The INAT fountain of youth?

[Blue 72 250]

After seeing an INAT modification in the latest thread from RgnProf, Just pulled the head - '72 250 I became inspired.

As many of you who run INAT carbs know, they have some “issues.” One of the chronic problems that I have experienced is a weak or nonexistent secondary. When I saw the modification in the photos from RgnProf I thought that I would give it a try.

I have parts carbs that I pulled the top of the secondary diaphragm from. The modification required a little drilling and tapping to fit a 1/4in npt brass nipple sealed with a little Teflon tape.
The result of the modification is a secondary that opens based off of throttle position, with little regard for manifold pressure.

I took her out for a spin and I found a world of difference. I don’t think the secondary ever opened before the modification. Now they open as soon as I hit 75% throttle.

Pro’s –
I control the secondary opening
It opens every time
Increased power (I would estimate at least 20 hp)

Cons -
Opening the secondary early in the rpm range will flood the engine
Not original
Gas Mileage reduced

So what do you guys think? Keep it or go back to stock?

[250 Coupe]

I'll have to take a look at mine. It will bark the tires on the 1-2 shift and I think every thing is stock.

Michael

[cth350]

there's a little bore in the venturi that controls the secondary. You should be able to move the secondary with your hand and then cover the bore. If that doesn't keep the secondary in place, look for a leak and/or rebuild the carb. (presuming you have the correct bore that is).

There is a little o-ring in the little bit of metal that connects the secondary diaphram's housing to the carb. It could leak. Also the diaphram itself could be punctured. Also, the vacuum bore could be clogged.

-CTH

[rgnprof]

Glad my post served as some inspiration, although I really can't take any credit for it. When I went through the Zeniths a few years back, the secondarys were working fine using the procedure outlined in the online manual.

My car has always run well wide open and accelerates nicely, after the initial off idle stumble. But, I do get poor gas mileage (11-13 mpg) and the car runs rich. Possibly due to this modification - I'm not sure?

I don't know why the modification was made as it appears to me that the secondarys have always worked appropriately.

ryan

[Blue 72 250]

Originally Posted by rgnprof

Glad my post served as some inspiration, although I really can't take any credit for it. When I went through the Zeniths a few years back, the secondarys were working fine using the procedure outlined in the online manual.

My car has always run well wide open and accelerates nicely, after the initial off idle stumble. But, I do get poor gas mileage (11-13 mpg) and the car runs rich. Possibly due to this modification - I'm not sure?

I don't know why the modification was made as it appears to me that the secondarys have always worked appropriately.

ryan

Unless you give her +75% throttle, the secondarys are prevented from functioning by the linkage. I assume that you hook up the vac line as equipped, or plug the modifications. The running rich is more likley a result of the jetting or the idle screw/idle stop setting. When you get her back together and running, I can send you a set of jets from one of my parts carbs.