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Drilling Out Main Jets

  • Thread starter Thread starter kbruce6057
  • Start date Start date
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kbruce6057

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Hello, rebuilding carbs, is it possible to drill out the main jets to 122.5? As you might guess I am running exhaust and filter mods. Just wondering if anyone has done it?
 
Jets are cheap. Don't screw around drilling them. I may have a set of 122.5s you can have.
 
kbruce6057 said:
Hello, rebuilding carbs, is it possible to drill out the main jets to 122.5? As you might guess I am running exhaust and filter mods. Just wondering if anyone has done it?
Click to expand...

Yes, it can be done but is not recommended unless you have plenty of past jetting experience. Most here will suggest you use a Dyna Jet kit for a very good reason. There is no guess work required and only minor fine tuning required.
You need to tell us what engine/carb combination you are running!
You also need to be comparing "apples with apples" when quoting jet numbers, either flow rate or hole diameter.
 
What he said. The most important part of a jet kit are the slotted needles. Just throwing in bigger mains won't help much, the rest of the rev range needs tuning too. You might be able to shim the needles up a little but get ready to pull the carbs lots of times to get it anywhere in the ballpark.
 
How would you know what diameter drill bit to use?
Drill bits don't come calibrated in flow rates.

You might get one "a little bit bigger" but what good is that?
 
I've drilled jets before, back when I was a poor student. Yea I know, it's not recommended but who cares? If you have a full drill kit, with both numbered and lettered sizes, it's a simple matter to go just a smig over where the orifice is currently. Slam the jet back in and go do some plug chops. I've even drilled pilots using a small pin as a drill bit; measure the diameter with a micrometer and work it in there. Again, not recommended but not much to loose. If it doesn't work, get some new jets.
 
I drill my own jets because I have to. You would need to buy a drill kit and make a fixture for them.
I would call Sudco and be done with it.
 
Drilling jets will give you a starting point for tuning, but the quality will be suspect. The easiest way to illustrate this - take a set of factory Mikuni or Keihin jets and squirt WD40 through the jet (with the little red tube). Then do the same with a home drilled jets. The cone of fluid will be consistent with the factory jets Repeat with all the factory jets and you'll understand why you don't want to drill jets.
 
49er said:
Yes, it can be done but is not recommended unless you have plenty of past jetting experience. Most here will suggest you use a Dyna Jet kit for a very good reason. There is no guess work required and only minor fine tuning required.
You need to tell us what engine/carb combination you are running!
You also need to be comparing "apples with apples" when quoting jet numbers, either flow rate or hole diameter.
Click to expand...

I have 1982 gs850 with cv carbs, vance & hines 4 into 1 and k&n pods.

Steve
 
Billy Ricks said:
What he said. The most important part of a jet kit are the slotted needles. Just throwing in bigger mains won't help much, the rest of the rev range needs tuning too. You might be able to shim the needles up a little but get ready to pull the carbs lots of times to get it anywhere in the ballpark.
Click to expand...

So if I can drill out the jets, all I need is the slotted needles? Can you buy them seperately? I just can't see $122 for a jet kit. How do you shim up the needles? I've got more time than money.

Thanks,
Steve
 
kbruce6057 said:
So if I can drill out the jets, all I need is the slotted needles? Can you buy them seperately? I just can't see $122 for a jet kit. How do you shim up the needles? I've got more time than money.

Thanks,
Steve
Click to expand...


The needle height is controlled by the thickness of the spacer on TOP of the needle clip. From the factory there is a plastic spacer, take it off and replace with a small stack of washers about 1/2 as thick. This is a good starting point from which to tune. Remove additional washers to raise the needle further.

Good luck.
 
kbruce6057 said:
I have 1982 gs850 with cv carbs, vance & hines 4 into 1 and k&n pods.

Steve
Click to expand...

Now you need a response from another member who has used this combination of mods, or somthing quite similar. They may supply the correct DJ kit # and the suggested needle positions.

