Edelbrock 1407 Choke Vacuum Leak, WHAT?

[savatreatabvr]

In the pics below a red arrow is pointing to a hole in the choke assembly. 1st pic is the front of course and second is the back, on the back there's an oil ring that fits into the hole and it butts up against the carb. This might be a stupid question but why is my carb sucking air through that hole? It's creating a massive vacuum leak but it doesn't look like anything was ever plugging it. I searched online for info and out of 50 or so pics I saw only one had a plug in the hole. I'm going to plug it anyways and see what happens but I wanted to get other members input on why this hole is even there?

[savatreatabvr]

Ok, I did some more research and people are saying NOT to plug the hole because it's there to cool the choke spring and others are saying YES plug the hole because it's an obvious vacuum leak! I'm leaning more towards NOT plugging it but it's obviously a problem so maybe I'll plug it with something that has a smaller hole. What you you think?

[tucsonjwt]

I say spend $12 and install a manual choke conversion.

[geezer#99]

So, which hole are you plugging?
THe one on the outside or the one in the carb where the gasket is?

[wilkin250r]

I took apart one of these chokes years ago, but my memory is fuzzy. I don't believe the hole is straight through, and the backside has a small o-ring where it attaches to the carb. I remember that specifically, because I lost that o-ring once.

And I seem to recall it wasn't a "massive" vacuum leak, there was something that impeded the flow, but didn't stop it altogether, and I remember seeing the logic in it. But my fuzzy memory can't recall the details.

If you've got a massive vacuum leak, you might be missing something, or assembled incorrectly.

[savatreatabvr]

Well the vacuum leak is massive enough to cause a very rough idle and a whistling noise while driving. As soon as I let off the gas the whistling stops, as soon as I hit the gas it starts so that's why I was assuming that hole was the leak. Another thing is my passenger side air/fuel mixture screw has little effect while turning and with the choke being on the same side it kinda made sense. I did a smoke test on it and the smoke came billowing out of the top of the carb and the hole but came out the hole first. I guess I'll pop off the carb, cover the holes in the intake, try another smoke test and I'll post the results.

[mud.man.rj]

Funny as it sounds, I have prob rebuilt about 70-80 Edelbrock carbs and always use the o ring for the vacumn port on choke, never thought about what it actually does, hv a closer look at the inside of it and think about what it's purpose really is, they put it there for a reason or wouldn't be there right. Try the edelbrock site, something should come up.

[mud.man.rj]

haha, just looked it up on an edel site and is used to supplement the choke heat spring in holding open the choke by pulling vacumn into the port and pulling on a small piston inside of the choke housing, ya of course that is what the piston is for that I saw a million times and didn't know what it does, Thanks, now we both know. So not needed for a manual choke but is for an electric choke.

[INSIDIOUS '86]

My money is on the adapter is leaking air and your throttle is closed to much rendering the idle screws useless

[savatreatabvr]

Originally Posted by INSIDIOUS '86

My money is on the adapter is leaking air and your throttle is closed to much rendering the idle screws useless

I was wondering if the throttle plates weren't closing enough because during the smoke test the smoke billowed out of the top of the carb, if they were closed properly shouldn't there be no smoke? Next step is pulling the carb and do a smoke test just on the intake manifold to see if the gaskets are leaking.

[INSIDIOUS '86]

Could it be your secondary's are open to far?

[savatreatabvr]

I did the smoke test with the carb off and the intake manifold taped off with masking tape and long behold, smoke start coming out of the driver side bolt that holds on the Edelbrock adaptor. Not billowing out just a little so I pulled the adaptor, removed the studs, got a piece of glass with 220 wet/dry sandpaper and straightened that thing right out. I'll be bolting it on first thing in the morning to see how it runs. I'll post a video of my vacuum gauge readings also.

[cadillac_al]

This is interesting and gives me something to research. This sounds like some kind of choke pull off system. I have always wondered why Edelbrock carbs don't have a choke pull off. What opens the choke when the engine starts??? This may be the answer. I have been trying to get my choke tuned to start as good as a Quadrajet. My choke doesn't seem to open when the engine starts but I don't hear any vacuum leaking in that area. I guess I should read the manual

[geezer#99]

Originally Posted by cadillac_al

This is interesting and gives me something to research. This sounds like some kind of choke pull off system. I have always wondered why Edelbrock carbs don't have a choke pull off. What opens the choke when the engine starts??? This may be the answer.

It's not in the manual. It's not the piston either!

[savatreatabvr]

It's an electric choke, the spring inside the choke casing heats up and either expands or contracts one of the two and move the choke plate on the carb. I seem to have better luck with manual chokes myself.

[Pyrotechnic]

The vacuum port is for the choke pull off. The electric choke will close the choke blade fully when the engine is very cold, which creates a very rich mixture when the engine is first cranking over. Once the engine is spinning fast enough to create some vacuum, the choke pull off will open the choke blade slightly because the super rich mixture is no longer needed at this point.

The choke pull off piston is built into the electric choke unit. If you take it apart you will see what I mean. Other carburetors will use a diaphragm type choke pull off that is much more obvious.

In order for the piston to operate under vacuum, there must be a pressure differential between the top and bottom of the piston. One end of the piston is referenced to manifold vacuum, and the other end is referenced to the atmosphere. The open hole you see on the choke housing is so that one end of the piston can see atmospheric pressure.

The reason you may feel it sucking air is because the piston type pull off does not have a positive seal like the diaphragm types do. The rubber diaphragm has no air leakage, whereas the piston type will have some leakage between the piston and the bore. Though less than ideal, the pressure differential still works to move the piston and it is much more compact than a diaphragm. Yes it is a slight vacuum leak, but your PVC valve also a controlled vacuum leak and is much bigger in comparison.

The short answer is that it is designed to work this way. My advice? A manual choke cable in the hands of a skilled operator is far better than any electric choke.