I turn mine on at a little higher rpm, but I turn it off at around the same around 5250. It is nearly impossible to not get it to lean or richen at the switch on/off point. It is too large of a change. I favor a rich spike over a lean one. It would be nice to have a smoother mechanism for. I look at the spike and I get a boost spike at the same time. I think this indicates that somewhere in the middle of on and off would be ideal at this rpm.
If you are getting fuel in your intake then I think that the cam timing is pretty off. This could be because of your exhaust cam also or exhaust back pressure.
It has to be that when the intake cam opens there is enough pressure to blow fuel back out of the intake port. I would think that your idle would be rough though and your manifold pressure would be jumpy.