[VFR750]

There are many ways in which Zeniths can be mistuned. also many ways in which you can fix them.

Can you provide more details on the jets in the car:
Idle jets,
main jets,
air correction jets.

It is very likely the initial backfires were from air leaks between the manifolds and cylinder heads. That is common.

Now you are running E10 fuel, and the idle and main jets should be increase a little to offset the loss in energy in the fuel. It sounds like you are too lean on the mains.

At this point in time you have to ask yourself: how much do I want to learn about carbs and do I want to learn how to tune them myself?

It can be expensive to pay someone to figure this out. It can also be time consuming to learn about them.

If you want to learn how to tune, then I'd recommend you get some form of exhaust gas measurement system that can tell you the Air-Fuel Ratio. You can buy a system that mounts to the exit of the muffler, or you can get an exhaust with a bung welded on to mount a wide-band 02 sensor.

Either way, without a way of measuring AFR, you will be guessing as to what to change and why.

alfa1750 on E-bay sells all kinds of parts for Zenith TIN carburetors. I've use him for several years now. Should be able to fix just about anything, and get almost any size jet.

Does this help?