Zenith 40TIN recommended jetting

[Forssling]

Hi All, I'm rebuilding my carbs.

What do you recommend? T-71 engine. The engine is going to be stock apart from K&N air filters. The carbs currently have the stock setup. In brackets I have a recommendation from another thread but I do not know if that setup is on a stock engine or not.

911 100 999 01
Idle jet, stock 47,5 (57)

911 100 929 00
main jet, stock 115 (165)

911 110 894 00
air correction jet, stock 195 (175)

Appreciate all input,
Jörgen Forssling
Gothenburg
SWEDEN

[Trackrash]

Assuming an otherwise stock motor, the only thing I would change is the idle jets. Probably 55s would do the job, but 57s may be better. If you get flat spots going through transition, go up to 120 or mains. BTW, I think the stock main jet is 105, not 115, but I'm not certain of that but for sure 165 mains would be way to rich. No reason to mess with the air correction, I would think.

The only way to know for sure, is to check you plugs, or better yet, install an Air-Fuel gauge.

[Forssling]

Thanks Gordon!

One thing that struck me is that here in Sweden we have an addition of 5% ethanol in our premium petrol. Would that affect my choice of jets?

//Jörgen

[Trackrash]

Only 5%? We are struggling with 10% here.

I don't think that will make much difference, but again, you will have to try your changes and see how it goes.

[Forssling]

Yes, we will probably get 10% soon. Would you recommend to put in new main jets as a part of the overhaul?

//Jörgen

[kium]

Hi Forssling,
I'm at the same point of rebuilding my Zenith carbs.
Have you run the car with the new jetting?

[Forssling]

Originally Posted by kium

Hi Forssling,
I'm at the same point of rebuilding my Zenith carbs.
Have you run the car with the new jetting?

No, not yet. In the middle of engine rebuild.

//Jörgen

[kium]

Me too, well, just finished it, waiting for the rocker covers that have gone to powder coat, but finished.
Now I'm on the carbs and gearbox. Jumping like a monkey from one task to another.
We'll see who tries the jetting first LOL

[timc]

Back in the day, Sotddard used to sell a 2.2 "Sports Kit", that included larger Jets and and I think different Venturis for the Zenith, Twin Pipe Sport Exhaust, and Filtron Air Cleaners. I found a used "kit" back in the early 80's and installed it on my 2.2. Nice difference. My point, I'd research and see if you can find out what sizes were in the kit, as it worked very nice with an otherwise stock 2.2.

For some reason I am thinking the mains were 155, but I could be wrong.

Quick Search on "2.2 Sports Kit"

1.) Custom machined 34mm venturis for the TIN Zeniths
2.) set of jets (result of John Truman's dyno work with the kit)
3.) pair of Filtron low retriction foam air filters
4.) 911E Marelli rotor to raise red line to 7000 RPM

I can't even believe I remember this......

[kium]

Sounds really interesting!
I'm surprised in general how little tunning and information there is on Zenith carbs.
I found this thread with goood info:
https://forums.pelicanparts.com/911-engine-rebuilding-forum/602778-de-detuning-2-4t-stoddard-zenith-sport-kit.html

The bottom line I get is that the more you squeeze the carbs to more on-off it becomes.
I'm sure there will be the "middle path"...

[Forssling]

Originally Posted by kium

Me too, well, just finished it, waiting for the rocker covers that have gone to powder coat, but finished.
Now I'm on the carbs and gearbox. Jumping like a monkey from one task to another.
We'll see who tries the jetting first LOL

Yes! Let me know how it goes. I'm waiting for the machine shop. Maybe I'll get the engine in car at the end of the year.

//Jörgen

[ed mayo]

On my 71 t that is stock except for 1 mm over bore I went up two sizes on idle jets, made for a smoother acceleration curve. It is very important to properly adjust accel pump discharge and aim the nozzles.

[Richey]

klum , where are the air baffle tins that fit on the cylinders ? They need to be installed before the oil return tubes . GOOD LUCK .

[kium]

hahaha, yes I know, Thanks for your advice.
I sent you the wrong picture. It shows the dry assembley recomended in Wayne's book.
Here's the good one.

[VFR750]

Originally Posted by Forssling

Hi All, I'm rebuilding my carbs.

What do you recommend? T-71 engine. The engine is going to be stock apart from K&N air filters. The carbs currently have the stock setup. In brackets I have a recommendation from another thread but I do not know if that setup is on a stock engine or not.

911 100 999 01
Idle jet, stock 47,5 (57)

911 100 929 00
main jet, stock 115 (165)

911 110 894 00
air correction jet, stock 195 (175)

Appreciate all input,
Jörgen Forssling
Gothenburg
SWEDEN

Jorgen,

For a stock 2.2 engine going a little bit larger than stock would be good. Maybe 50 idle and 125 mains.

57/165 jets are way to big for a 2.2/2.4 T engine.

Alfa1750 on eBay has a huge selection of parts for the zeniths.

I’d recommend an air/fuel sensor to measure the mixture. While some can tune with their ear and reading spark plugs, I can not. I use dual O2 sensors.

Air correction jet sizing is really tricky. You need O2 sensors and a place where you can really accelerate hard. Or a dyno ( rolling road). Air correction jets affect the top end mixture. 195 is probably too large for a 2.2/2.4 engine.

[Forssling]

Hi, this is the setup I ended up with (recommended to me):
Idle jet, (57)
main jet, (115)
air correction jet, (195)

We'll see where this takes me when starting up the engine hopefully before this year ends.

//Jörgen

[911 SLANT]

Make sure to check your float bowl levels. Made a big difference with my Zeniths.

[Forssling]

Originally Posted by 911 SLANT

Make sure to check your float bowl levels. Made a big difference with my Zeniths.

Yes! I bought the tool and has made the settings. Thanks!

//J

[iq207]

I have Zeniths on a 2.4S motor with 2.2S pistons (higher compression). This runs well as follows:
32mm venturi
60 Idle
155 Main
170 Air
Stock Emulsion tubes

I have not done Air Fuel Ratio testing, so only have the seat of the pants reporting.