Zenith IDA Conversion by Performance Oriented

[VFR750]

My Zeniths now have a Weber IDA Transition Ports.

Here are my Zenith TIN carburetors after throttle body service by Performance Oriented and reassembled by: Me


As the came in the box. New throttle plates, bushings, ports, fly cut flanges, IDA ports, and thinned shaft for more high end flow. Cleaned and replated hardware --> they look awesome.


New Ports verses original, enlarge #1, #2 and #4. Plug #3:



Fresh hardware by Jim Ruffi:


Angled acceleration jets, drilled out to 0.32mm, and Performance Oriented replaceable idle air bleed jet conversion shown:


Currently tuning them, I will have more data in the next few weeks. They are runnnig rich right now. However, transition is quite nice. Solid. AFR says no lean spots. Plus, tuning is MUCH easier without random air leaks in the throttle shafts.

[speedo]

How are they set up? I have a pair I rebuilt on a stroked 2.2-2.4 which are now running 60 idles, 125 mains and 30 chokes. Be careful with the old main jet holders, I had one break (40 years old) and when looking for replacements, found more seemingly decent holders...but under close inspection had hairline cracks. It is a real bummer to be replacing jets in carbs on an engine in a car...and have the holder break off leaving the last 3/8ths of an inch and the jet deep inside the carb. Ask me how I know

Speedo

[VFR750]

Just swapped in 57 idles replacing 60s. matched with 120 idle air corrector jets. 34 venturi and 162 mains.

On a 3.0l

[Elombard]

Interesting is this a "low cost" alternative to webbers/PMOS. Something new or has it been around a long time? I know Perf. Oriented does great work.

[VFR750]

Zeniths are less expensive than Webers. And certainly less expensive than new PMOs.

Initial cost is low, add in new venturis, jets for another four hundred or so.

However, you now have 45 year old carburetors. You can jump right in and use them, but, they are more fussy due to wear and age. They, like a low priced set of Webers, should be refurbished, like i just did.

I don't know of many others doing the IDA conversion to Zeniths. I am most certainly in a very small subset of the very small group using Zeniths on a big engine: 3.0 and up. So i may be the first in a long while. Other than paul himself with his Zeber project

Paul and i have discussed transition ports for years. It was time to fix the pops and spits due to throttle shaft wear and loosness. So i thought it was time to try something new.

Cheaper than new PMOs? Yes.
A lot more effort? Yes.
More interesting? Yes.

[VFR750]

Alfa1750 sells new main jet holders. I have a set. Makes swapping mains very quick!

[porschetub]

+2 on alfa1750 he has a good stock of parts for these carbs,recently purchased rebuild kits,they arrived recently and are very good quality @ a price that amazed me,cheap freight also.
vfr750 did you have the bodies bored and sleeved for the new shafts or just new shafts?,BTW they look very nice

[VFR750]

New pre-oiled bushings. Really nice and snug. No slop

[VFR750]

Update: did not like the 57s with the 120 idle air jets. Put the 60s back in and biased the idle AFR to 11.5-11.6. Also advanced idle (900 rpm) timing to 9 deg BTDC. Much nicer, and very tolerant to sudden throttle opening.

Next step is to drop from 162 mains down to 160 mains. 85F day means I'm running richer overall. 162s are my 25-50F day jets.

Once I get it the way I like it, I will start real data collection.

[VFR750]

Today was the first day of data collection. Overall happy with the conversion. Once I've processed the data I will post it in a separate thread. It is easier to search a dedicated AFR testing thread.

Per my convention, I tested 60/120-160/165-IDA

Idle Jet: 60
Idle Air Correction: 120
Main Jet: 160
Air Corrector: 165
Transition port configuration: IDA
Float level: bottom of meniscus at upper line
Emulsion Tube: 4.0 mm Zenith
Advance: 9 deg BTDC @ 900 rpm
No vacuum connection on 78-79 SC Distributor

No stumble on typical accels, and less of a hesitation under snap, full throttle application from only 2000 rpm. (which is a very tough thing to ask of an engine with individual stacks) Very Nice. But I have to confess idle was very rich. And that probably biased the results. Next data run will be with a leaner idle to see how it influences the answer.

So tonight, I did the final idle tune: turn idle screws until rpm drop, and back out 1/4 turn. Net result was an idle AFR of 11.8-12. Nice! Took it for a test drive (w/o data) and it behaved. That made me smile.

The single, best thing about re-manufactured carbs is the ease of setting up idle. No random air leaks affecting the answer. As you lean out the system, random popping and spitting just don't happen! Either the carbs are set up right, or they are not. Repeatable.

Wow, this is much nicer than before. And worth it.