Sam,Its supposed to make the combustion process more efficiant,I suppose because its quite flammable,you use less throttle,I dont really know yet.

Just done my first tank of fuel with the larger primary jet,got 39.8 mpg .Although,I did take Jonathans advice and try and drive everywhere on the first choke.When I had the carb off,I wound the return spring for the secondry butterfly around half a turn so that now I can feel when it is just about to open as its harder to push the pedal down.

Forgive me fir bring naive, but, how do you tell first and second choke?
And what difference?
I have the new jet so will fit tonight of I can work out where it goes

We are talking throttle butterflies really James.As you open yhe throttle,the firts butterfly to open is the primary .That will open so far,maybe 3/4 of its full opening,then the secondery one opens.Simples.
It may be easier to take the carb off to change the jet.

Take the hole carb off? I don't have one of those wanky spanners
Could I get away with just taking the lid off?
Had a baffling carb problem on a outboard recently!

Take this bolt out,and the jet is inside here.Trouble is,its sunk deep inside the body of the carb.If you can get onto it great,if you can undo it,even better,if you can keep it on the end of a screwdriver,your a magician

Get youself a 13mm crowsfoot and grind the sides to make it fit,easy peasy

James 2 barrel carb the primary is the front smaller one the secondary is the rear one .

Fuel is drawn in by the fast moving air stream on a big bucket carb at idle the air would hardly be moving and so metering the fuel would be difficult - if you have a small carb then the engine cant get enough air in at top revs , so the twin carb was developed the second carb opening up in the last 1/3 when the main carb starts to sun out of puff.
economical at low engine speeds not restrictive when you give it the beans.

If your re jetting the carb its the jet in the RHS as you peer in the bonnet opening - you need to go in through the "Drain Plug" on that side. and you only need to take the lid off

TOP TIP
BnQ sell silly "Hi torque" spanners there is a 10-12mm one if you heat the bend at the 12mm end you can bend up the spanner so its at 90* to the 12mm ring then take a few mm of the face with an angle grinder and hey ho new carb spanner! you can probably get it cherry red on a domestic gas ring i did mine with a mapp blowtorch

PS13mm crows foot usless as its 12mm af nuts on the newer carbs and they tend to be too thick

Dave,
bicycle pump with its nozzle parked on top of the primary emulsion tube?
Takes one or two strokes to pop the primary jet out into the bowl.

A wooden toothpick can be useful when refitting a jet.

Ken
( That's probably more than enough of the homespun tips for this month. )

Yes Sean,you are of course correct.I meant 12mm ,what a dick.(me not you)
Heres a pic



Fits the carb on my special and D*lly no problem at all.