"66 Holley carb leaking at idle transfer slots

[Robert B.]

I have been trying to get this carburetor running right and I've got the driveability issues solved but it continues to dribble a very small about of fuel from the primary transfer slots down into the intake, and some of the fuel seeps out around the throttle plates when the engine is shut down. And of course, when you restart it when hot it takes a few seconds of cranking because there is so much fuel in the intake.

The car is a "66 350HP coupe with A.I.R. (fully functional) - the carb is a correct list 3605 with a 4920 metering plate. The float levels are correct, the power valve is a 6.5 and has been replaced to isolate that as a cause.
What I don't understand is what would cause fuel to seep out from the transfer slots?

I just rebuilt a friends' list 3367 carb off his "66 300HP engine and when I dialed it in on my car it runs perfectly - no leaks or seepage, and the engine starts immediately when either hot or cold.

Any suggestions?

Thanks,

Bob

[Mike Z.]

Re: "66 Holley carb leaking at idle transfer slots

The problems you describe: I have found quite often on these carbs. Because of a more confined hood clearance on Corvettes, the carb is more susceptible to heat sink issues. I have sent out several carbs to "professionals" for rebuilding and re-coloring. Unfortunately, I end up going through them again because of the same type problems you are having. I chaulk up the expense paid to the "professional" to the price to re-color. It amazes me why someone in the business, doesn't run the carb (with liquid) to check for these things, or check the surfaces as I describe below.
I have found warpage on many surfaces: front bowl to accelerator pump plate leakage (the largest issue I have had), leakage in the secondaries into manifold-usually warped metering plate, leakage into manifold from the primaries-usually due to warped jet plate and/or main body.
I recommend you take the carb apart and check the straightness of each surface-I use a machinist straight edge. Once I discover any surfaces that are not true, I use a plate glass as a true flat surface and medium emery taped to it. Then I "re-surface" the affected area by running the surface over the emery until I have a flat surface (you know when you have completed the task, when you have surfaced/sanded all the coloring off-which you can't once the carb is back together). Sounds crude, but it works.
Until you get the seepage/leaks stopped, you are wasting your time jetting and adjusting-it will never be right, and the thing never wants to start when hot, unless you put your foot to the floor. Hope this experience helps
Michael Z

[Steve G.]

Re: "66 Holley carb leaking at idle transfer slots

If it is leaking at the idle slots at shut down you can be pretty sure that the fuel level in the bowl is too high. That can be a leaking needle and seat, a damaged float or improper adjustment. Misalignment of the bowls and plates will cause an external leak, but it won't make it's way into the throats via the idle circuits.

[Greg H.]

Re: "66 Holley carb leaking at idle transfer slots

Hi Robert.
I agree with Steve and the need to re-surface the throttle body housing and I have found an additional remedy to fixing this condition. I too have been frustrated by the leaking fuel on the throttle plates; I found the problem is inside the fuel pump.


Each pump has an internal check valve that is suppose to release / relax the line pressure after the engine is off and the pump stops working. If the valve does not relax, the internal line pressure will increase inside the hot fuel line and force the fuel past the seats into the throttle body.

You can check your pump with a pressure gauge to see if the valve will relax. I did this test with the pump off the car because of the potential fire danger. Replacing the pump was the only option on my 67, I am not sure about your year.
Good luck.

Greg

[Timothy B.]

Re: "66 Holley carb leaking at idle transfer slots

Greg,

I have never heard of that before, the C2 mechanical fuel pump has a inlet check and a outlet check valve, when one is open the other is closed. The pump pressure is a function of the spring above the diaphragm. I think the later fuel pumps have a return line but I can't comment on them. I would not be surprised if the internal valves leak down after the pump stop operation so you may be correct in your observation.

I have read about some people drilling a very small hole in the pump to relieve pressure and it's probably a good idea providing the hole is small so the pump still maintains it's 6 PSI in use.

Before tearing the carburetor apart try to lower the float as the 66 carburetors have a externally adjustable float so it's easy to try and also make sure the idle air bleed is open. How much initial ignition timing does the engine have, the fuel could be percolating and being forced out the slot. Is there any fuel from the booster venturi nozzle, I would think the fuel would be forced here first as it has to travel a long path to find it's way to the transfer slot. If the fuel is from the booster venturi it's a sure sign the float level is to high.

