One of the pages from my old website that brought me great feedback had to do with repairing the dreaded “missing dashpot oil” condition that afflicts Zenith Stromberg carburetters. So with the new blog, I figured it might be of some value to have this page updated and posted here where everyone can read it.
One of the greatest mysteries about Zenith Stromberg carburetters is exactly: “where does the dashpot oil go”? Usually, this is caused by a rotten, leaky o-ring in the nut that holds the needle assembly. This article will show you how to replace the o-ring.
You will need to have several basic tools for this repair:
- A small flat-head screw twister.
- A small Felix screw twister.
- A Zenith Stromberg needle adjustment tool. If you don’t have one, you can get by with a long hex head tool.
Here is a picture of the tools we will need:
The goal is to remove the needle adjusting screw pictured above. The needle adjusting screw has a rubber o-ring that deteriorates through the years and this is what causes the dashhpot oil to disappear.
Before you start taking things apart, please read this article completely and get familiar with all the parts. This is far from rocket science but the pieces are very small and can be easily dropped. Ask me how I found that out!
Enough talk, let’s get started…
Step 1 – Remove the Retaining Screw
Before attempting to remove the retaining screw, the needle adjusting screw has to be taken all the way out. To do this, introduce the Zenith Stromberg needle adjusting tool (or suitable hex wrench) and make sure the hex head is firmly in place. Now turn counterclockwise a few times. Now we are ready to remove the retaining screw.
The retaining screw must be removed in order to get the metering needle out of the way. Using a small screwdriver, insert it in the small hole on the side of the piston. The hole is on one side of the piston, as shown here:
Before we start taking things apart, please note the small notch on the side of the base of the metering needle. Note that it points in the direction of the hole (basically it points in the direction of the retaining screw). The screw has a taper and it fits inside that notch. This keeps the metering needle screw from turning.
Even though the diagram above says the retaining screw is spring loaded, I have yet to see one! All the carburetters I have taken apart have not had the spring. In your case it might be different, so just be aware of its possible existence. Take the screw all the way out.
Step 2 – Pull the Metering Needle Out
With the retaining screw out of the picture, gently pull the needle out. If it won’t come out, you might have to use the needle adjusting tool and turn it counterclockwise some more. It is possible the threads of the seat are still in contact with the needle. Eventually the needle should come all the way out.
This is what it should look like once it is out:
Step 3 – Remove the Needle Adjusting Screw
The needle adjusting screw is where the o-ring is. This is what we need to fix. Removing this can be a little tricky because what holds the needle adjusting screw in place is a little star washer that grips the walls of air guide valve rod. The best way to remove it is by placing a drift or wooden dowel in a vise. Then place the piston on the dowel and very gently tap with a small hammer. Use a small block of wood between the piston and the hammer so you don’t strike metal against metal.
IMPORTANT: Make sure you have the piston in the correct direction. You want to put the side where the needle was against the drift or dowel. You want to tap the piston down, thus forcing the needle adjusting screw upwards so it exits on the top of the piston.
Here is what things should look like:
As you can see, the little star washer is almost about to come out. Use care not to drop the thing, they can be hard to find! With another small tap the whole thing should come out. The needle adjusting screw should look like this:
Here it is! You can see the little rubber o-ring on the nut. The top part has the hex shaped recess where the wrench goes. The bottom is basically the threads that go into the metering needle. This is how it looks like:
The little star washer and the retaining screw keep the whole affair from coming out. Once the nut, washer and metering needle are removed the piston is pretty much hollow inside, as shown in the following picture (no the piston is not oval, when I cropped the picture it distorted).
Step 4 – Replacing the O-Ring and Assembly
As seen in the prior pictures, the o-ring has flattened and does not fit tight. This one is in decent shape, however I have seen some that literally fall apart when the nut is taken out. You can source a new o-ring (preferably one made from Vitron) and install it quite easily. This is also a good time to thoroughly clean the piston and make sure it is free of any carbon buildup. Finally a little Marvel Mystery Oil should be used inside the shaft so the new o-ring can slide easily – no point in messing him up during assembly after all this effort.
Putting the newly o-ringed nut will take a little care. Place it at the top of the guide rod and gently push on it. You want to make sure it is level. Now comes the tricky part: fitting the little star washer. Put the star washer on top of the nut and with a 5mm deep socket gently tap the socket. This should drive the star washer down and get it started. For example:
Since the socket will bottom out (it is not long enough) use a drift or dowel to gently drive the whole affair down. Just tap and eventually you will feel the nut bottom out.
The last step will be to insert the metering needle back in. You will want to get your hex adjusting tool and turn the nut as you push the needle in. Do this until it catches the threads on the needle. Of course, the needle will want to move around a little and this is where that mark or detent will need to line up perfectly with the hole where the screw goes in. Put the screw back in and tighten it a little. You should now be able to turn the hex tool and the needle should not turn anymore. At this point you should see the needle start to walk back into the piston. This of course is the way you adjust the mixture in your carburetters.
I hope this sheds a little light into how to fix the problem with damper oil disappearing. I don’t claim to be an expert in Zenith Stromberg carburetters – as a matter of fact I ditched them in favour of fuel injection. However this was a little trick I learned when I owned a Spitfire. I hope you find this article of interest and it can help you figure this mystery out!
Comments And Tips Are Welcome!
As always, any suggestions, clarifications or questions are highly welcome. This page will hopefully help someone else and with your comments this might be of further interest to others.
If you find article useful and of value (ie., it keeps your Zenith Stromerg’s running better), please consider a contribution to the tip jar… Any amount would be much appreciated!