I’m posting this as it may help others even though I doubt many of you run dual Holley’s that are mounted backwards as Ford did back in the 60’s. However, much of what I mention could apply to a single Holley mounted normally. Besides, even if it’s of no concern, it may be of interest just because it is Cobra related.

This whole thing started after I recently improved my braking. Although I always had a rough idle after braking hard, it has become much worse with the brake improvements. The engine drops from a 900 rpm idle to less than 400, and sounds like it’s about to stall.

I suspected fuel was sloshing up the primary bowl vents and substantially richening the idle mixture. I temporarily capped of the tubes and the problem disappeared, although not entirely, which I didn’t realize until much later. Fuel level was eventually found to be a part of the issue, although a small one.

The picture below looked like a good answer, but I don’t have enough air cleaner clearance even if I modify the proposed vent tube:

I decided to add vent baffles to both primaries. Below you can see the baffle in the exploded view from my 1966 Ford Manual. I didn’t think my carbs had these, but it turns out they do. I just didn’t remember them from when I had the carbs apart quite a few years ago.

At any rate, I didn’t think much of either of the two flat baffles on the left, and the white one on the right requires drilling the metering block and tapping in a screw type rivet. I decided to try the black “shortie” baffle, P/N 59-10. Doing a Google Search seemed to point out these extension baffles would not work with side-hung float bowls, however a few people said they would. I am now able to confirm the black ones do fit, at least with the floats I use (at bottom right with arrow).

I went for a test drive and the problem was mostly gone. Idle might drop from 900 to a little above 800, but sounded fine and was acceptable to me. However, many people have suggested that merely a float level adjustment would work, so I lowered the fuel level by ½ turn of the needle valve assemblies. Surprise, surprise, it helped and the idle only dropped to about 850.

For now, I’m leaving it there, but lowering the fuel level in the float bowls doesn’t appeal to me very much as it may uncover the Power Valve under hard acceleration. So I decided to analyse things…. i.e. "guess".

First was the force due to braking. Researching it with Google, it turns out that 1 G was a reasonable expectation for a vehicle with very good brakes. No doubt I can’t achieve that, but it seemed logical to use the figure.

As you can see in the figure below, under 1 G of braking, the fuel level (green line) moves to a 45 degree angle and certainly reaches the input of the internal air vent. Note that it is still highly unlikely the fuel would actually spill out over the top of the air vent, although I have proven that it does.

I also had this braking problem while braking while going down a steep hill. Not braking to come to a stop, but to keep the car from speeding up due to gravity. So no significant g-forces, but the angle of a steep hill would also let the fuel level enter the air vent, but nowhere near enough to spill out the top. I’ll get to more on this later.

Of course lowering the fuel level bothered me upon consideration of acceleration. Doing some math, it turns out my car ought to hit 0.71 G’s in second gear. (That’s only theoretical and is probably less.) First gear could yield 1.1 G’s however due to wheel-spin and that I don’t like shifting hard from 1st to 2nd, I decided to ignore it and stick to where the fun is…. 2nd gear.

The red line indicates the fuel level at 0.71 G’s and you can see how dangerously close it is to uncovering the power valve. However even after lowering the float that little bit, I don’t seem to have a problem (more testing is required to be absolutely certain).

I think the reason I don’t have issues with the Power Valve being uncovered is due to the presence of the float which I did not include in my drawing (if I did, it would be to the left side of the float bowl). As the fuel sloshes back against the bowl, the float rises with it but is limited in its movement. This causes it to displace more fuel and the overall fuel level increases somewhat. Speculation, but sounds quite plausible.

Now one final thing. It bothers me that I can’t find any documented evidence, whether in my books or on the internet, about anyone having these troubles with Ford’s Dual Quad setup. I believe that the reason I am having trouble is due to my air cleaner and filter assemblies.

If you look at the picture above, you can see how air can enter through the external air vent, travel along the top of the float bowl and exit at the internal air vent. Due to my small and restrictive air filters (foam with perforated steel bands around the inside and outside perimeters) there ought to be quite a pressure differential between the outside and inside of the air filter. See below:

To make matters worse, the air cleaner covers are very low with 3/8” at most above the primary internal vent tubes. The flow of air over the tubes (which are flat vs. angled for some reason) might increase the draw from the vent tubes.

Any fuel splashed into the vent tubes would be drawn into the filter areas and vanish down the carb throats.

On a side note I have identified a much freer flowing K&N filter that is an inch taller as well. Unfortunately I don’t have the hood clearance…. Or do I? Turns out I do except for the reinforcing web around the inside perimeter of the hood. I don’t want to cut into that, but I can make offset air cleaner bases with offset holding rods. All I need is to move the air cleaners forward ¾ of an inch.

Another project…. for another day.