It all leads to one thing, lean fuelling. It was all Dynojet kits when I was growing up. New pipes, or just a slip-on can? Needs up-jetting. And, although expensive when compared to buying a few jets, you knew your bike was going to run pretty well for the setup you had.
They did the testing for you to save you constantly removing a bank of carbs, swapping jets and then road testing it. That was enough of a selling point, and well worth the money, for most people.
Back to the point.
A little bit of searching online and I found 6Sigma racing jet kits for sale on eBay. Reasonable money, good feedback and they seemed to have a fair idea on what they were doing. Gotta be worth a shot so I put in a best offer, they accepted and the rest, as they say, is history.
Instructions are clear with good pics and diagrams of your particular carbs/bike. Shims are supplied for the needles, but as luck would have it, mine are adjustable. The kit came supplied with the drills (and screw) to remove blanking plugs, and to drill the slides (neither of which I actually needed), and a choice of main jets.
The middle cylinders take a slightly bigger jet to reduce combustion temperature, which is what we used to do on the old T150 Triumph. With an air-cooled engine, the outer two cylinders will always run cooler so if you can richen the centre cylinders slightly it makes for a better running engine.
The original 120 mains.
Underneath is the slide with the diaphragm. Just inside you can see the plastic retainer held in by the circlip. You'll need a long-nosed circlip pliers to remove it and then the retainer and needle can be removed easily. Note the diaphragm/slide assembly will only fit in one position due to the extra tab seen in the ten o'clock position. It might be slightly different on other models/makes of carb, but there's usually some form of location somewhere.
Lastly, don't use carb clean/solvents on the diaphragms. They are very fragile and will get ruined. On these carbs they come complete with slides and are very expensive!
Retainer, circlip and needle removed.
Now you can see the five grooves. Currently in the middle position, we're going to lower the circlip one notch to effectively raise the needle. This allows more fuel to mix with the air and create a richer mixture.
And moved one notch down.
Some of the new jets. The instructions will tell you which ones to use depending on air filters, exhaust and height above sea level you'll be riding.
Probably should have had more pics of the rebuild here! Anyway, the brass housing held with the steel clamp and screw is where the fuel enters the carb. This has had a new O-ring fitted. All fuel and airways have been cleaned with carb clean and blown out so hopefully they're good to go. Poor running will soon let me know if I've missed something.
Emulsion tubes go in from the top side of the carb and are actually held down in place by the main jet. The pilot jet is now fitted and covered by the rubber plug seen next to the main jet. The float and needle are also fitted. Always support the tiny brackets that hold the float pin when removing/refitting. They snap off easily (as someone managed to do to my GS550 carbs)!
Getting ahead of myself again, here's the finished carbs. Not concourse, highly-polished examples but clean enough, and looking good. The screws for the brackets, float bowls and carb tops have all been wire brushed and sprayed with silver (Galv) spray to spruce them up a little. A quick way of making them look reasonable again.
And the before pic, nice improvement.
Thanks 6Sigma, just a shame it'll be a while before I get to test them!