I took my Mikuni 44's apart this weekend to gasket match the cannon intake. The car now backfires quite badly out of both the carbs and the tailpipe while idling, downshifting and cruising. The AFR's are 11-12 while backfiring and the car pulls hard at WOT where the AFR is in the 12's . The plugs seem normal, timing looks normal, float bowl levels look normal and compression is 170+/-5 across all cylinders. The engine is a Rebello L30. The carbs have 39mm chokes, 150 fuels, 180 airs and deleted balance tube on the intake.

Any advice as to how I should diagnose the problem? Could this be caused by a vacuum leak?

Out both carbs?!?! I think you forgot to put one on again after the gasket match. hahahah

Vacuum leak is the place to look for sure. Snug them all down again.

Backfiring out of multiple carb throats. I'll have to double check to see if I left some parts on the bench.

Took them apart ? Or just separated them from the intake?

Gasket match what?

Manifold to head?

Highest vacuum demands are when-

Idling, decelerating , cruise

I took the carbs off the intake, the intake off the head and I took the top off the float chamber. I smoothed out the transition from the carbs to the manifold. I did not do anything with the head side of the manifold.

Rubber o rings still there between the carbs and manifold

Sounds like the intake may not be on tight - it was tough to get my short Mikuni intake onto the head. Might be worth a recheck.

Yes and I replaced the o rings with new.

The previous owner had done a hack job on the carb side of the intake. The head side is still stock dimensions.

This is a photo from before porting the carb side of the intake...

90% done...

I didn't take a photo of the finished product but it made for a nice smooth transition from the carb to the plastic spacer to the manifold if you look down the throat of the carb.

Top off the float chamber?

I opened up the float chamber to adjust the floats. I used new gaskets when I reassembled them.