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Car runs better with vacuum leak?

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Stick

Stick

Freshman Member
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As some of you may know, I'm finishing swapping a stock 1275 from a 73 midget into my bugeye racecar. As I'm now using the carbs and engine from the '73, there were more vacuum lines and emissions things (vs the race carbs that had none at all). So, I figured I would simply plug all of the vacuum hookups since I don't have anything to hook them to.

See below:

IMG_1813.jpg

As you can see I have the vacuum hookups off the carbs run into the stock "Y" and then to open air

IMG_1814.jpg

The intake manifold is also from the racecar, so where there seem to have been additional vacuum hookups on the manifold on the Midget, they are plugged on this manifold.

IMG_1815.jpg

Here is another angle of the current setup.


So, the question is, why does it run better this way than when I have the 3rd leg of the "Y" plugged? I made a piece of hose with a bolt sealing the other end (good tight fit) to fit over the open end of the "Y" but when I put it on the car does not want to run as well. I tuned the car with the vacuum plugged, yet it never liked to run well that way.

When the vacuum is sealed the car will spit and kick when you give it gas, be hard or impossible to start when warm, and sometimes want to die at idle. Removing the plug seems to help tremendously with all of these problems, but I don't understand why. Seems to me the opposite should be true.

Any ideas?

Thanks,
Nathan
 
On my '69 vintage 1275 the hose coming off of the timing cover can connects to the Y pipe. I do not have a hose coming off of the valve cover like you have.
 
Just a WAG, but the vac line is behind(post) the needle and ahead (pre) of throttle disc. This should produce a leaner mixture entering the intake manifold because air entering through the vac port contains no gas and slows the air flow across the needle and seat.
ISTM that if you plug the vac line and go to a leaner needle, you should acheive the performance.
OTOH, the air entering through the vac port may be causing better atomization of the fuel through the interference of the two different angles of intake air.

What happens if you eliminate the Y?

Have you tried reducing the size of the vac line to partially reduce the air entering through the vac ports? How does this affect performance?

Or put a velocity stack(s) on the vac line(s).
 
The hose you have connected from the oil separator on the front of the timing cover to the top of the valve cover should be connected to the wye fitting on the carburetors. You need to add a hose from the top of the valve cover to the charcoal adsorption canister. If you don't have the charcoal adsorption canister connect it to the air filter back plate in order to filter the air going into the engine. The PCV system is designed suck clean air through the engine crankcase and to maintain a slight vacuum in the crankcase. This helps oil to NOT leak out the rear main seal. The carburetor needles are set rich to have the proper mixture with the added PCV air. The Haynes manual shows this set up.
 
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