Question for Tom Endy

[Carb Guy]

What should the early #19 cap jet flow test at. Having trouble with a early double venturi and tried different rates on the cap jet, but just wants to stumble when giving it the gas. Any help would be greatly appreciated. Thank You.

[Tom Endy]

Originally Posted by Carb Guy

What should the early #19 cap jet flow test at. Having trouble with a early double venturi and tried different rates on the cap jet, but just wants to stumble when giving it the gas. Any help would be greatly appreciated. Thank You.

When I flow test jets, early or late, numbered or not, I follow the guidelines that came with the video our club purchased a number of years ago. The cap jet in a Zenith has the widest window of all the jets installed. It is flow tested and sized to 150 - 185 cc per minute.

The cap jet gets its fuel from the secondary well. The comp jet, which is smaller than the cap jet and has a narrow window, controls the amount of fuel that enters the secondary well. In order to increase the amount of fuel offered by the cap jet, you open the GAV valve. This ports fuel into the secondary well by bypassing the comp jet. If you open the GAV valve several turns the cap jet will deliver fuel to the full extent that is was sized to (150 - 185 cc per minute).

As a suggestion, I would remove all the jets and GAV assembly and make sure all the internal passageways in the two housings are open and none of the jets are plugged. It is important to flow test all the jets.

Tom Endy

[Carb Guy]

Thanks Tom. Like you I have been rebuilding carbs for many years, but never have run into a carb like this that just wants to stumble when stepping on the gas. I thought that the fact that it calls for a number 19 jet on these early double venturis, which have the sized opening on the bottom of the jet rather then the top like the other cap lets that it might call for sizing it different. I have flow tested the # 19 jets at 174,179, 181 and have tried each one with no luck at running better. the main jet is at 159 which is where I usually run them about 160 and the cap jets I try to stick close to 173 on the regular cap jets. I have checked all the internal passages. Just a mystery to me.. Thanks again Tom. Dick H.

[Richard Lorenz]

I have 2 original double venturis with which the inner venturi is distorted into an oval shape as the result of swelling of the pot metal. I have not tested them on a car, so I don't know whether the oval shape is good or bad.

The stumbling that you have sounds like a lean fuel condition. When accelerating, extra gas is supposed to be pulled out of the secondary well until it is empty. It would seem that the cap jet should therefore be one that flow tests at the high side of the recommended range.

[d.j. moordigian]

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Originally Posted by Richard Lorenz

I have 2 original double venturis with which the inner venturi is distorted into an oval shape as the result of swelling of the pot metal. I have not tested them on a car, so I don't know whether the oval shape is good or bad.

Richard,
I thought that was the original shape,....of the one I pulled a part, too
clear which ever jet (front one) is there.
Dudley