Author Topic: drilling out main jets  (Read 10093 times)

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Offline hymodyne

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drilling out main jets
« on: August 31, 2005, 07:25:23 am »
a converstion with a fellow at morotcyclecarbs.com  points me in the direction of drilling out my main jets to address a bike that is runnign lean. Is that something I can do myself with metric bits, .40-.45.?

hym
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eldar

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Re: drilling out main jets
« Reply #1 on: August 31, 2005, 01:23:35 pm »
Well you could do it I suppose but you would want a very fine bit otherwise you may end up with burrs. I guess I would not drill a jet. Change clip positions if you can or get larger jets. If you drill and they dont work, then you are stuck getting new jets anyways. You may es well keep what you have in good shape. Did you put on pods or different exhaust?

Buffo

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Re: drilling out main jets
« Reply #2 on: August 31, 2005, 01:52:53 pm »
I dont like the idea of drilling jets. I almost had to but thanks to Cville, 2tired and motorcyclecarbs.com I was able to avoid that detail.  jets are not that expensive. but you can always try...if it doesnt work you would have had to buy new jets just the same.

BTEAS

Offline MikeDeB

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Re: drilling out main jets
« Reply #3 on: August 31, 2005, 02:14:07 pm »
There is nothing wrong with drilling jets.  I've done it a number of times and it's worked out great.  Use a quality steel bit and a steady hand.  Drilling the soft brass jets generally won't leave burrs but what you have to avoid is making the holes egg shaped (elongated).  One pass with the drill is all that is needed.  Keihin jet sizes are in metric diameter of the orifice.  For, let's say, a 135 main, you'd use a bit that measures as close to .135mm as you can.  I recently did a friends CB750 and we used a bit that measured .13493 using digital calipers.  For mains you can use a power drill with the jet mounted in an old carb body (slide & needle removed) braced on the bench or in a vice.  I've used a PanaVise with nylon jaws to hold the jets with great success.  For pilots, use a pin vise to hold the bit.  The holes are too small to use a power drill and you run the risk of elongating the holes.  You can use 3in1 oil as a cutting fluid but the brass is very soft and drills cleanly without it.  Take a set of digital calipers to your local hobby shop and you should be able to find a number of drill sizes that will work great for drilling jets.
Mike (Old SOHC/4 #2641)
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Offline Noel

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Re: drilling out main jets
« Reply #4 on: May 31, 2006, 08:42:45 pm »
Old thread, I know, but I just wanted to ask a question and add my two cents.

The question is "What's wrong with drilling jets?". I was under the impression that some jet kits for modern bikes include a drill bit for just that purpose.

The two cents is that I just finished drilling out the mains on my 500. Used a #57 twist drill, chucked into a vice, with the jet twisted down onto it. The jets came out with perfect mirror-finished holes and the bike runs very well. After some riding and plug chops, I may well go to the #56. (57 ends up being about 109, 56 about 118.)

Moreover, I haven't the slightest idea where to find Keihin mains in other than the stock size. None of the usual sources were helpful, but the drill bits were readily available for a few bucks down at the local CrownAce. Even if the jets had been available, I'd be in the hole for 20, 30, 40 bucks, rather than three.

So again, what's the downfall of drilling them yourself?
'73 CB500

cb500 maniac

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Re: drilling out main jets
« Reply #5 on: May 31, 2006, 11:36:03 pm »
im with silicone on this one. speaking as a machinist, if you were to open up the diameter of a jet, you should get yourself a lathe, chuck it up and remove unwanted material with a strait-flute reamer. the mental image of some yokle with a makita screw gun drilling out something as precision as a carb jet is disturbing to me. for one: twist drills are not as accurate as you think, sure they work good for drilling out the cavity in your mom's last tooth, but they dont cut perfectly on center and can only be trusted within a few thousandths of an inch to a sixty-fourth depending on the diameter of the drill. B: twist drills dont leave a nice surface finish, and a drill that small going thru brass, to obtain a decent finish you would have to turn it insanely fast, upwards of 4000 rpm. does your DeWalt do that? i think not. If you MUST practice home-dentistry on your carb jets, use kerosene as a cutting fluid, brass does not like to get hot, and thicker oils do not evaporate off and dont posess the cooling properties that kerosene does. Im switching over to a header and pods this weekend, and i will be buying the recomended size jets (As soon as i find out what those are) for my 500-four. as i've said to so many people, so many times,  "someone who went to school a lot longer than you said to do it this way, and the owner of the company agreed, why dont you?"  here's to putting our faith in engineers and leaving the duct-tape and safety-pins to MacGuyver!

