relationship between main jet sizes and their diameters

Hello Commando enthusiasts,
Does anybody know diametrs of main jets to concentric carburettors? My 750 commando is weak on the "top". A lot of settings was done. I thing i need bigger main jets. I have 32 mm ports, 32 mm concentric and only 210 main jets. manual says 230 for 32 mm 750 or 260 for 32mm 850. For me is a little bit expensive to order new main jets only (from england or germany to czech republic) so I woul like to drill mail jets to bigger diameter. Thanks a lot

I think you need 240's or even 260's.

Althouth nobody would ever ever do this, (right hobot?) get a set of number drills. These of course will cost more than a couple of jets. Take the drill that will just go into it and go to the next size up and drill it out, althouth nobody would ever, ever do this, particularly myself, NOT a dozen times or so. The worse that will happen is you will be too rich on top and you have to get another set anyhow.

You could take the jet into the hardware store or drill bit supply store or what ever you have over there, and size it so you only have to by one drill.

There must be someone or a shop around you somewhere that has a drawer full of these. I have not known them to wear out like needles and needle jets.

Yes, I know that it could sound stupid but I have a friend who is an "oldschool" moto mechanist and he has maybe the biggest set of micro drills of the world so why do not test it? Only thing is to know diameters.. I have a few czech Jawas nad CZ's and for them is no problem to buy anything in each village but for Norton is complete different situation in our country. Only from england or germany and the best way with more things. Now I do not need anything else...

The number on your mainjet relates to the flowrate of the jet.
So a main jet 250 can supply 250cc of fuel per minute.
This means that you will have to do your maths to drill out any mainjet to the next size, and i personally would not recomend to do go this route.
Jets can be ordered from many sources , including Burlen fuel systems in the UK.

If you are using petrol, forget about drilling main jets, I wouldn't even do it with methanol fuel, even if I had a good flow meter. It is OK to make your own needle jets for alcohol, you correct with the needles, but with petrol it is not so easy, it doesn't hide up tuning errors the way alcohol does.

acotrel said:

If you are using petrol, forget about drilling main jets, I wouldn't even do it with methanol fuel, even if I had a good flow meter. It is OK to make your own needle jets for alcohol, you correct with the needles, but with petrol it is not so easy, it doesn't hide up tuning errors the way alcohol does.

Click to expand...

He's not asking to guess the size, he's asking if someone with a 230 to measure the diameter. He'll then drill out his jets to that size.

I found this for Amals:
JD75
Flow in cc's Jet Diameter
per minute in inches


100 .034"
110 .035"
120 .037"
130 .038"
140 .040"
150 .041"
160 .043"
170 .044"
180 .045"
200 .048"
220 .050"
240 .052"
260 .055"
280 .057"
300 .059"

Thank You all very much.

Hey Arpi, would it be too expensive if a state side hillbilly just sent ya some bigger jets? Pretty tricky to drill jets accurately [for me] with little drills but could try opening up yours up bit by bit till decent then start taking chances of over rich, then back off a bit or two. I've lucked out on as my Amals and Mukuni carbs came jetted pretty good so don't know much fine tuning yet, just that ya can fudge fuel mix a bit with float level. If they have epoxy in your area fill in the hole if over done and drill again.

Some people have soldered up jets and redrilled them. If you can't afford to buy a couple of jets which have been properly flow rated, you probably shouldn't be buying expensive petrol.

Standard drill tolerances in this size range are on the order of +0.004/-.001.
Probably pretty tough to hit a particular size.
I suspect they are more or less reaming the jets to get any precision on the hole sizes.

Jim H.