Zenith 40T carb questions

[911Tfan]

Hi,
I have a 1970 911 T with Zenith carbs. I have some backfiring especialy when i let off the gas and want to synch and tune them the best i can. I own a book about weber carbs and woner how similar they would b in constuction. Could i use the weber book at all for a schematic or anything? I read the book and the description of how the webers are designed and i am guessing that the triple choke Zenith is a similar but cheaper copy. Can i take out the idle mixture screws and shoot carb cleaner in there? Or maybe take out the air bypass screws and shoot carb cleaner in there too? i already sprayed some down the throats but it didnt change much. Id like to test the accelerator pump to see if they are working, etc... as much as i can without taking them off the engine.

Any ideas?
Also who rebuilds zeniths for a reasonable cost?
Thanks

[neilca]

The two carbs are really different in how they function. For instance the idle screws in the Weber adjust the amount of air in the idle circuit. The Zenith the same screws adjust the amount of fuel introduced into the idle circuit. The Weber has one accelerator pump and the Zentih has three. The Zenith also has a high speed enrichment circuit via the accelerator pump circuit.

The backfiring on deacceleration is probably a vacuum leak. Check the hoses on the deacceleration valves located on the outboard side of the carb. If your accelerator pumps were not functioning you would have a stumble off idle.

Hope that helps,
neilca

[rw7810]

email me for a good two page weber set-up guide... it can also be used with Zeniths.

rw7810@att.com

PS: to check the accel pumps, take off the air cleander and looking down you should see a little J faucet like tube that suirts as you press down on the pump actuator rod. There is a $25 vial you can buy to measure if it is squirting the right amount. You should take out the idel jets and insure they are clear.

[911Tfan]

Thanks guys those are good suggestions i will try.

[James Brown]

not to be bias but the carb world order is: PMO, Zenith, Weber.
zenith's are just not respected/understood, can I get an amen!!!

[digitaldgp]

rw7810,

Thanks for the set-up guide. I'll use it when I tune the carbs.

After the body is back together!!

[911Tfan]

I know the carbs need a rebuild but i am figuring that will happen in the winter when i have the car garaged. i tried setting up the carbs by the 101 projects book and a haynes book i have and neither made it run any good at all. in playing around with them i discovered that if i open the air bypass screws 7 turns the backfiring all but disappears and it runs pretty good. it also seems that the idle mixture screws dont do a lot and it runs best whn they are screwed all the way in, basicallynot doing anything.

[crater64]

Originally Posted by rw7810

email me for a good two page weber set-up guide... it can also be used with Zeniths.

rw7810@att.com

Hi,

Do you mind if I e-mail you for a copy of that as well? My Zeniths are already in decent tune but that's no guarantee they'll stay that way.

Thanks,
Ed

[rw7810]

Ok, but you need to send an email to the above adddress........

[crater64]

Done, thanks.

[VFR750]

Originally Posted by rw7810

There is a $25 vial you can buy to measure if it is squirting the right amount. You should take out the idle jets and insure they are clear.

Where do you find the vial to measure the .5ml +/-.1 ml discharge from the tube?

I searched chemistry supply websites, and can find anything that small. The best I could find is a graduated eyedropper, but it seems to be too long to simply fit between the tube and the butterfly valve.

Mike

[french911]

I found that increasing the idle jets to "55" made a world of a difference. Also take off and seal all aux enrichment system, blank off the vacuum on the carbs and run about 5 degrees BTDC on the distributor at 900 revs. Sounds like yours need a good clean also.

Barry

[french911]

Sorry forgot to add, copy of factory set-up for Zenith 40TIN carbs can be found at Porsche Leather Work

barry

[James Brown]

******** has the tools for Zenith (float gage, acc. pump vial).

[James Brown]

s-t-o-d-d-a-r-d

[deathpunk dan]

Interesting thread. My car has Zeniths on it now, but I have a set of pretty nice Weber 40s I rebuilt last year in anticipation of installing. I thought conventional wisdom was to install the Webers for best performance, but is this not necessarily the case?

I just want to use whatever will work best on a stock 2.2T.

Barry, can you comment on your Zenith steup suggestions? IE what is the effect, better drivability? smoother transition from idle to main, better acceleration?

[VFR750]

Originally Posted by James Brown

s-t-o-d-d-a-r-d

James,

Thanks!

I assume the little plastic one with the metal holding strap is the one I need. Does it have markings on it?

BTW, your conversion got me thinking about Zeniths.

Mike

[James Brown]

The glass one does but it's $80! the plastic one does not ($15), you can find a mil vial for a few bucks and save some $$$$
Love my Zeniths! they just work....

[french911]

Originally Posted by deathpunk dan

Interesting thread. My car has Zeniths on it now, but I have a set of pretty nice Weber 40s I rebuilt last year in anticipation of installing. I thought conventional wisdom was to install the Webers for best performance, but is this not necessarily the case?

I just want to use whatever will work best on a stock 2.2T.

Barry, can you comment on your Zenith steup suggestions? IE what is the effect, better drivability? smoother transition from idle to main, better acceleration?

Hi

When I started on my 73T after a top end rebuild it was difficult to find a good tickover, it had flat spots and was a pig to start and keep going until warm. I stripped the carbs and gave them a service kit. While they were off I removed all the auxiliary enrichment and blocked all the holes, I also blocked the vaccuum on both carbs. I figured that I did not need the retard on overide.When I put them back on the car I checked float levels but I have to admit I did not check the pump volumes I just trusted to the setup procedure as per the factory spec sheet ( see previous post). I also put in "55" idle jets. Back on the car I put a Pertronix ignition (best value from Pelican) and reinstalled the carbs. I set the timing to 5 degrees BTDC at 900rpm and found the idle mixture screws at out by 21/2 turns to be almost there, I think eventually they went to about 31/2 turns after road tests. I set the balance of the carbs at idle using a carb synchroniser as per the specs.
Now the car starts like a dream after a couple of pumps on the pedal, idles sweetly after a minute or so, it is free revving and smooth through the range upto 6k revs, and I can get 16litres/100km although not often because I tend to drive like I stole it. In the early days I was looking for Weber or PMO but now I am more than happy with my Zeniths.

take care

Barry

[Christien]

pm me your email address and I'll send you the Zenith repair manual, or at least the important stuff on parts and adjustments. It used to be hosted on a website that has since disappeared - luckily I archived the whole site before it went missing. It's pretty much indispensable for working on Zenith carbs.