Jet location clarification

Ok..i have a Gurtner carb and im not exactly sure where the jet is. It's down to two parts on the carb. The first is a screw that comes off from the outside which has a hole running through it and two holes on the side. It has a little 24 on it. the 24 makes me think this is a jet. The other thing is right where the fuel joins the air to mix. it's a little brass nipple with a small hole in it. I'm not sure how i could get it out (small needle nosed pliers wont fit inside) though i didnt try all that hard to remove it. Which of these might be my jet? Also, why is it that it's so bad to drill a jet larger? i always see it mentioned that one should never drill a jet to make it larger.. any particular reason for this? thnx.

RE: Jet location clarification

Ron Brown /


The thing with the # 24 on it is a jet. You do not say what your machine is but my Motobecane uses a # 54 main.

If you look in the top of the jet, opposite the hex part, you will see a fairly big hole drilled part way then a tiny hole into the section with the hole drilled sideways. The tiny hole is the jet orifice and is the hole described by the number on the jet. It was made with a drill and therefore can be drilled bigger, but why do you want to do this?


RE: Jet location clarification

Yeah but Ron... the one with the #24 on he said was screwed in from the outside.

Now I have never heard of a "Gurtner" carb....

But the jet you want to concentrate on is the MAIN jet... and that is INSIDE the float bowl on the bottom of the carb (on any carb I have ever seen).

You must remove the carb... remove the float bowl... then in the middle of the carb you will see a little brass "nut" ...which is the main jet... it is probably screwed into the bottom of the part you were trying to grab with your needlenose pliers.

And the reason it is not a very good idea to try to drill it is these holes are very tiny

If you go to a hardware store the smallest drill they have (usually a #60 (I think)) is actually quite a bit bigger than the one in most peds.

So you pretty much have to go but a set of special small drill's..and these are usually too small to even chuck up in an electric drill... you have to do them in a "pin vise" and hand turn them...(no problem..brass is soft)...even then..the range of sizes may not be fine enough.

The reason why people say "don't do it a typical guy goes and drills the hole almost twice as big (dia)...which allows almost 4 times as much fuel....TOO MUCH... (for gas)...alcohol? will have to do the research...(I wouldn't do it...but that's me)

Oh...and it's kind of hard to "undrill" something....if you know what I mean

Maybe that's what Ron's "watchmaker bushing tool" is for?

To figure Area of a hole ...Area = (Dia./ 2) (squared) x pi

...............jet of .020 dia / 2 = .010 (squared) = .000100 x pi = .0003142 sq in

...............jet of .030 dia / 2 = .015 (squared) = .000225 x pi = .0007069 sq in

........................... or 2.25 times as much fuel with only .010 diff in dia.

RE: Jet location clarification

Ok. Well thats what i thought it must be. the one screwed in from the outside didnt look like it was small enough to really regulate fuel control. Well now i suppose i should go and try and get it out to see what size it is so that i can order a few more to experiment with..


RE: Jet location clarification

Ron Brown /


The Gurtner carb is the one used on my Motobecanes. The main jet is in fact external.

A picture would be easier but here goes. The jet is a length od 8mm hex brass rod, this is turned down leaving enough hex to put a wrench on. The hex has a tapered shoulder on the top edge to seat in the carb body and is slotted on the bottom for a screwdriver. The top end of the jet has threads to screw it into the carb body directly below the diffuser. Between the hex and the threaded part, the jet is turned down somewhat to allow gas flow around it. There is a hole drilled through this area at a right angle to the jet axis to allow gas to enter the jet body. The jet is drilled from the bottom and top, leaving a short, solid section within the threaded portion. This is the area which is drilled to form the jet orifice. The bottom of the jet is plugged with a ball bearing leaving a cavity between the intake holes and the ball as a "crud" sump.

The major advantage of this, seems to be that with a screwdriver, you can remove the jet on the road and blow it out if it gets plugged. It worked for me anyway.

As for drilling, my "why would you want to" comment was supposed to dissuade him, especially after he had seen how small the jet orifice realy was. Your caution is well advised, I have a set of number drills from # 60 to #80 and I think that one drill size is greater than one jet size.

Yes, I suppose bushing is like un-drilling. The bushing is really cheap but the tool is expensive!


External main jet.

Boy... that is interesting... I had never heard of that.

It sure would make it easier to clean out that jet.

To get to the main jet on my PA-50 requires that you separate the frame from the engine...then take off the carb.....big pain!....but I've done it enough that I can do it in 45 min now.

RE: Why you shouldn't drill

Because the jets are bored, not drilled. These are precision holes both in diameter as well as suface finish.

Using a common drill bit will give the passage a rough finish that will not promote the proper vaporization of the fuel. There are such things as carburetor jet drills, but they are very expensive. It's not something you chuck up in the Black & Decker VSR and attack the jet you've clamped in a vise. You would have to use a lathe to ensure an accurate job.


RE: Why you shouldn't drill

Ron Brown /


While I agree that in general, drilling a jet is not to be recommended, when you get down to the # 60s and 70s drills, the finish is as good as the quality of the drill bit. Cheap drills are hard to find in these sizes. Boring bars are impossible to find.

The main jet restricts only the flow of gas. Emulsification and atomization take place later, in that order.

As for drilling vs. boring jets in general, if you look at any 3 idle jets from a Honda 4 cylinder with Keihin carbs, the jet orifice, which is counterbored into the body of the jet much like the Gertner Main jet, you will find at least one of them is drilled at an angle such that the hole in the jet looks significantly oval. Given the distance from the end of the jet to the drilled orifice, only a drill bit could flex enough to do this. By the way, the jet still works fine in terms of metering gas.

Sometimes, you have to do what you have to do. You just have to do it carefully and let someone else do it if you do not think you are able.


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