Carburetor jet size

[billh]

The main jet size on this Kelvin carb is marked size "160"  Anyone know what the units of size : thous, tenths of thous, mm this figure relates to? The jet appears to be between  0,5 and 1mm ( not measured). I suspect the jet is worn and therefore I'm considering making  a new replacement but only if I can resolve "160" to a real life measurement. Any ideas? Thanks in advance.

[BEngo]

1 hour ago, billh said:

The main jet size on this Kelvin carb is marked size "160"  Anyone know what the units of size : thous, tenths of thous, mm this figure relates to? The jet appears to be between  0,5 and 1mm ( not measured). I suspect the jet is worn and therefore I'm considering making  a new replacement but only if I can resolve "160" to a real life measurement. Any ideas? Thanks in advance.

  Been there, have that T shirt🙂

 

There is a chart showing jet numbers and sizes on the internet. It took a lot of finding.  One of the carburettor specialists  sites IIRC .  My copy is not in phone on boat but I will dig it out when I get back home ( next week). The chart matches correctly for a size 130, (as measured with pin gauges and cross checked against a drill) fitted to my  model J.  There is no logical equivalence indicated, such as 160 =1.6mm  or 130 = 0.13 in.  IIRC a 130 equates to a number 55 drill.

I have not made a spare one yet because I have not found a source of a short straight piece of 1/8 in brass tube to make the rest of the jet.

 

You could always make one a handful of drill  sizes smaller than the existing and go larger drill-step by step till it runs nicely.  The J will run,  albeit much  more slowly than 'normal'   on  a very rich mixture, but as it runs out of petrol it speeds up.  In my experience they fire the priming but don't run when the mixture is too weak.

 

N

 

[BEngo]

https://www.allensperformance.co.uk/carb-kit/jet-size-chart/JetSizes.pdf

 

See also pdf attached.

 

NJetSizes.pdf

 

 

[Tracy D'arth]

Allen's chart gives 160 as 1.094mm diameter.

 

[billh]

Thanks both for the information. Here's a link to a cut away carb drawing:

http://www.sky-net.org.uk/kelvin/petrol/ricardo/man/index.html

Useless fact: The G4 (60HP) engine had 4 carburetors ,1 per cylinder, trying to balance that lot would be a bit of a job😄, I remember struggling with twin SUs on  Austin A110 and Jag XJ6. I don't know if any G4s still exist?

[MtB]

3 hours ago, Tracy D'arth said:

Allen's chart gives 160 as 1.094mm diameter.

 

 

Dammit I've just broken the last of my drills that diameter...

 

 

Point being, I suspect the tolerances on the holes in jets is far finer than a drilled hole would be. 

[Tracy D'arth]

Normally you turn  a peg that diameter and ream the jet until it is an interference fit on the peg.

[BEngo]

MtB  is right. Small drills tend to be oversize.  Normally you drill the hole a bit small and then ferkle it to size with a jet reamer.  "Size" being determined by a jet gauge, aka a pin gauge which, as Tracy suggests, you could make.

 

That said it does no matter too much.  A petrol engine will run and produce rated power on a quite wide range of mixtures.  If it is a bit rich/lean because of  tolerances in the jet then it only matters if fuel economy is vital, when you need to be running bang on stoichimetric, to use all the available fuel, or you want absolute power - when you want to be running a rich mixture to use all the available oxygen. Drill tolerance holes are OK for general production work.

 

The gas industry has it's own set of sizes of jet reamer, apparently.

 

Never heard of a running G4, but you never know.  Willie Dow might be your man in thd know.  There are  couple of running sleeve valves though.

 

N