HOW TO - resize carburetor jets

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HOW TO - resize carburetor jets

Post by kyle_pc_75 » Sun Sep 05, 2010 7:35 pm

Leatherneck suggested I do a "how-to" on resizing carburetor jets, since I had to downsize the idles on my Zenith. It's really pretty simple and takes about 20 minutes a jet once you get it down.

Tools you'll need:

- Jet gauges
- Jet reamers
- Pin vise to hold reamers (mine came with the reamers)
- Dial calipers (to measure the reamers)
- Electronics "helping hand" or bench vise
- Soldering gun/iron (I prefer the Weller on-demand guns as opposed to constant hot irons)
- Solder (not sure if it matters what kind, I just use the same as I do for electronics)
- Flat, fine file
- Welding gloves (to handle the hot jet if you need)
- Carb cleaner
- Sewing needle and string



First off, clean the jet you're going to resize. The carb cleaner and some needle and thread work great for getting rid of any varnish in the jet. I don't know if this matters, but it can't hurt when considering the solder needs to "bond" with the brass.

Next, gauge the jet so you know what you're working with! Many are not sized as marked. Here's what I started with...marked a 115, it's really about a 140.


Assuming you want to downsize the jet, you'll need to solder the hole partially closed. Obviously if you want to upsize, you can skip the whole soldering part and simply ream. When soldering, make sure you heat the jet and don't melt the solder and try to dribble it in the hole. It won't stick. Hold everything about like this, that's the tip of the soldering gun on the right:


Once you feel it's melted into the hole nicely, let it cool just a bit and then heat and melt it from the other side of the jet to make sure all sides are sticking nicely. It will look something like this when done:


Next, file the solder flat with the jet. If you didn't get enough heat throughout, you'll end up with ugliness:


I reheated and refiled and when the surface looked flawless I was happy.

Now it's time to ream! I like to use a reamer slightly larger than the hole I want. The thicker ones are easier to use as they don't wobble as much. You'll have to use the dial calipers to find the size reamer you need. I was after a 115 (1.15 mm) jet, so I picked a reamer that measured 1.20 mm.

Try and get the reamer as near center and straight on as you can. I don't think perfection is necessary, but I like my work to be neat. Be gentle when reaming. Some of the reamers are very thin and bend and wobble quite a bit when any downward pressure is applied. This will lead to crooked and wallowed holes, something you don't want!


Ream a bit, measure the hole, ream a bit more until you get the size you want. At this point I like to see how far in the reamer went before the hole went through. I figure if I get about 5 mm of solder lengthwise inside the jet, that's enough. If not, I'll start over.

When you're there, file the surface down flat and ream again to get a nice, smooth opening. I like to use one of the large reams to then slightly chamfer the edge of the hole.


After you're done, clean the jet thoroughly! Compressed air (your mouth), carb cleaner, and the needle and thread again will help get any bits of solder or brass out of there. No sense running that junk through your engine.

If your jet was marked, now's a good time to remove the marks (unless they are now correct). A dremel is good for that. It's nice to remark them with an awl or something similar, but a handy storage/organizer case and the gauges work also.

That's really about it. If your carb is hard to find jets for, this is the ticket. Good luck. Hopefully this helps, let me know if you have any questions or suggestions.


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Re: HOW TO - resize carburetor jets

Post by Qurtys Lyn » Sun Sep 05, 2010 7:47 pm

Nice write up, Kyle.

Mods can we get this put in the Stickies post?
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Re: HOW TO - resize carburetor jets

Post by Leatherneck » Sun Sep 05, 2010 11:26 pm

Kyle thanks for the write up. Added to stickies ... 98#p858398

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Re: HOW TO - resize carburetor jets

Post by fusername » Mon Sep 06, 2010 10:30 am

very nice write up. works on fixing idles or air jets on kadrons, can't remember which, as you must wedge somethign into them to pull them from the carb body as tey are "permanently' installed. once I have some more cash I am going to get a reamer/guage set as I don't trust my jets, or anythign that i didn't make myself honestly (except DGs extinguisher mounts, those things are BA) aanyways, I plan to go through my huge bin of solex jets and wrap them in masking tape and label the sizes. i know if i awl someting on the side i will end out reaming it again later.
this is the trick i plan to use to size down my 180s and bigger solex jets. honestly where did these come from? I think i have something in a 320. hotrodsurplus didn't belive me, and I forgot all aobut them till someone came by my house with a limping empi carb that needed some love and I turned them up in the bottom of my bin.
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Re: HOW TO - resize carburetor jets

Post by bratjie » Fri Oct 20, 2017 12:43 am

Hi, I know this is an old thread but just a question if I may, I have 180 main jets on 2 x 40 DRLA Dellortos. Now I would assume that the hole or reamer for 180 was a 1.8mm reamer. If I need to go and play to say 200 or 220 I could use a 2mm drillbit or 2.2mm drill bit, is this correct?

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