Larger Venturi for Zenith TIN40 on a 2.4L? Options

[Tdskip]

The 74 914/6 build has a 2.4 L MFI engine in it that came to me running Zenith 40 TIN.

I spoke with a Zenith specialist and he said that a 34m Venturi would be a bette match for that engine displacement and it’s an easy change to make along with some jetting changes etc.

This seems a whole lot more cost effective than going to Weber 40ida that are $3,000 +/- just for cores.

Anyone here have experience with this they can share?

Thanks!

[Ninja]

Thanks for all of the responses.

Not looking to over do it here, just get enough flow to better match the larger displacement. She is to lean know and I'm running out of mixture adjustment.

I do not know what cams the engine has.

Very helpful chart, appreciate you posting that.

New (old) member here. My last member name was technicalninja...
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The Hawk is right!

Street based cars require carbs set up for midrange power.

One thing that grabbed my attention is you're running out of "adjustment".

Are you trying to adjust mixture across the board with the mixture adjustment screws?

They only adjust in idle and when you're on the progression slots (little tiny bit of adjustment on progression). So, the mixture screws help during idle and low speed, low throttle opening operation.
This is 50% of a street car's performance envelope.

Past light cruise the jets themselves come into play.

I don't know anything about Zeniths but carbs are carbs.

If the optimal venturi is 30 (I agree with Hawk again here) I could make 27.5 work fine below WFO. The 27.5s might give me slightly better power below 4K and the proper venturi would have slightly better power above.

On yours I'd try bigger idle jets if the lean spot was below 3K, next step up on mains if it was lean between 2-6K and smaller air corrections if the lean spot was above 4k.

Are you using a wide band O2 sensor to verify mixture?
In my book it's the only thing that is really accurate.

You provision the mains and air corrections to the venturi size and changing venturis will often require main and air correction jetting changes. A venturi change from 27.5 to 30 would NOT be a big jump on either, maybe one step.

I'd jet to the venturis you have now before changing them.

You have the opposite problem that most members have. Almost all of the carb kits come with venturis for power/race operation, and they don't work well below 4K, REQUIRING a venturi change.

I had a customer with a built 460 ford (9.5comp, medium cam) who was using a 600cfm carb (baby small block carb). The computations showed a 1000 cfm carb for power and an 800 cfm carb for street use.
He never took it past 4K...
That tiny carb significantly improved low end torque and worked just fine for the customer. He WOULD not have liked a 1000 cfm carb at all.
This was in a Cobra replica (4 speed manual) and is the most violent car I've ever driven. At any speed below 100mph you could turn it sideways with throttle application alone!

Small carbs are NOT inherently bad!
Most of the time they are better than "oversized" carbs...