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.
Required Tools:
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:
So we don’t get our wires crossed, let me post a schematic of the carburetter. I will try to refer to all the pieces parts by the terms described in the following diagram:
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.
Conclusion
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!
It’s useful for me to repair my Carb. Thanks.
Awesome news… Good luck with the carbs.
thanks for the info and the pictures. very helpful! the manual just doesn’t do it justice…
No, the manual does not illustrate this “detail”. I am very happy to know this has been of help to you!
I wonder about the delrin washer. It did not appear there was such a washer when the carb was disassembled not was there anything in the rebuild kit. Is it the same as the fitting that appears to be pressed into the unit? The needle has a lot of slop and wiggles around when the unit is assembled. What material is the delrin washer made of?
Gerry
Thank you for the question. Let me see if I can answer your question.
I am not sure exactly, I follow what you refer to as the “delrin washer”. Delrin is a hard thermoplastic and I don’t think I mention it specifically by name in my article – correct me if I am wrong. Instead, I presume you are talking about the O-ring as described in the diagram on the second illustration of the article. If you look at the diagram, the O-ring is labeled on the left hand side of the cutaway diagram of the carburetter’s plunger.
I suppose this is what you are talking about. If so, I agree with you that the O-ring is NOT supplied in any of the “kits”. That is what makes this whole procedure such a “mystery”. All the kits I have seen, include everything but this very important O-ring. I suppose this O-ring is something that can be found at a hardware store however what you are going to find there is likely made of ordinary rubber. With today’s modern fuel formulations (especially if they are laced with ethanol) you will need to source an O-ring made of Viton which will offer a longer life.
I put this article together from some old junk parts I had lying in the garage and wrote it so others might benefit from it. If anyone is reading this comment and know of a source and size of the O-ring please let me know so I can update the article and others might benefit from it.
Finally, regarding the “slop” you describe: again, I am not following you. The O-ring is used to seal and seat the needle adjustment screw. This is what basically screws to the top of the metering needle assembly and keeps it from moving around. In addition, there is a slot on the metering needle that allows the spring loaded retaining screw to “lock” the whole affair together. If the O-ring is not on the needle adjusting screw I would suppose the entire assembly would not fit tight. Finding a proper Viton O-ring is the trick here.
I hope this helps answer your questions. If not, please give me some more details.
Hope you can help me? I can’t figure out how the whole assembly goes back into the body of the carb. What I mean is, the two large holes on the bottom of the assembly. Which way do they face? I didn’t pay attention when I pulled it out!!! Do they face towards the air filter or what?
If memory serves me correctly, the rubber diaphragm can only be mounted in a certain way: there is a small indexing mark that enables the proper orientation on the center of the diaphragm. Once that is properly located and the ring secured with the 4 phillips head screws you can look at the outside edge of the diaphragm.
I believe that you will see there are also indexing tabs on the outside edge of the diaphragm that will fit only one way on the body of the carburetter. This will only go but one way and will properly align the whole affair. If this is not aligned correctly, I don’t think the plunger will work correctly.
I am writing this from memory. I no longer use these relics. I hope this helps…
Thanks. This article helped me a lot. I would never dared to tap out the needle adjustment screw and retaining clip if I hadn’t found this guide.
You wrote that you hadn’t seen any spring together with the spring loaded retaining screw. That’s because the springs is inside the screw. The tip of the screw can move inside the hollow screw and the spring inside pushes the tip out against the small notch on the side of the base of the metering needle.
I guess that’s why you can tighten the retaining screw and still adjust the needle when everything is mounted.
Mats,
I am very happy the article helped. Ah!! So that is where the spring is! Good to know.
Also Mats, I wanted to thank you for your donation. I’ve been running this blog for several years now and this is one of the most read pages. You are about the only person kind enough to make a donation. Knowing that my article has helped someone enough to wish to make a donation is very comforting.
Thanks again!
The new o-rings are in place. It all went smooth, but I have the kind of needle adjustment screw with a raised blade instead of a hexagon recess, so there was a little challenge.
When tapping the adjustment screw and retaining clip down with a drift, the drift hits the top of the raised blade, so the retining clip never goes the whole way down to fixate the screw. Therefore the base of the needle could move 0.8 mm (or whatever height the raised blade have) up and down with the adjustment screw. I guess this would have made it a little bit tricky to adjust the mixture in the end.
The solution was to find some kind of tube among all my junk (I rarely throw anything away) with suitable dimensions. I was lucky to be able to cut of a short piece of brass tube from a discarded gas burner. Outer diameter 8.5 mm and inner diameter wide enough to go clear of the raised blade, and 10-12 mm long. With the tube in front of the drift it was easy to tap down the retaining clip and fixate the screw. I have a picture if you get the idea to complement your excellent guide at a time.
Sorry to hear about the lacking donations. I think it’s a great way to appreciate and support all the good non commercial initiatives out there. The principle is simple. If you find something that has a value for you, well then pay something. I hope it gets more used in the future.
Regards, and thanks
Mats Sellner
Mats,
Thank you once again for your follow up and kind words. This page just keeps getting better and people continue to visit here to get very good information on something that has really never been documented very well. Your idea of using the brass tube is brilliant. And, you are correct in stating that if this is not all driven correctly it would be difficult to gain proper adjustment on the needle.
Finally I really appreciate your kind words about donations. I established this blog not for profit, but knowing somebody thinks enough of this page to kindly donate sure is nice. Once again, thanks and happy motoring!