Z650 mis-fireing

My bike has a mis-fire, and is popping through the exhaust on throttle down, I have changed the airbox to carb units for new ones that are sealed correctly, so where should I look next?

There's a good chance that your exhaust gaskets are loose or leaking. If air can get into the exhaust system near the exhaust ports it will serve to provide the oxygen need to burn off the rich mixture that ends up in the exhaust pipes during throttle down conditions.

KZCSI

Sure sounds like a good start. Also check to see if you are running rich too.

What year? And does it have the air-suction-system? The air suction system looks like two pipes coming out of the top of the valve cover with hoses going up under the tank. This system can cause the backfires on throttle down if it's not working right. You can just remove it and cap off all of the pipes and hoses.

Does the engine misfire at 1000RPM idle? If it does, try this: adjust the idle as low as possible, but high enough that the engine still runs; spray carb cleaner on the intake manifolds where they seal against the cylinder head and where they seal against the carbs; if the idle drops or the engine dies, the intake manifold to cylinder head seal is leaking and the intake manifolds should be replaced. The rubber on the manifolds dries out after being exposed engine heat for 20 - 30 years and lets raw air in when the throttle is closed. This is not noticable when the throttle is open because air can be drawn in through the carbs thus reducing the effect of the air leaking in. Ed

posted before how my kwaka was mis-fireing well today it was cure day, she was booked in to the local bike hospital, well she is now back and running like a dream, even the mechanic said how nice it was, I had asked him to road test it as it got worse as it warmed up.
first the carbs were stripped and cleaned, water was found in one carb (possible condenseation as she lives outside) and general muck was in all carbs.
the inlet manifolds needed work, and the carbs were balanced.
one of the coils had an intermitent fault that got worse as the bike got hotter, so a new coil was fitted ( I have been advised to change the other) so it will be done.

So she is back at full steam now. I hope it's a nice day tomorrow.
Thanks for all the advice.