Offline TwoTired

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Re: drilling out main jets
« Reply #6 on: May 31, 2006, 11:51:50 pm »
Twist drills do not make round holes.  And, they often push metal rather than cutting it, if not maintained sharp.  Have you never deburred a hole?  Where did the burrs come from?  Also, twist drills make holes bigger than their shank size.  Round holes are made with reamers or are cast, possibly stamped in a die, as such.

The jet orifice is not simply a hole size.  If you use magnification, you will see that the jet is ramped both into and out of the restriction orifice, and the orifice wall is rounded, not flat.  The reason is that these forms help to keep the fluid flow in a laminar, non-turbulent manner over a wide range of fluid speeds.  Stepped orifice walls will introduce turbulence, often in a modal fashion related to fluid speed or the pressure difference between the orifice inlet and outlet.

You've probably seen this effect with a garden hose nozzle, where at low pressure the outgoing stream is solid, but when the pressure is increased, an air pocket begins to appear at it's output.  When the pressure is increased more, the solid stream begins to flail about wildly.

Drilled jets often flow LESS at holes sizes larger than smaller sized properly formed orifices, when subjected to high flow rates.

Precision, well formed, holes are seldom achieved with $3 tooling, unless you are not measuring, or not looking very closely.

It is possible to use jet reamers and jet gauges to get much better precision when modifying jet orifices.  Still, these may not flow as well as properly formed jets.  But, there may be no alternative if a suitable source can't be obtained.

I wrote the above before there was prior response.
Remember, you did ask.  And, I am not simply trying to get on your (or anybody's) case, or, deliberately annoy you.  FWIW
Cheers,

Lloyd... (SOHC4 #11 Original Mail List)
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Offline Lumbee

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Re: drilling out main jets
« Reply #7 on: June 01, 2006, 07:21:48 am »
...OK, I'll come to Noels defense here.  I think you guys overestimate the sensitivity of the carbs on these bikes to how perfectly round the hole in a jet is.  I've drilled out jets before (with my craftsman hand drill no less!), and the bike ran OK with them.  In the interest of full disclosure I did not run the bike long with those jets as I was trying different jet sizes and settled on another jet size that I did not dirll myself.

Correct me if I'm wrong, but all the jet hole does is get the gas from the jet to the emulsiton tube (is that what is called??) where the "spray" of fuel is actually created.  So its hard for me to see how a jet thats not perfectly round is going to have a profound effect of the end result which is a proper air fuel mixture.

...let the flogging begin...  :-\
« Last Edit: June 01, 2006, 07:23:23 am by The great "Lumbee" »
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Offline dusterdude

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Re: drilling out main jets
« Reply #8 on: June 01, 2006, 08:55:19 am »
Well son, now that you've gone ahead and broken regulations, and pissed on 30 plus years of high paid technicians in large corporate buildings that spent innumberable hours perfecting the specific drilling and cooling techniques required to produce a spec on jet diameter, now's not the time to ask what's wrong with it.

 When gasoline contaminants acrrue at thrice the rate in your stepped out jets and she coughs and sputters sending you lurching over the side of a cliff, don't blame the honda engineers who did everything they could to save your life before you defied them and "busted out" on your own dangerously deadly road.

I believe the one young brazen daredevil mentioned a steady hand concerning non regulation unaccountable self modification jet drilling of said two wheeled motor operative vehicle.
Well, there's only one thing dumber than a wobbly drill press in a backyard hacking a prsitine stock jet to pieces, and that's some cowboy giving it a hand drill job after sucking down 3 cups of black jitters java in the morning.

