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Zenith 40 TIN jetting etc
Posted by: DaveO90s4 ()
Date: November 29, 2017 06:47PM

Hi all,

I've referred to my carb problems in a separate thread but I think it better to start a new specific thread re my Zenith 40 TIN carbs

Engine is 2.7L +.060 ( so about 2.8L) in my 914 Porsche, with Otto 20 (or maybe 30) cam, triple port inlet manifolds and a pair of Zenith 40 TINs with the following specs:

Venturi:- 34 mm
Mains:- 140
Air:- 200
Idle:= 55
Emulsion:- unknown
Accel pump vol:- unknown but all jets definitely squirt.

I have yet to do everything i know I need to do (ignition timing, float level, accel pump squirt volume, balancing etc etc). I will.

As a first step I have checked that mains and idle jets are not blocked. Yet to check air jets (scared of dropping one down a venturi...)

Engine idles OK though a bit lumpy. Pulls nice and hard at moderate to high throttle openings. At minimal throttle openings engine hesitates. Quite pronounced at 60 mph on a flat road.

Being a complete novice at carb tuning is there anything obvious that looks amiss with the above jetting and could be a contributor to the hesitation off idle, assuming all things that I've yet to check out are OK?

I'd be particularly interested to know what others with Zenith 40 TINs might be using in terms of jetting.

Hope to receive sage advice.

Thanks all
Dave
Bne

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Re: Zenith 40 TIN jetting etc
Posted by: MattNall ()
Date: November 29, 2017 07:05PM

This will be interesting... Very few have used the carbs due to the lack of different sizes of Jets, etc. here in the US.





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: Zenith 40 TIN jetting etc
Posted by: texas yenko dude ()
Date: November 29, 2017 08:46PM

I have only ever owned 1 set of Zenith's and did not run them and sold them. I have run IDA's on the street and the race car. I've probably had 8 motors with IDA's.

The off idle and minimal throttle condition you are having is a lean condition.


If I was to tune them like a set of IDA's, which is where I would start, your idles are too small and your air's are too big. The bigger the air, the leaner it gets. I would start with 65 idles and drop the airs to 180 and see what you get. Next step would be to drop the airs to 175's. On IDA's, 3 air sizes = 1 main jet size.

You did not state your CC ratio, but your 140 mains are close with 34mm chokes. Close enough that you will be able to get it right with adjusting the airs.

Texas YENKO Dude
YS-070
YS-199
YS-320
Southeast of Disorder


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Re: Zenith 40 TIN jetting etc
Posted by: DaveO90s4 ()
Date: November 29, 2017 10:03PM

Hi Matt and texas yenko dude

Yes, it will be interesting!! I see that the jets that I removed and looked at are marked 'solex' so I'm hoping either zenith or solex jets will do the job.

And thanks for the 'too lean' diagnosis. Now that I know what the idle, air and main jets actually look like I'll go through the packets of new and used jets that came with the engine and see what I can see.

I'll see if I have some 65 idles and 180 airs. If not I'll see if I can hunt some down. (Mind you, if its hard to get in USA it'll be all but impossible to find in AUS).

Thanks and cheers

Dave O
Bne
Oz

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Re: Zenith 40 TIN jetting etc
Posted by: DaveO90s4 ()
Date: November 29, 2017 10:31PM

Looks like everything I need is available here.

[stores.ebay.com]

Dave

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Re: Zenith 40 TIN jetting etc
Posted by: vairmech ()
Date: November 30, 2017 04:25AM

The first thing I would do is make sure that the accelerator pump is working right from the start. If it works off a cam take the clearance out, if there is an adjustment then make the stroke longer.
I disagree on the idle jet size but the air correction could be a little smaller, 180-150, and you really need to know what the emulsion tube is. What works for a Porche does not work on a Corvair! The IDA works fairly well with an F11 emulsion tube. I would have to take apart my Zeniths to see what emulsion tube is installed.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Zenith 40 TIN jetting etc
Posted by: MattNall ()
Date: November 30, 2017 07:48AM

Great prices on those parts too!





MODERATOR
Sea Mountain, between Charleston Harbor and Coos Bay! SW Oregon Coast
Click HERE for My Website...Click HERE for My TechPages!
..............................110-PG.................................................Webered-Turbo

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Re: Zenith 40 TIN jetting etc
Posted by: DaveO90s4 ()
Date: November 30, 2017 10:59AM

Hi Ken,

Thanks for your input. I can see that the accelerator pump is squirting enthusiastically in all ventures. I have yet to hang a tiny vial in each Venturi to measure the volume though.

I agree it would be nice to know what emulsion tubes it has. But at this preliminary stage I'm quite keen to not remove the carbs from the engine to disassemble them. It took me about 2 hrs to get just one of the 12 carb mounting nuts threaded to its stud. Engine access is not wonderful in the 914 engine bay! Maybe next time I'd be faster :-)

Cheers al
DaveO

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Re: Zenith 40 TIN jetting etc
Posted by: vairmech ()
Date: November 30, 2017 06:59PM

I think the emulsion tube is accessible from or through the top under the air corrector jet.

Ken Hand
Handy Car Care
248 613 8586

Vairmech@aol.com

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Re: Zenith 40 TIN jetting etc
Posted by: DaveO90s4 ()
Date: November 30, 2017 08:30PM

Hi Ken,

Thanks for that info. I'll re read my zenith 40 TIN manual.

Have ordered a range of jet sizes too.

Thanks
DaveO

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