Jet size

[TezzerTel]

I'm just looking at my carbs and with people saying that replacement jets can be very hit and miss I'm just wondering what size a 100 main jet should actually be so I can measure what I have?

[Nurse Julie]

If they are OEM Keihin, they will have the size stamped on the side of the jets Terry.

[Sesman]

I believe 100 denotes 1mm bore, but could be wrong. Eg, a 40 idle jet denotes 0.4mm.

[TezzerTel]

Yes I was thinking that 100 could be the size in hundreds of a millimeter

[McCabe-Thiele (Ted)]

Might be difficult to measure the narrow orifice.

[TezzerTel]

I have access to a fair bit of equipment 😉

[Sesman]

Get yersell some carb cleaning probes as these are size graded. As Ted has said, differentiating from say a 98 and 100 is pretty difficult, but it’s not impossible.

[Sesman]

I have access to a fair bit of equipment 😉

Steady Roo😁

[Laverdaroo]

wasnt me!

[Sesman]

That had real promise what with orifice, probes and equipment.

[taysidedragon]

That had real promise what with orifice, probes and equipment.

Oooeer missus!

[Murf]

I have been thinking and reading about carb jets because I am short of 2 #100 jets for my 022a 550 carbs. I have been trying to get 2 genuine ones so they will definitely match the 2 I have.  From what I read the basis for carb jet size number vary between manufacturers and can be based on any of the following,
Size of orifice in thousands of an inch,
Size of orifice in hundreds of a millimetre,
Area of the orifice
Amount of fluid that can flow through the orifice in set conditions.
   I am sure there are others as well.

I did see that as may be expected the shape of the entrance to and exit from the jet will alter the flow rate of the jet.    This could explain why many after market jets even with the correct orifice size don’t appear to work as well.

I also saw this about  Keihin carbs and I quote:-
 On Keihin carbs the jet# is the area of the hole, not the diameter. So a #60 jet has twice the area of a #30 jet. while the diameter is increased by 141.42%

I would like to think this is helpful,   Or does it just add more confusion.

For me I am just going to keep looking for some genuine jets, because they will turn up and in the meantime I will keep cursing the PO that lost them.

[McCabe-Thiele (Ted)]

Might be worth a PM to either Bryan or Ken, they might have a secret stash.

[Sesman]

I think you may be little confused over jet diameter and area. The discrepancy you perceive is simply due to the square law.eg A=pi r sg or indeed A=pi d/4.

Eg a 30 jet will have an area of 0.07069mm and a 60 jet will have an area of 0.28274.

So a 60 jet has twice the diameter of a 30 jet ie 100% increase in diameter, but an 4 x increase in area of 400%.

[Murf]

Thanks
 That tells  me it is very easy to get things very wrong trying to drill out your own jets. 😄

It’s a pity Keihin don’t still make the jets I need and a pity after market copiers can’t copy very well.