Well I think that's enough to let the lesson sink in.
damn,i fell out of my chair and broke my ass.man it hurts :D
mark
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Offline dusterdude

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Re: drilling out main jets
« Reply #9 on: June 01, 2006, 08:56:14 am »
also,if anyone is interested,snap-on sells a tiny drill bit kit that is perfect for jets,been there,done that myself many moons ago.
mark
1972 k1 750
1949 fl panhead
1 1/2 gl1100 goldwings
1998 cbr600 f3

Offline Steve F

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Re: drilling out main jets
« Reply #10 on: June 01, 2006, 09:39:06 am »
"...........For, let's say, a 135 main, you'd use a bit that measures as close to .135mm as you can........."
Actually (not to be picky or anything like that), a #135 jet is 1.35mm (or .0532") a #100 is 1.00mm (or .0394").  ;)

Offline crazypj

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Re: drilling out main jets
« Reply #11 on: June 01, 2006, 11:28:13 am »
I probably mentioned (several times?) I was a precision machinist at one time. How exactly does everyone think that jets are made? I dont have details but I'm pretty sure its done on an automatic feed lathe loaded with bar stock, not even CNC or anything. probably less than 1 in every thousand gets checked for flow accuracy and even that has certain design parameters (i.e. +/- percentage) I have drilled jets, reamed jets and made jets, carbide drills are accurate enough. If you have EPA issues then you pretty much cant do ANY modifications from stock, the fines range from $2,500 for individuals to $10,000 for companies (Bike shops).
Yamaha has just paid out about $16,000,000.00 of a $24,000,000.00 lawsuit. If you check the finish with a 10x or 12x magnifier, you'll see hand 'drilled' jets really are'nt inferior to mass produced ones.
BTW, K&L Supplies now lists Honda push in jets so your local bike shop can get them for about $2.50~$3.00 retail ( as long as they deal with K&L ;D)
PJ
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Re: drilling out main jets
« Reply #12 on: June 01, 2006, 12:52:33 pm »
I drilled out the jets to compensate for pods and didn't have any problems.  All you need is a simple hand drill (looks about like a pencil) and it takes only about 5 seconds.  Stay away from the makita!  The main problem I had was finding metric bits.  There are all sorts of precision bits for little screw sizes like #18, etc.  Just find a conversion chart.  Jets are cheap, but this method can get you in the ballpark or even get you buy just fine.

Raising the needles is a pain and it won't solve a fuel limitation; it just alters the time you get it.

Offline Lumbee

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Re: drilling out main jets
« Reply #13 on: June 01, 2006, 12:57:23 pm »
Quote
Just find a conversion chart.

I keep this one bookmarked...

http://mdmetric.com/tech/cvtcht.htm
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Offline Noel

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Re: drilling out main jets
« Reply #14 on: June 01, 2006, 05:45:38 pm »
TwoTired: Don't worry about offending me. I did ask, and I'm glad to have your well thought-out answer. Seeing all those downsides about imperfect holes and decreased flow rates and increased turbulence would make me pretty worried about what I did, if it weren't for the fact that my bike is running so nicely now.  ;)

At least now, if I ever drill out a jet and it doesn't work, I'll be able to tell myself why.
'73 CB500

Offline oldfart

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Re: drilling out main jets
« Reply #15 on: June 01, 2006, 06:02:02 pm »
For, let's say, a 135 main, you'd use a bit that measures as close to .135mm as you can.

I'll be the first to point out the obvious...your math is a bit off, or perhaps it was a slip off the keyboard.  :-)  A 135 main is 1.35mm, not .135mm.  What Lloyd says is true.  This thing about jet shaped (tapered) holes is what makes Keihin jets different from their Dynojet counterparts.  The Dynojet jets have the exact same number, and size, but a different angle of entry and exit, and some fellas (and I believe them) claim to be able to tell the difference.  Now I know we aren't talking about Dynojet stuff here, but the analogy is apt and relevant.  The fact is, there are two jetting systems.  Most of Honda's street bikes use the old fashioned diameter based system we have just described wherein the marking refers directly to the diameter of the hole.  However, since a change in diameter does not correspond directly to a change in flow (the ratio from memory is something like 4x, that is, doubling the hole size permits four times the flow, or something like that -- correct me if necessary), the diameter based system is not very handy for those machines that are going to get tweaked a lot, i.e. race bikes.  However, a more scientific system is employed in racing carbs and incidently in most Mikuni carbs, wherein the diameter is a flow rating, in say, ccs per minute.  Three things become apparent.  First, that doubling (for ease of thinking --I know no one would double their jet size) a flow-rated jet would in fact double its flow.  Second,that this is another way of saying that a mathematical change in this system is directly linked to a corresponding flow change.  And third, that a certain increment of change will no in any way be similar between the two systems.  To make things a little more interesting, the threads on the two jetting systems was purposely made different so as to discourage mixing the jets up.  However, I have seen unsuspecting (and not too observant) folks get them mixed up anyway as they will in fact mix if the threads are forced a little...