[Steve G.]

Re: "66 Holley carb leaking at idle transfer slots

Greg,

I have never heard of that before, the C2 mechanical fuel pump has a inlet check and a outlet check valve, when one is open the other is closed. The pump pressure is a function of the spring above the diaphragm. I think the later fuel pumps have a return line but I can't comment on them. I would not be surprised if the internal valves leak down after the pump stop operation so you may be correct in your observation.

I have read about some people drilling a very small hole in the pump to relieve pressure and it's probably a good idea providing the hole is small so the pump still maintains it's 6 PSI in use.

Before tearing the carburetor apart try to lower the float as the 66 carburetors have a externally adjustable float so it's easy to try and also make sure the idle air bleed is open. How much initial ignition timing does the engine have, the fuel could be percolating and being forced out the slot. Is there any fuel from the booster venturi nozzle, I would think the fuel would be forced here first as it has to travel a long path to find it's way to the transfer slot. If the fuel is from the booster venturi it's a sure sign the float level is to high.

Float bowls are always vented to the top of the air horn so percolating can't build any pressure. Main metering takes place higher in the throat so siphoning is less likely.

Edit: I shouldn't have used the term "siphon". They can't siphon, they just flow. When spilling into the throat they will drip from the lowest point.

[Timothy B.]

Re: "66 Holley carb leaking at idle transfer slots

Steve,

If the fuel boils the vent will keep the bowl from building pressure like you suggest but the fuel will expand and be forced up the main well and out the booster before it will travel through the idle feed, up to the top and by the idle air bleed then back down and out the transfer slot. Now that's not to say there is something else going on causing fuel drip from the slot but I don't know what would cause that.

If the float level is high, the same thing will happen with the fuel dripping from the booster nozzle then down onto the throttle blade where it runs out the side of the carburetor.

[Robert B.]

Re: "66 Holley carb leaking at idle transfer slots

Thanks for all of your feedback. Based on what I heard, I ran another test to rule out external components. I started the car, let it warm up until coolant temperature had stabilized, and it was idling normally at about 700 RPM. I removed the sight plug from both primary and secondary fuel bowls to make sure the fuel levels were below the threads. Both levels were OK. I left them out to make sure the fuel levels did not change, and shut off the engine. The fuel bowl levels did not change, but within 60 seconds I could see fuel dribbling from the area of the idle transfer slots. I immediately pulled the carb off the car to find out how much fuel was leaking and found the intake manifold was all wet as was the gasket. So now the carb is on my bench, and I'm not sure where to start. All components look to be in good shape. The metering block has some slight surface corrosion, but it does not appear to come close to penetrating the passages. I would expect that if the power valve were leaking that it would cause these symptoms, so I will replace it (again) just to be sure. Beyond that, I am not sure where to go next!

Appreciate any thoughts -

Thanks,

Bob

PS - Initial timing is 10 degrees BTDC, dwell is 29.5 degrees. Since this problem does not exist with the other Holley off my friends '66 300HP car, I know the problem is within the carburetor.

PS2 - I am using the blue Holley gaskets on the float bowls and metering block. No evidence of any leakage externally.

[Steve G.]

Re: "66 Holley carb leaking at idle transfer slots

Timothy,
Not to belabor this, but in the interest of the most complete info for our archives, the reason they drip from the idle slots rather than the main nozzles is that the idle circuit and the main circuit share a passageway exiting the bowl. All the fuel travels up to the top then past the idle well before going on to the main nozzles. When not running and fuel spilling it runs down the idle circuit well rather than continue on to the nozzles.
Holley.JPG

Venting of the bowl into the airhorn is more than a suggestion. The reason it is vented there is to minimize the effects of a restricted air filter. Atmospheric pressure pushing down on the fuel in the bowl is higher than air pressure at the outlets of the nozzles, so fuel flows. Pressure drop at the nozzles is not an absolute, but rather a determined degree of drop from the pressure of the air coming in to the airhorn. If the filter is plugged pressure in the airhorn is reduced so pressure at the nozzles is reduced as well. If the bowl was vented outside the airhorn/cleaner pressure on the fuel would remain unchanged and a greater amount of fuel would flow because of the greater difference between the two points. Because the bowl is vented inside the air cleaner, the pressure on the bowl is reduced as well.

