Topic: Oriface Size

[Trav-i]

I was wondering if anyone knows what the actual decimal size is for a 115, and 120 main jet.  My brother-in-law and I are looking to drill out some jets we have to try to do a little more troubleshooting on his 750.  We are also looking to drill out our spare slow jets as well (possibly a 50 or higher).  This is not going to be a final fix we are just trying not to buy extra jets we don't need.  As soon as we find the right combination we will buy the proper jets. 

Any info would be really appreciated.

Travis

[CB500_k2]

It is my understanding that the jet size in mm in jet numerical jet size divided by 100, so a 120 is 1.2 mm.

[Trav-i]

Thanks for the tip didn't think of that.  Hopefully this will clear up his problems.

[Soos]

Mind you that only works for Keihin carbs.

Mikuni are different.
Probably most other mfg's of carbs use some arcane method of their orifice measurement designations.

[crazypj]

You can look up number drill sizes as well.
#56 is about 116.5 (1.165mm) It's a 3/64" drill bit (0.0469")
#57 is about 107.5 (even though its listed as 1.10mm)
  I have a metric micrometer reading to thousandths of mm, so I measured the actual drill bit, bit of a pain with numbers that small as thermal expansion changes measurements significantly ( this morning mic is 0.002mm 'off' because weather has changed and its several degrees cooler, Micrometer is set for 68deg F)
1.15mm = 0.0453"
1.20mm = 0.0472"
Still trying to find something 112.5, probably have to buy the jets as I can't find anything that size (even in jet drill kits)
 I just bought five of each, 1.10mm and 1.15mm drills and a 5 x 0.75 die as I'm going to make some jets with 'modified' flow patterns, the actual hole is going to be in similar position to Mikuni high flow jet with entry/exit modified
PJ

[Alan F.]

here's a decent chart, there are dozens of these around the web, if you search a while...

[crazypj]

The chart only gives inch sizes though, I measured the actual drills to get mm sizes

[TwoTired]

Um, you are aware that a drill bit makes a larger hole than the bit is.  If it didn't, you couldn't get the bit out of the hole.

Cheers,

[crazypj]

Thanks for pointing that out TT because I had forgotten
 Its one of those things your aware of but discount  (although I did buy the #57 hoping it would cut far enough over to get F/A mix right on CB378, maybe I'll try a HSS #57 drill or make a miniature reamer?  )
1.10mm High speed steel is 1.096mm (checked 5)
1.15mm High speed steel is 1.148mm (checked 5)
1.20mm High speed steel is 1.186mm
#57 Carbide is 'exactly' 1.075~1.080, I have two, mic'ed both (lists as 1.10mm)
#56 Carbide is exactly 1.165mm (probably cuts as if it was 117.5?)
 Anyone got a pair of  Kei-Hin  112.5 main jets (5 x 0.75mm screw size)
PJ

 Screwed up last night, Kei-Hin are 5 x 0.80mm pitch, Mikuni is 5 x0.75, I have some of them

[Soos]

I think I do. Might have them being used, but I don't think so.
MIGHT have keihin 112.5's, but more likely aftermarket copies.(bought 10 sets a while ago, 100-135)
I know I have 4 or 5 sets (used) from carbs, but I don't think any of those are 112.5's.
I'll check though.


You wouldn't happen to have any 85, to 100(except 90's) mains you would like to trade would you?



l8r

[crazypj]

I may have.
 Got a bunch of different sizes from around 72.5 up for Kei-hin carbs.
 Originally I had lists of the bikes they came out of but lost a lot of my notebooks about 10~12 yrs ago
 I have very little in the 105~115 range (probably because that's the sizes I was using) 
I'll check later.
PJ

[Trav-i]

Thanks for all the advice.  I also know that a drilled hole is anything but round especially when done by hand or with a drillpress.  That's why we are going to chuck them up in one of the lathes in his machine shop.  We are going to use an old emulsifier (i think that's spelled right) tube to hold them while we run a bit through.  Still not perfect but should at least give feedback on his bike.  Then we will get the rights parts for a final fix.

Thanks

[Soos]

Sorry PJ...
All I have in the 105-115 range is 110 and 115's.

