the brass bit in the photo is for the power jet fitted in the us market was connected to the float bowl with a rubber hose supplied extra fuel at full throttle,was banked off in the float bowl in the uk
The USA, 38mm, "49 state" models, 90-92, came with "zero" sized power jets, too.
I think the Canadian ones came with activated powerjets (but the wrong size).
The power jet circuit is designed to deliver a more atomized fog of fuel and the power jet increments of change were finer for finer tuning.
The power jet circuit, as on the 750's 38s, delivered fuel due to airbox inlet air "vacuum". Without the airbox's restriction, the power jet circuit will not lift the fuel up the side tube. So - if running individual or dual filters, the pj circuit doesn't deliver any fuel.
In 1988, most AMA 750 Supersport teams were using dynojet jet kits. By the end of 91, hardly anybody was. The power jet circuit and smoothly tapered needles were track proven to be better performing.
The proper method of tuning the 38's with an airbox is as follows:
Special fine tuning for these carbs.
Use whatever main that make the bike pull the hardest at 9k-10k. Pay no attention to 12k and 13k.
That's the right main jet.
Adjust the needle for best full throttle 5k-7k power.
That's the right needle height (assuming you did the previous main jet test).
Adjust the float height for the best full throttle / 3k-4k power.
That's the right fuel level (assuming you did the previous main jet and needle testing)
Adjust the fuel screw for best idle and cruise (whatever it happens to be).
If you are 3.5x+ on the fuel screws, you should try a larger pilot jet. If 1.5x or less, you need to try a smaller pilot jet.
Adjust powerjet size for best power at 12k and 13k.
That's how we tuned them and won AMA Nationals.
When it's right for EC997 dyno and real world, and you run them on a dynojet style dyno, it will show an imaginary burble at about 10.25k.
It doesn't exist in the real world.
Nice to see people still practicing carburetor artwork!
Marc Salvisberg
FPT