Pilot screw broken .....drill sizes

[Steveh]

I have 3 of 4 carbs with the tip of the air/fuel/idle adjustment screw broken off in the carb body...I have searched the forums for ways to get them out...I have tried most methods..freezing, heating, needles, picks, everything that sounded possible...
I decided to try and drill them out..* *A 1.5mm drill was smaller than the hole in the 'good' carby.* Used a pin vice and drilled through and brass tip is gone!* *Now I think the drill bit was to big and the 'good' carby had already been drilled before.

So...does anyone know what the size is of the passage where it meets the carbys throat?

Anyone cut a carby in half to show a 'section' where the tip sits normally? or a drawing with measurements

This fantastic guide....* * * http://www.xj4ever.com/inside%20your%20carbs.pdf * * .....shows on page 19 and 20 the pilot screw tip where it meets the carbys throat.....This is the hole I would like to know the size of.

The BS34 carb is very similar to these pics* *
Thanks for any help

[WMarshy]

If I had a gauge pin to slip down in the hole I could tell you very easilly because I have by carbs out and on the bench... unfortunately I dont have any pins.

Even if you knew the correct size of the hole the needle seats against, how would it make a difference? Cant exactly add material very easily to the ones that are already messed up...

If you can wait a little while I will be offering new idle screws for sale...

[BA80]

Pretty much any time you have to remove a broken tip the orifice will be a little larger. The screws are adjustable, any difference can be adjusted out if you properly adjust the idle screws.

That's exactly why I think counting turns is a lousy way to adjust them other than a base to start with. Unless you have brand new carb bodies, and even then, no two carbs are EXACTLY alike.

I've removed broken tips from all my carbs and my bikes have run great with proper adjustment.

The pin vise drill works best for me. I gauged the bit on a carb that didn't have a broken tip in it by fitting the bit in there.

[MarkD]

I had 2 broken off needles, and worked like a dog to try different ways. In the end, I took a common push pin used for bulletin boards( they come in all colors), and used pliers to squeeze it to press out the broken needle. I will try to post a pic of the type of pin I used.

[MarkD]

link to pic:

http://commons.wikimedia.org/wiki/File:Push_pins.jpg

[bikerphil]

That hole is smaller than a #60 drill, my guess would be more like 1/32" (67-68). I agree with others, if the hole is larger, it will still be tunable, just a bit more sensitive. I'm running a few drilled at 1/16" and I had to shorten the spring slightly. The mixture tip can also be filed to a different taper if need be. GL

[fredintoon]

- - - I decided to try and drill them out..* *A 1.5mm drill was smaller than the hole in the 'good' carby.* Used a pin vice and drilled through and brass tip is gone!* *Now I think the drill bit was to big and the 'good' carby had already been drilled before. - - -

Hi Steveh,
I read that the hole should be 1mm. The smallest drill bit that my drillpress chuck will grip is 1/16" (1.58mm) so that's what I used.
What's just as important as the drill size is re-drilling it dead centre.
I set my carb body up on the drillpress work platform and used a countersink bit to centre the chuck on the needle bore before re-drilling the hole.
Carb seemed to work OK with a new needle though it had to be turned in a lot further.
BTW, that's a very helpful link, thanks for posting it.