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UnicycleMode
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7,584 Posts
Never actually drilled jets, but I do know older guys that have, and they use a pin gauge to determine the ID of the jet in question and find a drill bit suited to the size up from it. Jets are very precise and it would take a very very good drill press or similar machining equipment. Come to think of it a lathe would work well for it to have a precise hole drilled correctly.




Something like this depending on the jet sizes:

http://www.ezbore.net/Pin-Gauge-Set-011-to-060-p/nog2011.htm
 

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lɐʇuǝɯᴉɹǝdx&#4
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3,517 Posts
I've never done this for carb jets, but I've drilled plenty of tiny holes for model building and jewelry.

I wouldn't mess with gauges and a lathe or whatever. If you have a full set of wire gauge bits, you can use the shank end to test for size, pick the next one up, and bore it by holding the bit in a pin vice. This set on amazon comes w the bits and pin vice, for less than $20. http://www.amazon.com/Mini-Micro-Drill-Vise-Chuck

In fact, I'd strongly suggest against using an sort of power tool with a bit that small- they snap easily, and unless the chuck centers precisely, the bit will wobble. That doesn't matter with a larger bit, but with those bits that look like wire, the wobble can be more than the diameter of the drill (part of why they snap). When I worked in a jewelry factory, they had us drilling silver with wire gauge drill bits using a small drill press, would snap one every 10 holes or so. It sucked. Drilling with a pin vice takes longer but is a much more controlled and sensitive process; it should be very easy to keep the bit on course through the original hole. It also minimizes the forces used; you can just hold the jet with your fingers on a phone book or other surface you don't mind drilling into, which means you won't mash the threads trying to clamp it. Hell, you could probably drill em without removing from the carb in some cases.
 

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Registered
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740 Posts
Dynojet numbers refer to the metric diameter of the orifice in. So a 150 is 1.5mm, 162 is 1.62mm, ect

Mikuni the number refers to cc of fuel flow per minute. This not only has to do with the orifice size, but also the way it tapers.

A general rule of thumb is to add 10 to the mikuni size to get a dynojet equivalent.

I wouldn't mess around trying to drill jets. It would be extremely difficult to get the precision needed, making tuning almost impossible. You'll end up chasing your tail so to speak since you don't know for sure what "size" you just made. Drilling also reduces or eliminates the taper, which can cause the fuel to mist out differently causing other effects. Not saying it can't be done, or won't work, but you're making it a lot harder on yourself to save a few bucks.
 

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Is my bike ok?
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15,025 Posts
Discussion Starter · #5 ·
Wow, thanks guys.

Rat, I never knew of pin gauges. :fu: for making want to spend money on a new tool.

Seb, pin vise was idea,too. I use one to hold guitar string for cleaning carb jets. Agreed by hand is probably th way to go.

Razed, had no clue about the taper. And since I working on Mikuni's right now you probably just saved my ass. Thanks man. And rep for the PM. HUGE help.
 

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lb/hp is what it's about!
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10,448 Posts
You probably shouldn't drill them out with a machine unless you can get that drill spinning around 6000rpm minimum and have no more than a couple tenths of runout. You could do it by hand in a pin vise but you might end up making the hole oversized since I doubt your hand has less runout than even your lathe. Also run a higher risk of breaking the drill in a pin vise because one little bump with your hand grabbing it the wrong way and it snaps.
 

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lɐʇuǝɯᴉɹǝdx&#4
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A pin vice is effectively free floating; runout doesn't apply. Even if you do manage to wobble enough to go oversize on the bore, the exit hole won't be oversize. You'd just have a bit of taper, metering should be good. I could see concerns re doing multiple carbs identically, but really, the variance would be quite small unless you totally ham fist the job.
 
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This was common back in the day & most of us older guys over here have done it more than once, I still don't think its the best option though for a number of reasons.