Not affiliated, just like 'em. Here is a link to Z1, 1.08 per jet.
http://www.z1enterprises.com/SearchResult.aspx?All=True&KeyWords=jets%20keihin


l8r

[Cvillechopper]

Don't be surprised if the result with nice parts is significantly different that with the drilled jets.  The tiny imperfections (no matter how careful you are) will make the air/gas flow properties very different from that of a smooth bore. 
That said, it will certainly get you close.  I've drilled jets out using a cordless, wood bits, and a vice and they worked enough to get by until the ordered jets came in.

Good luck

[crazypj]

I wouldn't worry about the holes being inaccurate, how do you think they make them in the first place?
 Difference is, unless you get high tolerance jets (which I think may only be available from Mikuni?) they are made on a production line by the thousand. ( I mnain difference with high tolerance jets is they are actually checked for flow- at least 1 in every 500)
You will probably have more accurate jets drilling them yourself with a new drill bit. I've looked at high tolerance jets and jets I've drilled out under same magnification and the surface finish is pretty much the same (high polished, no machine marks @x10 magnification) High tolerance jets are the ones with computer style 'square' numbers.
 I've seen some really nasty aftermarket jets, holes off center, burrs, not round, etc
 Looks like I'll have to go to Sudco for 112.5's,
 I only have one of each of the following,small head screw in main jets #100, #102, #105, and two #90, but, it does mean Honda use them in something. Will probably have to do some searching as it could be almost any (British) Honda from late 70's to mid 90's, probably in the 125 to 200 cc single (the #102 is from an XL100 or XL185, both use same jet and I modified the hell out of both bikes at one time)
PJ

[TwoTired]

I wouldn't worry about the holes being inaccurate, how do you think they make them in the first place?

I've closely examined the stock Honda supplied jets.  There are no tool marks from a twist drill or mill.  This leads me to believe they were form molded in a press or punched.  This might explain why they are so brittle (anybody snapped one off?)  The  metal is work hardened in the process of force forming.

It also would explain why the orifice is ramped and rounded at the orifice diameter.  The lack of "squared" edges at the orifice promote laminar flow without adding air bubbles into the fuel stream.  Such complex shapes are better formed with a die, rather than deal with wear factors of a complex cutting head of this shape, which would be far more costly to maintain in a precise tolerance.

But, I'm just guessing...

Cheers,

[crazypj]

Hi TT,
The majority of round head main jets certainly look like sintered material (although some are obviously 'billet' and still marked with the Kei-Hin stylised 'K')
Several I've checked do show tool marks, doesn't matter if they are matte or shiny (and not because I drilled them out  )
 They have a cut thread and what really looks like  a drilled pilot hole, the jet size looks drilled to me but it is probably burnished somehow?
Brass is pretty brittle when you get to small diameters, particularly when there is a hole drilled in it.
Of course, we don't know what grade brass it is either.
Powdered metal technology would tend to maker them slightly more brittle though.
 Good points, got a great high tech discussion going, don't know if its too much information?
 Does anyone know for certain how the flow is measured?
 I believe (at least for Mikuni jets) its a 20" head and flow in cc/min. (I think it may have been 500mm but a non exact conversion was used?)
 Read it somewhere in the 70's in some technical paper but have never seen anything since
PJ

[TwoTired]

Hi PJ,

I read somewhere that Mikuni jets were all flow tested and given a number corresponding to the flow.  Keihin jets were stamped with their orifice diameter.
I've no experience with Mikuni, however.

About the jets you found tool marks on.  Were these Honda distributed?  Or, ones found in stock unmolested carbs?

The 500/550 mains don't have any threads on the earlier style (non-PD) carbs.
I'd worry more about mains fuel flow than idle fuel flow.  I expect drilling slow jets is not so significant than when the mains are delivering at WOT.

However, if you aren't on the DYNO or at the track, it's all about looks, anyway.

Cheers,

[crazypj]

Hi TT,
The screw in jets are from stock carbs, but, some are probably from the early/mid 70's ('sandcast' vintage jets  ) They do show tooling marks on the larger diameter before the jet 'proper' begins. It looks like a stock blank was made then finish drilled later (which seems sensible, run a few million blanks then drill to size and 'stamp' numbers)
 I can only find a couple of genuine 550 jets (which is strange as I modified quite a few)
 They don't look like sintered type though, but they are vastly superior to the two aftermarket jets I never used ( because the holes were so far off center)
 I did find a set of Hitachi jets for an XS750 while rummaging around, they are a bit weird, nothing else fits Hitachi carbs
PJ