You could end up spending far more money than the $122 cost of the jet kit, say on a good range of ACCURATELY sized drills, or worse still some holed pistons.

I have a comprehensive jet drill set. Each replacement drill I purchase is measured to varify the accuracy of its size. It is quite common for these smaller drills to be slightly undersized/oversized than what they are sold as!
Correctly sized drills can also be used to confirm that a PO hasn't previously drilled out a jet.
Even new OEM jets have been found to vary in size from what they are marked as.
 
gday 49er, im trying to work out, when you remove the washers, from the needle, in the slide , doesnt that lean the mixture as it drops the needle further into the main jet?, regards.
 
mac1170 said:
gday 49er, im trying to work out, when you remove the washers, from the needle, in the slide , doesnt that lean the mixture as it drops the needle further into the main jet?, regards.
Click to expand...

Giday Mac, no it will richen the mixture by raising the needle. IMO, you shouldn't run the needle without the nylon washer as it acts as a damping mechanism when the throttle is released suddenly, especially on manual slide carbs. Cheers.
 
Last edited:
There is a spring under the needle that always holds upwards force on the needle. There is also a piece that goes on top of the needle that the needle gets pushed up against from the force of the spring underneath. If you add washers to the top of the needle the needle will be lowered (and the spring will be more compressed) because the needle hits the piece that goes on top of the needle at a lower position because of the extra washer on it. I hope that makes some sense..

Nick
 
Nicholaschase29 said:
There is a spring under the needle that always holds upwards force on the needle. There is also a piece that goes on top of the needle that the needle gets pushed up against from the force of the spring underneath. If you add washers to the top of the needle the needle will be lowered (and the spring will be more compressed) because the needle hits the piece that goes on top of the needle at a lower position because of the extra washer on it. I hope that makes some sense..

Nick
Click to expand...

Thanks for the clarification Nick.
On the manual slide carbs (VM's) there should be 2 nylon/plastic washers. From memory the thicker one, about 2mm thick ,sits between the clip and the slide. The other thinner one sits on top of the clip and is closest to the positioning/tensioning spring. If you remove or fit a thinner washer between the clip and the slide, the needle will be lowered deeper into the jet, causing a leaner condition.
There is some damping done by the washes as the top one causes a sideways/offcenter condition at the needle when the throttle/slides are fully open. This is designed to reduce wear on the needle and jet surfaces during normal operation.
Ian
 
all I say is if you drill any jet . sand the original number off of it .

that may help a fellow mechanic after the bike is sold a time or two and the mod has been forgotten.
 
Why bother drilling. There are some good supplier very close to you. Just go to www.sudco.com or www.psep.biz or my favorite www.carbparts.com to get everything you need plus the technical support is invaluable.
I used to drill jets until I found these guys, now I have complete EBC jet selection boxes just like a dealer has. It's amazing how cheap these selection boxes are compared to what the dealers get for a single jet or needle.
 
trippivot said:
all I say is if you drill any jet . sand the original number off of it .

that may help a fellow mechanic after the bike is sold a time or two and the mod has been forgotten.
Click to expand...

I agree and do just that.

As for buying an assortment of jets instead of drilling, there is a good case for that too, if you're tuning lots of different combinations on your machine, or are tuning many others.

A comprehensive set of drills allows you work on any carburettor. You can confirm that a 102 main jet does in fact have a 1.02mm oraface or any other size for that matter. Having confirmed that, you have a starting point for future tuning, especially if you have just bought a new to you GS.

I have drilled mains on Dellorto carbs only to find that I was now too rich. I then soldered the hole up and re-drilled to an intermediate size and tried again. Once the right jet size was found, I would buy an OEM set for competion. Delortto's have push on main and air correction jets that attach to each end of emulsion tubes.

For my last carb rebuild, I bought 4 GS 750 K&L kits. I was impressed with the apparent quality until I measured the replacement mains. They were all stamped with 102. Two were correct the other two were 0.98mm. If I had fitted these to a stock 750, I would have wondered why two pots weren't performing correctly, at mid to top end.
 
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