Adjusting Zenith 40 TIN with vacuum gauge

[Forssling]

Hi All,
I have a stock newly rebuilt 2.2 T engine with Zeniths. The carbs are overhauled and the float levels are set. I run 123-ignition distributor. I have adjusted the acc pumps to spec. The auxiliary system is blocked off. I have checked the system for leaks with starter gas and found none.

I run:
Idle jet, (57)
main jet, (120)
air correction jet, (195)

I am now breaking in the engine per Waynes book. I'm on the third oil. I have some popping in the carbs and exhaust, mainly on deceleration.

When working on V8:s I have successfully used a vacuum gauge to find the sweet spot on the idle mixture screws.

So my question is, can I do this on the Zeniths? If yes:

1) Where do I hook it up? (the vacuum gauge)
2) Can I adjust each carb bank from the same vaccum source?

Thanks for all input!
Jörgen Forssling
Gothenburg

SWEDEN

[targa72e]

Unlike your V8 with one carb for the whole engine your 911 effectively has 6. One for each cylinder. Each individual barrel needs to be adjusted individually for airflow and mixture to get the correct tune. The link below has information on how to adjust. Weber carbs have essentially the same procedure.

Zenith 40 TIN Carburettor Manual

Can be a fiddley process.

john

[Forssling]

Originally Posted by targa72e

Unlike your V8 with one carb for the whole engine your 911 effectively has 6. One for each cylinder. Each individual barrel needs to be adjusted individually for airflow and mixture to get the correct tune. The link below has information on how to adjust. Weber carbs have essentially the same procedure.

Zenith 40 TIN Carburettor Manual

Can be a fiddley process.

john

Thanks John for your input. I have this already. But I guess what you are saying there are no way to use a vacuum gauge to set the idle mixture screws?

KR // Jörgen

[Matt at Pelican Parts]

Hi Forssling,

Beautiful 911 you have there!
We have a great article on syncing the carburetors using a Unisyn. It's written up for Weber but you can apply much of the same info for the Zenith.

You can also check out this older thread where a couple users gives some tips on adjusting them and in what order.

-Matt

[Nditiz1]

Maybe I'm just a young guy and never used the Unisyn tool, but friends don't let friends use a Unisyn. Get a Syncrometer from either here or someone else. Actually get two so you can check both sides at the same time. That also tells you if the carbs are opening in sync from the linkage as the vac measurement should match. Don't get the Latest Rage off Amazon either, its garbage. Get a good German or STE brand.

[targa72e]

HI Jorgen,

You might also check out the tuning section for webers from Performance oriented

Performance Oriented

Pretty much the same procedure for Zeniths. Adjust overall, adjust individual repeat until overall is where you want it.
I think the issue with trying to use vacuum is that you would need a vacuum port on each cylinder. Because you only have one cylinder pulling vacuum on a port it is not smooth at idle. Vacuum works on a V8 with a single carburetor because the manifold is a common plenum design and with all 8 cylinders pulling vacuum under the carb you get a averaging so you have a relatively smooth vacuum reading.

john

[Forssling]

Thanks All for your replies! I don't have a problem understanding the balancing of the carbs. I got that worked out pretty good.

What I find difficult is the setting of the idle mixture needles. Some say to screw them in to where the idle drops then out. The factory manual says to where "it runs the smoothest". Since there are six of them I would like to find a more definitive way (as a vacuum gauge).

I understand I have to balance the carbs afterwards once again after setting the idle mixture screws.

Appreciate all your input!

//Jörgen

[Tyson Schmidt]

Clean the transition ports before diving too deep into tuning. These carbs are notorious for carboning up the transition ports.

You can access them by removing the large slotted screw in plugs in the side of each barrel.

[Forssling]

Originally Posted by Tyson Schmidt

Clean the transition ports before diving too deep into tuning. These carbs are notorious for carboning up the transition ports.

You can access them by removing the large slotted screw in plugs in the side of each barrel.

Thanks for your input! I have had them totally disassembled, cleaned, ultrasound cleaned and cleaned again. So I hope that they are clean

//Jörgen