As RaZed says one is measured by flow & one by hole size, I actually thought it was the other way round to how he describes but hey ho the result is the same :)

Drilling DJ jets never works out well in my experience as a larger hole also reduces the length of the taper which is matched to the needle & can cause some odd issues, the same applies to Mikuni jets but to a much lesser degree DJ jets & stock needles or the other way round don't tend to give good results (there are exceptions)

With Mikuni jets & needles in most Mikuni carbs raising the needle one notch equates to half a size on the jet so 2 notches is roughly one size, you can raise the needle with a small washer if adjustable needles are not fitted

Jets holes are extremely smooth & I have yet to see anyone match stock for that no matter what they have tried, I'm happy to be proved wrong here :)

Drilling can be useful to help determine where you need to be with jetting & that is what I have done a number of times before but every time I have changed over to stock jets in the correct size the bike has run smoother, I now just keep a box full of jets in a range of common sizes & work with those

Just my opinions guys :)
 

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ShamanFighter
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22,340 Posts
its a very Dinosaur method of tuning but once you get the set up it can come in handy. I have personally Done it to my own machines but would never do it to customer retail work. Ive had great luck. I was Taught by a world renown Carb Genius. My Bike ran so good on the dyno it blew the shop owners mind. Almost a near Perfect A/F Mixture Id have to Go Way up in the Montanyas before Id Lean out. It ran a Better A/f Mixture than a Fuel Injected Ducati. Ill Dig in my Paperwork to see if I can find my Jetting Paperwork. Nowadays I just do it By the seat of my pants.
 
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Is my bike ok?
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Discussion Starter · #11 ·
Thanks for all the input guys.

There's no way I'd ever tackle drilling jets as a method of tuning carbs for other people. I am fitting some early GSXR carbs (Mikuni flat slides) to my '80 CB750 in my sig. Where to start with the jetting is a crap shoot at this point. I figured if I could get myself in the ball park by drilling jets it would save me tons of money rather than ordering $20-30 worth of jets possibly several times.

Carbs are carbs, but these are totally new animals to me and I have no idea what they like or how they react to changes. But hope to find out soon enough. :clown:
 
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Hey shiny I probably cant help much with the jetting you will need but do know a fair bit about the Mikuni carbs off old gsxr's etc so if you get stuck feel free to shout up, there are quite a few similar carbs all with slight differences.

You really need to know exactly which ones you are dealing with, this is usually my start point as gsxr, gsxf & bandit carbs get swapped around so much its easy to get in a mess buying bits for one set then finding out you have something different

Carb size, the year/model of bike & the model number off the side would be a good start to help id them if you don't already know 100% :)
 

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Is my bike ok?
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Discussion Starter · #13 ·
Actually, that would be fantastic since ai really don't know exactly what I've got. I just hung them on the bike and they started and revved beautifully. I was actually quite surprised how fast the bike started cold. It's never started that well with choke plates and accel. pumps! I keep seeing things like "BST36SS" ( or something like that....) , but I haven't found any numbers like this on mine. I'll take a peek and see what I can find. Thanks a ton for the offer. :)
 
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No worries, most but not all have a part number on the side just above the float bowl, that along with the size will give us a good start point to find out what you have, if there is no number we can work it out anyway it will just take a little longer :)
 
G

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I have had a quick dig around for you & an educated guess is.......

Most likely ......Bst34ss from an 87 gsxr750 stock on those is, 112.5 main jet, pilot jet 35 & the float height is 14.6 + or - 1mm .....plastic tops & side mounted choke suggests this is what you have to me

2nd guess ...... gsxr1100 from 86 to 88, stock is 130 main jets & 32.5 pilots on those, same float height, these are pretty much the same as the 750 carbs, but I know there was a change in float type somewhere in here

The only other possibility that comes to mind is Gsx1100F (katana) 112.5 main jet on these, there is a nagging doubt about these somewhere in the back of my head i'll have a think & search on them

Having a look at the pilot jets may give you a clue as they don't get changed too often, this is all US settings & sizes by the way, I can find UK stuff if needed

I hope that gives you a start shiny :)
 
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I am glad to be able to help :)

32.5 pilots suggests 86 to 88 gsxr1100 the needles would have been 5D29 & the clip in position 2 as stock
(needle 4C09 on the 87 gsxr750 but don't know a clip position)

110 mains means someone has been messing as there is nothing oil cooled Suzuki that uses those as stock & they are definitely NOT gsx1100F (katana) carbs
 

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Premium Member
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2,225 Posts
Given how precise carb jets are, I find it OK to drill jets to get an idea where you're going, but a good idea on multi cylinder bikes to ultimately buy jets in the right sizes once you know what those are. Otherwise you may find any one of your cylinders a size up where the drill slipped, for example.
 
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