Warning to 911T owners

[Einar Irgens]

Most 911Ts were were delivered with Zenith carburetors, at least in Europe. Zeniths are ok, but have one design flaw that causes conserns for many owners. Having seen this described in a number of threads, I figure a warning for some owners might prevent this potentially expensive mishap from occuring.

On the front of each carburetor there are three spring loaded screws. They primarily control the amount of air/fuel mixture entering the engine at idle and low revs.



Typically, if the enigine starts fouling, spitting or similar, the owner will assume that a carburetor adjustment is needed, or that some kind of grit or particles are stuck inside. One of the potential particle traps are the screws. If these haven’t been touched for a while, it is quite likely that there is a layer of corrosion or sticky fuel residues on them. Unless they can be rotated by hand - take care! The screws have a o-ring groove just outside the threaded part. When torque is applied with a screwdriver, the screw can break at this slender portion. If this happens, the remaining part may be a real pita to remove.



The only sure way I know to prevent this from happening is to remove the carburetors from the car, lay them on the side and squirt release oil into these tiny holes, right below the throttle shaft.



Spraying oil from outside will not help because the o-ring stops the oil from reaching the threaded part. It might be possible to get a few drops of oil into the hole while in the car, but you want to make sure that the threads are soaked. The carbs should be left overnight and lubricated several times.

On the last pair of carburetors I discovered a new version of this problem. One of the screws appeared to be flat at the tip. First, I did not understand how this could happen, since the hole that it is supposed to enter is cone shaped. While checking and cleaning the carburetor I discovered that the little hole is plugged shut by the tip of a screw. It has stuck inside the hole and broken off while unscrewing. Most likely, this person has not realized where the tip did go, so he has bought a new screw and tried to insert it. The tip of this screw of course never made it through the hole, but stopped against the remaining tip and was deformed.



Here is the thick end of the broken off tip. I am still trying to remove it.



I suppose a way to prevent the screws from sticking is to every now and then rotate the screws back and fourth a little.

If anyone has something to add or correct, please feel free to comment.

Einar

[1QuickS]

There are two versions of the Zenith mixture screws: the early version has a small diameter where the o-ring goes and the later version has a larger diameter which takes a smaller o-ring. The reason for the change is probably to minimize the tendency to snap the screw at the small diameter when closing the screw with too much torque.

The mixture screw you show is of the later version with a larger diameter at the o-ring interface, check your other screws as they may be the earlier version with a smaller diameter body. I base this comment on your progression pattern which looks to be an "early" version. See below for more on this comment.

These screws are made of hardened steel and since the broken portion is recessed into the bore where the o-ring goes it becomes darn difficult to center-drill them for extraction. Also, the threads for these mixture screws are very fine so an errant drilling will cause much damage to them.

The reason the screws have a fine pitch is to overcome the steep taper of their tips, this steep taper was used to minimize the potential of seizing the tip in the hole in the body when closed too tightly. A steep tip angle would be difficult to fine tune idle mixture with coarser threads so a finer pitch was incorporated.

I have successfully extracted more than a few seized/broken idle mixture screws and air balancing screws from Webers but I have never attempted removal of a seized mixture screw from a Zenith, I know my limitations. I have replaced Zenith throttle bodies when a broken mixture screw was discovered.

Be wary about swapping Zenith throttle bodies, the progression hole pattern varied depending upon when they were made and to where they were sent. There are early & late progression drillings and then you have USA and ROW models, a total of four different patterns and no model designation to tell one from another.

Perhaps EDM is a solution to explore for removal of that screw.

[Einar Irgens]

Well, I'll be damned. I have not noticed the difference until now. I guess the newer version is recommended?



On one carburetor the screw broke, and I am pretty sure it was the skinny version. Strangely, removing it was easy. After leaving the carb on the shelf for a couple of years, wondering how to fix it, I put different kinds of tools into the hole, trying to twist the screw by the tiny protrusion left where it broke. No luck. Then, when using this Schaeder valve cap, to my big surprise the remains unscrewed easily and came right out.



That was a big relief. However, taking actions to prevent the screw from seizing is of course safer.

[1QuickS]

It definitely pays to be lucky!

Congratulations on getting that broken bit out.