Prewar Workshop: a new carburetor jet

Today’s workshop article is about a carburetor with a broken jet. The car performed very badly and was leaking fuel through the break in the jet. The jet was in 2 pieces and needed replacement. It had been broken in half and a copper pipe was inserted to fix the break, but this broke again unfortunately.

It’s a brass part with a few bores. One main bore, 3 different fuel jetting sizes and a few threads. A quite simple part but takes some careful measuring to get it correct.

Gauging the hole sizes accurately was the most important and most time consuming task. I did this with a full Imperial drill set I have. In the simplest way possible, I tried each drill in the hole until one size larger didn’t fit anymore.

There were 3 different hole sizes and one of these was under a 45 degree angle. Supporting the part in the rotary table was another challenge. If there wasn’t enough support, these small drills can easily snap off in a hole.

The threads were also quite unusual sizes. I single point threaded both the inside and outside thread for this reason, as I couldn’t find the correct tap and die for these.

The end result is a carburetor jet that the car ran on correctly and it starts by a push of the button. Please enjoy the photo album, which is again supported by comments with each photo. If anyone still has any questions about this part, please comment or contact me at [email protected]

The photo album can be viewed here: >click<

Publié:

mardi juin 6th, 2017

Inconnu

07 Juin 2017, 19:04

Jos, very nice, I will keep the photos for referencing. Back in college, I would have chosen this as semester project for Manufacturing Technology 104(M.E.). You probably know, wouldn't a hole gauge set be easier to measure the sizes?

jos

07 Juin 2017, 12:47

Thank you for the comments. It's difficult for me to judge wheter viewers want more technical details or more pictures. It's been about a year since I've made this part so I can't recall the exact sizes and holes anymore. But the thread was a 60 degree 7/16" thread with what I believe was a 13tpi pitch, at least not a BSW fine or course standard. The inside thread was the same. I grind my own single point tools for these threads. Since it's brass, this is quite easy to cut. I wouldn't dare doing that in stainless.

The holes were about 2mm for the smaller holes and 3,5 for the slightly larger. Scale is about 16mm diameter and 120mm long total.

I will follow up in more details in a following article about the thread cutting because I do that different than most people I see, good idea for a workshop article!

Inconnu

07 Juin 2017, 09:19

...one more thing, depending on the scale I'm very impressed with the thought of single point thread cutting on an internal tiny thread. That's the sort of thing I like to read about and marvel at.

Inconnu

07 Juin 2017, 09:16

Nicely made but personally I'd like to see more facts. If you didn't know how big a jet was it's hard to tell from the photos whether the jet holes are 0.8mm or 8mm. Why the lathe to cut the thread? You say they were unusual but in what way?

Don't get me wrong- I love to read these stories and workshops but more facts and relevent information make it better for the rest of us to learn from it rather than it just being a "I did this, isn't it pretty" type article.

That said, please can we have more of these workshops!

Inconnu

07 Juin 2017, 07:50

Nicely done - very satisfying fix.

Prewar Workshop: a new carburetor jet

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