Honda-SOHC
SOHC.co.uk Forums => CB500/550 => Topic started by: TezzerTel on January 20, 2025, 07:59:55 PM
-
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?
-
If they are OEM Keihin, they will have the size stamped on the side of the jets Terry.
-
I believe 100 denotes 1mm bore, but could be wrong. Eg, a 40 idle jet denotes 0.4mm.
-
Yes I was thinking that 100 could be the size in hundreds of a millimeter
-
Might be difficult to measure the narrow orifice.
-
I have access to a fair bit of equipment 😉
-
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.
-
I have access to a fair bit of equipment 😉
Steady Roo😁
-
wasnt me! ;D ;D
-
That had real promise what with orifice, probes and equipment.
-
That had real promise what with orifice, probes and equipment.
Oooeer missus!
-
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.
-
Might be worth a PM to either Bryan or Ken, they might have a secret stash.
-
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%.
-
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.
-
Absolutely Murf.
It certainly demonstrates why a small change in jet size can have a significant effect on fuel flow and engine performance. Eg a reduction in jet size from say 100 to 98 results in approximately 4% reduction in fuel flow. Though I’m betting that the relationship between jet size and change in mixture is not completely linear. I guess it’s something developers establish empirically, thus making it a ‘mixture’ of science and art🫣.
TBH it must be a real headache as other factors relating to the general condition of the engine and carburettor package could have a greater impact on engine performance than a small change in jet size, hence the need for experimentation.
-
Have you tried NRP Manchester?
There is also an outfit in Canada…I can’t recall the name, but their website used have two impressive pairs🫣
-
Also Allen’s performance….
https://prod.allensperformance.co.uk/spares-and-settings/keihin-spare-parts/spare-parts.html
-
Canada one is sirius consolodated but they dont do genuine, they are the only place i have found the push in pilot jets for the pd carbs.
Shame is their website is totaly crap
-
I nearly placed an order with NRP as they have some 103 jets (just listed as made in Japan) but they have such terrible feed back of late that I didn’t bother. I will give Allen’s a call and see what they have. David Silver did have 100 keyester jets, if I go aftermarket then I will buy all four so they match.
I can then put my 2 100 and 2 115 jets that came with the bike in my hall of horrors, this bike has added more exhibits than the 500 Triumph did, which I had hoped was impossible. 😢😂