Weber Taller Secondary Venturis

[Nitrometano]

Hi,

My 1972 911T have a 2.4 engine with 40 IDA Webers, MSD 6A, Crane Cams XR700, Blaster 3 coil, Bursch headers.

I can replace the stock secondary venturies for the taller secondary venturis? How the change of venturies can affect the performance?

Thanks,
Nitro

[jluetjen]

How (will) the change of venturies ... affect the performance?

I doubt that it will at all given your engine configuration. The key factor is the cam design (forget about all of the electronics for now) -- I assume that you still have T cams?

My understanding is that the tall secondaries help the part throttle response when you're running long duration cams with significant overlap. So the tall secondaries may help if you had S cams, but for T's, I doubt that they make a big difference.

Can anyone else confirm or deny my understanding?

[jpnovak]

I agree. I think it typically lengthens the vacuum runner length on the main carb jets. This helps smooth out the airflow from the reversion of high overlap cams. Reversion is usually only a problem at low engine speeds which usually occur at part throttle. Using the tall manifolds serves the same function. An analogy would be the RSR high butterflys for MFI and moving the injectors to the top of the stack.

[Nitrometano]

Really I don't know if the engine have another cams. I own the car since 7 years and I never repair the engine. I found in the papers that in 1976 2.4S pistons installed. Since that, I don't find another part replacement until 1998 hydraulic chain tensioner was installed. I notice that some flat spot exist between 2,000 RPM and 3,800 RPM. And I was thinking is the taller secondary venturis will help to reduce or eliminate that flat spot.

[jpnovak]

A flat spot can probably be diagnosed as improper float levels. Or you are running lean on your idle circuit.

The S pistons will clear the cams. The only way to tell is to measure them using a degree wheel and a dial indicator. map the intake and exhaust and post the results.

[Nitrometano]

I have the dial indicator, but I dont know how to use the degree wheel on the engine.

What I need to measure the float level?

[jpnovak]

If you have Wayne's 101 projects book it details a way to meaure the float levels. Another way is to buy a float level guage. You can get one on ebay or from PMO carbs.

A degree wheel will attach to the end of the cam. YOu take a lift reading starting from TDC every 10 deg or so. This will create a graph. You can determine the type of cam based on the max lift value, the duration (width of the curve) and the degrees of overlap.