Zenith problems
 

Hi,
I had a mechanic work on my 1970 911t because of a lot of backfiring out the exhaust. He rebuilt the carbs but there were issues. Floats were hard to find, then overflowing occurred and a fuel pressure regulator was added. Overflowing ceased but car surges while idling. Unsettled idling. Also at light throttle car surges badly and makes it uncomfortable to drive. Under hard throttle it accelerates ok until it sputters and sort of breaks down. I've been messing with the idle mixture screws and air bypass screws to no avail. If I make it rich enough so it doesn't backfire, it surges at low speed. As it was before the rebuild, the air bypass screws don't really seem to have any effect. The idea was to rebuild the carbs instead of buying new ones to be cheaper but I think I made a mistake. The auxiliary enrichment has been bypassed/disconnected as a few Pelicans have suggested. The floats are supposedly working, the car starts very easy compared to before the rebuild.
If anyone has any ideas I'd be glad to hear them.

There are many ways in which Zeniths can be mistuned. also many ways in which you can fix them.

Can you provide more details on the jets in the car:
Idle jets,
main jets,
air correction jets.

It is very likely the initial backfires were from air leaks between the manifolds and cylinder heads. That is common.

Now you are running E10 fuel, and the idle and main jets should be increase a little to offset the loss in energy in the fuel. It sounds like you are too lean on the mains.

At this point in time you have to ask yourself: how much do I want to learn about carbs and do I want to learn how to tune them myself?

It can be expensive to pay someone to figure this out. It can also be time consuming to learn about them.

If you want to learn how to tune, then I'd recommend you get some form of exhaust gas measurement system that can tell you the Air-Fuel Ratio. You can buy a system that mounts to the exit of the muffler, or you can get an exhaust with a bung welded on to mount a wide-band 02 sensor.

Either way, without a way of measuring AFR, you will be guessing as to what to change and why.

alfa1750 on E-bay sells all kinds of parts for Zenith TIN carburetors. I've use him for several years now. Should be able to fix just about anything, and get almost any size jet.

Does this help?

Are you running the stock air cleaner? If not those carbs won't run right unless you change the jets.

Is your motor completely stock? Including the muffler? Stock T fuel pump? Distributor original and timing and points in spec?

Any play in the throttle shafts?

Can't imagine flooding or "overflowing" IF the floats and needle and seats are OK.

My 2,2 T motor with Zeniths always ran great, until I started modifying the motor.

Overflowing canbe caused by the big threaded cap over the float valve

When the 6-point hex is machined, the flange can be deformed. This leads to a wavy surface that is trying to crush the aluminum gasket. It will leak.

I sanded that surface smooth and the problem was solved.

Surging at idle is too rich.

What mods does your engine have?

These are a simple carb and easy to work on and modify.

Seems odd they went rich after a rebuild ? keep the mixture screws @ 2.5 turns out, balance the throats using the air bleeds,if you are rich you will most likely find adjustments do little or nothing,check for wetness around the ventures @ 900rpm idle it ,if so you will be rich or have spill over from the fuel bowls.
VFR750 is correct about the float valve caps I had the same issue,easily fixed.
I run 3psi and find that pressure works for me,i'am running 30mm venturies ,52 idles and 125 mains and stock air correctors....starts and runs nice.

As far as I know, the motor is stock, but I don't know how many owners there were before me. I have had it 10 years. I don't know what size the jets are but I can ask the mechanic if he changed them and to what. I am running the stock air filter assy. The mechanic said he ran the intake manifolds on a belt sander to make sure they were flat. He also repaired a small chip/crack with something called fusia (sp) as well. I have been using the same gas for a while now and it ran ok before. I will try an AFR meter. and also look into the throats while it's idling to see if it's wet. He installed a fuel pressure reg and it's supposed to be running at 2.5 pounds. The muffler is a Bursch Turbo but that's been on the car 10 years. I'll have to check into the threaded cap over the float valve. It's already been expensive. lol. I probably could have bought reman. carbs already. Just hate to give up at this point especially if it's only a small thing remaining to fix. Thanks for all the good ideas!