carb adjustment and backfire

[cabz]

In my experience, the most likely causes are a slow jet flowing much less that the other 2, or a partially blocked pilot outlet passageway.

Next is a crack or leak in the intake manifold.

thats what I am leaning torwards as well, I have everything set as well as I can right now . it should be good enough to log some miles on and adjust more as I get a read on the motor during the plug reading phase.

[tz375]

Reading plugs is good for WOT at high loads, but not much use at lower throttle openings.

Try swapping two of the carbs over and see if the problem moves and while they are off, have a really good look at the intake manifolds.