And that brings us to percolating. When the fuel gets hot it starts to vaporize in the bowl. The vapors rise out the vent and into the airstream, richening the mixture. To combat that we have hot idle compensators. They can be a mechanical linkage that opens a vent (outside the air cleaner) on the bowl at idle or a vent o a bi-metal spring that opens when it gets hot enough. I don't recall which, if any, Holley used at this time, but I seem to recall some of them with a mechanical linkage type

Bob, I don't know of an easy or often found problem to point you to. What I would do is fill the fuel bowls and watch closely to ensure you know where it's dripping from and that it's not dripping or running down onto the slots. Then pull the bowl and the block and follow the fuel path and look for your leak. Use an air hose and identify and trace back the idle passages if you're sure that's where it's coming from. I think there are only three ways out of the bowl, power valve, accel pump and main metering jets. BTW, is it on both sides?

Accel pump circuit doesn't share a passage with the idle jet, but that's why I would want to ensure I knew where it was coming from.
The pump circuit is full of fuel to a height that could leak down and perhaps look like it's coming from the slots.

Check that the gasket on the power valve is not compromised.

I'm sure you're aware, but I'll say it anyways. When working on carbs and gas do it in a well ventilated space. Don't use a light that can drop and break. I've seen where the spark in a light switch ignited a room, a dropped trouble light burn a shop down and a pilot light in a furnace blew out the frt windows in the building severely burning the two guys working there.

[Timothy B.]

Re: "66 Holley carb leaking at idle transfer slots

Timothy,
Not to belabor this, but in the interest of the most complete info for our archives, the reason they drip from the idle slots rather than the main nozzles is that the idle circuit and the main circuit share a passageway exiting the bowl. All the fuel travels up to the top then past the idle well before going on to the main nozzles. When not running and fuel spilling it runs down the idle circuit well rather than continue on to the nozzles.
[ATTACH=CONFIG]56981[/ATTACH]

Venting of the bowl into the airhorn is more than a suggestion. The reason it is vented there is to minimize the effects of a restricted air filter. Atmospheric pressure pushing down on the fuel in the bowl is higher than air pressure at the outlets of the nozzles, so fuel flows. Pressure drop at the nozzles is not an absolute, but rather a determined degree of drop from the pressure of the air coming in to the airhorn. If the filter is plugged pressure in the airhorn is reduced so pressure at the nozzles is reduced as well. If the bowl was vented outside the airhorn/cleaner pressure on the fuel would remain unchanged and a greater amount of fuel would flow because of the greater difference between the two points. Because the bowl is vented inside the air cleaner, the pressure on the bowl is reduced as well.

And that brings us to percolating. When the fuel gets hot it starts to vaporize in the bowl. The vapors rise out the vent and into the airstream, richening the mixture. To combat that we have hot idle compensators. They can be a mechanical linkage that opens a vent (outside the air cleaner) on the bowl at idle or a vent o a bi-metal spring that opens when it gets hot enough. I don't recall which, if any, Holley used at this time, but I seem to recall some of them with a mechanical linkage type

Bob, I don't know of an easy or often found problem to point you to. What I would do is fill the fuel bowls and watch closely to ensure you know where it's dripping from and that it's not dripping or running down onto the slots. Then pull the bowl and the block and follow the fuel path and look for your leak. Use an air hose and identify and trace back the idle passages if you're sure that's where it's coming from. I think there are only three ways out of the bowl, power valve, accel pump and main metering jets. BTW, is it on both sides?

Accel pump circuit doesn't share a passage with the idle jet, but that's why I would want to ensure I knew where it was coming from.
The pump circuit is full of fuel to a height that could leak down and perhaps look like it's coming from the slots.

Check that the gasket on the power valve is not compromised.

I'm sure you're aware, but I'll say it anyways. When working on carbs and gas do it in a well ventilated space. Don't use a light that can drop and break. I've seen where the spark in a light switch ignited a room, a dropped trouble light burn a shop down and a pilot light in a furnace blew out the frt windows in the building severely burning the two guys working there.

