Drill bits and jet sizes

[dbeardslee]

When I installed the Mikes XS pods old Betsy developed an annoying flat spot between 2 and 3k. I'm thinkin' I need to go a little bigger on the pilots. I still have the stock pilots, and I was thinking about drilling them out rather than buying new ones. Anybody know what size drill bit to use to get a 47.5 pilot?

[TopCatGr58]

Hey DB,

Drilling them isn't very accurate, especially with the small size of the pilot vs. the mains! Not to mention causing irregular vortices from the drill bit marks in the brass, etc.!?

How many turns OUT are your pilot screws?? Have you tried turning them out a 1/2 turn each and retesting response, and then perhaps another 1/2 turn, repeat, etc.?! Been stated that don't need to go up in pilots until you are over 3 turns out on the pilot screw!
T.C.

[dbeardslee]

TC - I colortuned 'em and to be honest, I don't know exactly how many turns out they are. I pay more attention to the color in the combustion chamber. I do know they're nowhere near three turns out. If I start turning 'em out farther it'll be running rich and old Betsy likes to foul plugs when she runs rich. I can live with it the way it is, but I've got some old pilots so I thought I would give it a try and see what happens.

[dbeardslee]

I checked the thread Carburetor Jet Tips again unfortunately it only goes down to a #50 jet size. As I look at the table it looks like one drill bit size is roughly equivalent to one jet size. So I'm thinkin' it's a #77 bit that I need to use. Anyone know if that's correct for a 47.5 jet?

[red bandit]

A #77 drill is .018
47.5 jet is, if I calculated right .0187
45 jet is .0177 for comparision.

[dbeardslee]

Red - Thanks for the info. Sounds like the #77 will be 'close enough for government work'.

[planedick]

Drilling

Not likely you will get a hole that is .018 anyway. Most drilling gets a hole a little larger than the drill anyway.

[red bandit]

What Planedick said, drills may/will drill bigger, especially if you use an electric drill on that size drill bit. Use a pin vise or hold drill in vise grips and spin jet by hand with your fingers.

[dbeardslee]

I was thinking about drilling a hole in some 1" X 3/16" steel strapping that I have in my shop, tapping it for the jet, and clamping it to the table on my drill press. Should be able to get it perfectly perpendicular to the table and in line with the drill that way. Hopefully it's not some off the wall thread size.

That's one thing I like about the local Ace Hardware. They have a little board with a bunch of different size nut and bolt studs attached and labeled. I just keep trying them until I find the one that fits whatever I've got in my hand, read the size, and get what I need.

I had the same thought about holes normally being a little bigger than the drill bit which should work out pretty close to bang-on with the #77. Here's another question - When drilling out the pilots do I just need to drill the center hole, or do the holes in the side of the jet need to be drilled too?

[red bandit]

Just drill the hole in the center. Unless you have a VERY SMALL chuck on your drill press the chuck won't go small enough to hold that small of a drill bit. I would not use power tools to drill it. I would turn it in my fingers. A piece of computer paper is .004. If you are going from a 45 to 47.5 you are only taking out .001 that's .0005 on each side of the hole. These are my thoughts. You do what you want.

[dbeardslee]

Red - might just give that a try, although I usually save the easy stuff that works for last