gruber

when i got my 72 in 1990, i noticed it would smell up the garage unless i opened the hood.
(as lars says), the heat shield is a more elegant solution. gas quality can effect your results. since gas boils at 140-150F, any brand carb can bubble over

Coves4me

Jim, if I were in your situation, rebuild it!! The gas leak may be coming from any number of places under the carb, so why not pull it and tear it down. Rebuilding a Holley is easy if you pay attention to what you are doing. As a tip, have a disposable or digital camera handy to take pics of the assemblies as you take it apart. This alone will ease the blood pressure upon re-assembly. Next, buy a one-gallon container of Berryman's Carb cleaner. Looks like a paint can and has a handy lift out basket for soaking parts. The stuff is nasty, but it works effectively. Also, get some spray brake cleaner and a can of spray carb cleaner. Both remove gunk and grease effectively, but the brake cleaner will not leave any residue when it dries.

A good carb kit such as a Jiffy kit or Holley lit is essential to a good rebuild. The parts are of a higher quality than the generic fits all types. Mr. Gasket makes nylon washers for the screws holding the float bowls on and Holley makes replacement Nuba blue gaskets for the float bowls. I highly recommend both. Also, cleanliness is paramount as you want to clean the gasket off all surfaces especially the float bowls. I find that soaking the float bowls in the carb cleaner followed by a brass bristle brush works fine. Gaskets that can be particularly difficult to remove are the ones behind the secondary metering plate after you remove the secondary float bowl. The clutch head screws can be removed and re-installed with a screwdriver to fit. Be careful, as you remove the secondary metering plate as there may be a thin metal plate sandwiched between two gaskets. You can deform it easilty if you're not careful. Soaking in the carb cleaner is recommended first. Lastly, during assembly don't overtighten your screws as the Holley carb is made of pot metal and screw holes can strip out easily.

A Holley carb rebuild book that can be had at Barnes and Noble or Amazon is invaluable when re-building. Peterson publishing sells several. Also, Lars may be right when he suggests a metal heat shield under the carb. I ran one under my late-model Mustang Holley 4180C carb for years and never had any substantial leaks. They do help. Good luck.

67wave_saver

I have the same problem. I had my carb rebuilt by a professional. Still leaks after shut down. lars was right. After the engine cools down a little I stuff a paper towel on each side of the carb. As the gas leaks out the shafts it is absorbed into the paper towel. This at least keeps by intake manifold from staining.

joec

Well after hearing my wife and kids nail me everytime I parked the vette in the garage, I bought the holley heat shield that Lars talked about.. I still haven't put it on since it looks that I need to do some healthy mods to make it fit, but will soon.. I'm having some timing issues, so I need to get that worked out first..

My hope is that Lars will start selling his own version..