Carbs popping back and slow return from 3000rpm

HI all,
I know there has been someone with a similar problem, but let's see if anyone has ideas,
I've just rebuilt the engine, 0.5 oversize rebore with new pistons etc, also valves recut. The engine eventually started but straight away it was popping back through the carbs at lower revs and also slow to return from 3000rpm. What could have changed with the carbs as before the strip down 2 weeks ago the setup was fine, ok maybe just a little popping back when it was stone cold but that would stop after 2 mins.
I haven't touched the carbs at all while rebuilding the engine, but correct me if I'm wrong but it sounds like a weak mixture
Any help would be great,

Cheers Pete.

Has the exhaust been tightened sufficiently down ? Easy way to suck air in and cause a backfire ?

Popping back thru the carbs sounds like the timing is a bit out, I would check that first.

Yep I would check the timing too. First check the compression in the cylinders as that is a good indication that the timing may be out.

I checked the timing several times so should be ok. I noticed when I went out for a test ride the popping back in the carbs stopped but around 2000rpm a real lack of power and had to stop every 5 mins to let the engine cool down (not good after an engine rebuild )
So it would seem I must have a serious problem with the mixture being weak. I'm left scratching my head on this one though.
I purposely didn't touch the carbs as they were set up ok before the rebuild.

Pull the spark plugs and see what colour they are. If they are way too light then you will have to adjust your carbs.