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?

A quick update on my Mikuni issue. I ended up having a small set of vacuum leaks around my carb/manifold o-rings. The o-ring comes quite close to the edge of the outside surface of the canon manifold in one spot on every runner. I added a small dab of RTV in that area. I also rechecked the timing and found that it was 5 degrees lower than it should have been! Not sure how it got that way but I fixed it and now she idles fine.

I ran a time trial event last week at the local track and it performed flawlessly.

Thanks for all the help everyone!

So opening up the runners put you on the ragged edge of sealing with the O- rings?

Nice job finding that

The o-ring is close to the outside of the manifold so the porting does not effect things. Here is a photo of the RTV smear in the notch I'm referring to.

Not the neatest job of applying the RTV but at least it got me on the track. I'll tidy it up later.

Todd at Wolfe creek racing pointed me at the solution.

I think the timing was 75% of the solution. Vacuum leak was the other 25%.