Steve,

I agree with most of you comments, the original poster more than likely has a gasket seal problem. If the power valve gasket was the problem the fuel leak would be below the throttle blades and not seen from above. If the accelerator pump was leaking at the metering block to main body gasket the fuel would be sucked into the small power valve vacuum feed hole and again be below the throttle blades.

The gasoline leaving the fuel bowl does share a common passage for the main circuit and idle circuit and this is the main jet. Fuel travels through the main jet into the main well where it's sucked (by manifold vacuum) through a hole and into/through the idle feed restriction that's located low in the metering block. Vacuum pulls this IDLE fuel up to the top of the carburetor where it passes by and is mixed with air from the idle air bleed then travels down the idle well to the transfer slot and curb idle holes that enter the main body. Main circuit metering occurs when air flow through the booster venturi creates a low pressure signal and fuel is pulled from the main well and blown (by the emulsion well bleeds) toward the nozzle in the booster. To completely different circuits but they do share a common fuel feed, (the main jet).

I agree that the bowl vents need to be located such that they see the same pressure as the venturi and the purpose of the hot idle compensator venting vapor to atmosphere.

I had a problem with the primary metering block gasket seal on my Holley once that drove me crazy, the problem turned out to be the four screws were not tightened enough, gotta be careful here though. Now, after I assemble a Holley I use my miti-vac through the small power valve vacuum hole and pump up to make sure the seal is good. My experience is that it will never hold a perfect vacuum but the miti-vac will pump up and very slowly leak down (45-60 seconds). I called Holley and spoke with there tech help and relayed my experience and he told me as long as the tool will pump up it's fine. If there is a gasket seal problem the miti-vac will not pump up.

[Steve G.]

Re: "66 Holley carb leaking at idle transfer slots

Steve,

I agree with most of you comments, the original poster more than likely has a gasket seal problem. If the power valve gasket was the problem the fuel leak would be below the throttle blades and not seen from above. If the accelerator pump was leaking at the metering block to main body gasket the fuel would be sucked into the small power valve vacuum feed hole and again be below the throttle blades.

The gasoline leaving the fuel bowl does share a common passage for the main circuit and idle circuit and this is the main jet. Fuel travels through the main jet into the main well where it's sucked (by manifold vacuum) through a hole and into/through the idle feed restriction that's located low in the metering block. Vacuum pulls this IDLE fuel up to the top of the carburetor where it passes by and is mixed with air from the idle air bleed then travels down the idle well to the transfer slot and curb idle holes that enter the main body. Main circuit metering occurs when air flow through the booster venturi creates a low pressure signal and fuel is pulled from the main well and blown (by the emulsion well bleeds) toward the nozzle in the booster. To completely different circuits but they do share a common fuel feed, (the main jet).

I agree that the bowl vents need to be located such that they see the same pressure as the venturi and the purpose of the hot idle compensator venting vapor to atmosphere.

I had a problem with the primary metering block gasket seal on my Holley once that drove me crazy, the problem turned out to be the four screws were not tightened enough, gotta be careful here though. Now, after I assemble a Holley I use my miti-vac through the small power valve vacuum hole and pump up to make sure the seal is good. My experience is that it will never hold a perfect vacuum but the miti-vac will pump up and very slowly leak down (45-60 seconds). I called Holley and spoke with there tech help and relayed my experience and he told me as long as the tool will pump up it's fine. If there is a gasket seal problem the miti-vac will not pump up.

My points were only that it would be unusual, if not impossible, for a fuel flow caused by high bowl level to make it past the idle circuit and into the main nozzles rather than travel back down the idle circuit and out the transfer slots before ever getting to the booster as you had mentioned and that high pressure in the bowl from fuel perc forcing fuel out was not a possibility because the bowls are vented.

What puzzles me about this problem is how the fuel is exiting low in the throat without going up and through the main/idle circuit. The Op's fuel is flowing after the engine shuts off, so it is gravity only moving it. Consider the height of the fuel in the bowl. How is it getting past the metering block to the transfer slots without going up above the fuel fuel level? There are no passages below fuel level that would take it through the metering block. High fuel level in the bowl raises it above the passage level and that is typically what happens with a flooding situation. I would think that even a leak between the block and the body would show some sign of esternal leakage. This is why I am questioning a leak from the pump circuit that is running down and then visible at the slots. The pump circuit is filled with fuel above the level of the fuel in the bowl and held there by the check ball.