As to drilling jets, I don't think anyone yet has hit on the real problem.  Whenever a jet is drilled, PLEASE file off the number so the next guy isn't driven to drink trying to figure out what happened!  Of course, career technicians get in the habit of checking jets against a sizing wire just for good measure, and against themselves as well, but...

Take a look at the link below and you will see the jet kit most powersports techs use.  It was created for gas furnace work, but it works so very nice.  :-)  It's not really good practice however, and good techs drill jets only when they can't easilyget them. 

http://home.earthlink.net/~motorstuff/tools02.html
« Last Edit: June 01, 2006, 06:06:40 pm by oldfart »
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Offline MRieck

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Re: drilling out main jets
« Reply #16 on: June 01, 2006, 06:15:01 pm »
 Buggered slots can cause flow problems too...probably more than a drilled out jet. I've seen jets (and sparkplugs, oil pan drain bolts etc, etc)that appeared to be torqued down with a 3 foot breaker bar with a 6 foot length of pipe slide over it.
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Offline Bob Wessner

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Re: drilling out main jets
« Reply #17 on: June 01, 2006, 06:33:25 pm »
oldfart,

About a 1/3 of the way into your explanation I started getting dizzy. By the end,  I think I actaully understood it.  ;D

Thanks, from another old fart.

We'll all be someone else's PO some day.

Offline Bodain

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Re: drilling out main jets
« Reply #18 on: June 01, 2006, 09:07:35 pm »
<GRIN> Not sure why you would need a metric bit.

Well when I put the 550 back together it was obvious that it was starving for gas under full throttle. A run to Ace hardware picked up a bit.
Got home and drilled them. It is no longer starving for gas under full throttle.

If they are too small you will buy larger ones anyway. The way I see it. You got nothing to loose.
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Offline Noel

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Re: drilling out main jets
« Reply #19 on: June 01, 2006, 10:12:06 pm »
If I could find 110 mains for the 500 I might try them just to see if they're an improvement over my "homemade" 110s.

And without meaning to offend anyone -- I say this with a smile -- some of the conversation reminds me of the scientists who mathematically proved that the bumblebee cannot fly.

So you guys keep proving that drilled jets don't work. I'm going to go ride.  :D
'73 CB500

Offline Lumbee

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Re: drilling out main jets
« Reply #20 on: June 02, 2006, 05:51:00 am »
try this for jets for the 500 550's...

http://www.crc2onlinecatalog.com/misc_carb_parts.htm
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Offline crazypj

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Re: drilling out main jets
« Reply #21 on: June 02, 2006, 08:16:02 am »
Heres a scan from 1976 ZEUS Precision handbook. British people can get one from WH Smiths as its still in print ( although the price has changed from 0.65p to about £3.50) Still a lot cheaper than the Machinery Handbook at around $30.00+ for CD version. It has a great deal of other information on threads etc Its a 6"x3"x1/4" thick, so wouldnt cost much to mail around the world.
(I'm beginning to wish I had shares in some of these companies ;D)
PJ
« Last Edit: June 02, 2006, 08:20:39 am by crazypj »
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Offline Noel

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Re: drilling out main jets
« Reply #22 on: June 02, 2006, 06:06:56 pm »
Thanks,"Lumbee". They have exactly what I wanted. I'm going to keep drilling until my plugs look happy and then order a matching set of factory jets. 
« Last Edit: August 13, 2006, 10:05:50 am by Noel »
'73 CB500

Offline mmtsquid

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Re: drilling out main jets
« Reply #23 on: August 06, 2007, 06:07:35 am »
Where can I get the drill kit?
I've scoured home depot and lowes to no avail......
77 CB550K4

Offline Lumbee

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Re: drilling out main jets
« Reply #24 on: August 06, 2007, 06:10:18 am »
...harbor freight has a tiny drill bit set...
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