It was NOT the synthetic oil
 

Once upon a time, I had a Honda CB350. That bike averaged 60 MPG in a mixture of city and highway driving. I didn't specifically try for mileage, but did not drive it hard either.

I wanted to find if Mobil 1 synthetic would get better gas mileage. So I started with a pair of baseline MPG tests:

30 MPH 87 MPG
60 MPH 60 MPG

Changed to Mobil 1 and reran the MPG tests:

30 MPH 103 MPG
60 MPH 63 MPG

These results did not make sense until I realized that I was getting better at holding a constant throttle position. This was especially tricky at 30 MPH, because the throttle was barely off idle.

This could be called an A-BC test. It shows the need for careful testing of changes (see the How To Properly Test sticky in Ecomodding Central).

CONCLUSIONS

1) The Honda CB350 responds very well to a steady hand on the throttle.

2) I still have no idea if the Mobil 1 oil caused better gas mileage in that bike.

I too had a CB350 for a while but never saw 60mpg, I tended to average 45-50mpg, the thing with synthetic oil is that alot of it will ruin the wet clutch of a motorcycle if it has friction modifiers in it, so just a word of warning, I've replaced two wet clutches for this reason.
But you are right, the CB350 has constant velocity carburetors, so a slight twitch of the throttle would make it open up the diaphragm and thus open up the needle for the main jet in the carburetor, dumping in extra fuel.