[Timothy B.]

Re: "66 Holley carb leaking at idle transfer slots

Steve,

Fuel flow caused by a high float level would show itself first as a drip from the booster nozzles as opposed to a drip from the idle circuit/transfer slot. The fuel path for the idle circuit and main circuit only share a common source, (the main jet), after that they are independent for the purpose of this conversation. Fuel for the idle circuit gets raised above the float level by manifold vacuum where it gets mixed with air (siphon break) then back down the idle well to the curb idle and transfer slot holes, it does not then run to the main well. The main well is located inside the metering block under the center cup plugs and it's fuel feed is the main jet and the power valve channel restriction.

The only explanation I can offer if the fuel is coming from the slot is that the metering block gasket is leaking and allowing fuel from the idle well to empty or the idle air bleed is clogged causing a siphon. I am not sure if the base gasket could cause this either but they all would be suspect. I am not sure about the accelerator pump leak but you never know, lots of times the problem may originate at a place you can't see so you may be right. If those floats have a external adjustment I would lower them one turn then run the engine just to try and eliminate float level as the problem.

There has got to be some sort of gasket leak from warped surface and poor gasket seal. PIA, but once you get them sorted they run nice.

[Robert B.]

Re: "66 Holley carb leaking at idle transfer slots

Thanks for the feedback guys, and it helps to have some direction to isolate this problem.

So I decided to reinstall the primary float bowl without making any other changes, and with the carb on the bench, filled the fuel bowl to see if it would leak. It did. When it was a little over half full fuel was seeping out in the area generally between the two primary throttle plates.

So I removed the bowl, replaced the power valve and the accelerator pump and repeated the test. It leaked as before. That eliminated the power and accelerator pump diaphragm as possible problems.

Since I had another 4920 metering block off another carb I installed that block and repeated the test. It leaked just like the other one.

I then decided to remove the primary bowl from my friends carb which is a list 3367 with a correct 4743 metering block, and inspected both the metering blocks and the main body of the carburetors to see if there were any differences. As near as I can determine they are identical - both the metering blocks and the main body.

I then checked the original 4920 block for straightness with a straightedge, and it is in excellent condition - shows minor warpage but not enough that the gaskets would not seal. Besides, during each of these tests, there was absolutely no external leakage.

I then installed the 4743 metering block from his carb on my carb, filled the primary fuel bowl and checked for leaks. It has been sitting on my bench now for over an hour, and the throttle plates and entire underside is dry as a bone - no leakage, seepage of any kind.

This makes be believe that there is something different in the way a 4920 is drilled, and I'm now wondering if it really is the correct metering block for my carb, even though the judging guide specifies it as being correct for a '66 350HP with K19.

Am I missing something here?

Thanks,

Bob

[Timothy B.]

Re: "66 Holley carb leaking at idle transfer slots

Bob,

From what you report, the only thing I can conclude is the metering block is warped to a point where the gasket does not seal properly. The good news is that you found your problem. If you get a chance to look at a new Holley metering block they are reinforced in areas that can warp and nothing like the old blocks.

Have you put a machinist straight edge on the main body to check that surface.

[Robert B.]

Re: "66 Holley carb leaking at idle transfer slots

Bob,

From what you report, the only thing I can conclude is the metering block is warped to a point where the gasket does not seal properly. The good news is that you found your problem. If you get a chance to look at a new Holley metering block they are reinforced in areas that can warp and nothing like the old blocks.

Have you put a machinist straight edge on the main body to check that surface.

Well, you are correct. I tried installing the metering block with a double gasket between the metering block and the main body, and so far it does not leak. A straight edge test does confirm that the block is slightly warped, but one would think that it wouldn't be enough to affect the seal against the main body. I'm thinking these's really no reason I couldn't use a double gasket in that location if it continues to not leak. I'm going to leave it overnight with a full fuel bowl to make sure!

Bob