Zenith throttle shaft screws stuck

[Einar Irgens]

These screws really stick after sitting for fifty years. I have soked them in penetrating oil, but still the heads deform like this. Can this be drilled out without destroying the shaft?

[1QuickS]

You might be able to get these out if you have a mill to secure the part and control the drill bit. Even with that you will probably drill off center and damage the threads in the shaft. Since the throttle valve is tilted with respect to the throttle body you will either need to tilt the head of the mill or tilt the throttle body to allow drilling on the axis of the screw. Mill away the screw head to provide a flat surface to start your drilling & use a drill point countersink first to avoid drill wandering.

Removing these screws is best achieved by following these steps:

• Use 5/16" diameter (8mm), ball-ended die grinder to remove screw that extends past the shaft on the back side of the throttle valve
• modify a screwdriver to minimize the taper at the end and to have a tight fit within the screw slot, this is accomplished using a bench grinder
• apply heat to the screw using a small flame from an oxygen/acetylene torch

Be careful to not press too hard with the screwdriver, this will bend the shaft.

[Einar Irgens]

Thanks for your reply, Paul.

It it correct that the screws are deformed at the tip to keep them from loosening? And could this be the reason they stick?

I am considering not taking out the discs anyway. I wanted to check wether they have "ears" from wear caused by drifting along the shaft. How common is this? Can it be checked without dismantling?

Einar

[1QuickS]

Screws are deformed to keep them from coming out.

Edge wear on throttle valves can be viewed by removal of main venturis and then looking at a light through the throttle bores, hold bottom of carb toward light and view from the top of the carb body. Use the throttle stop screw to slightly open the throttles.

It does not matter too much if the throttle valves have a little edge wear, the real issue is if the shafts are too loose in the bearings that support the shafts. Too much clearance from wear will result in an erratic idle due to the shaft moving around in the bearing which results in variable air flow into the engine, the variable air flow changes idle mixture which causes the erratic idling problem with spitting up through the intakes as a audible result.

[Einar Irgens]