You can still leave both nitrous and fuel in the intake just have the nitrous solenoid farther from the intake to avoid this lean spike.

 

 

The vacuum carb sounds like a loss in signal. That happens with nitrous if the carb is on the bigger side for the engine na. Like a 48 on a 650.Yea you can run them and they work, but if it relies on vacuum it will already suffer a weak signal and nitrous only makes that worse.

 

We had the same problem on a raptor with a large hit. The owner said he asked the mechanic to move everything over to the motor side of the carb on the intake, cured it.

 

If you get the line length right it shouldn't go lean on activation or system shut off. Especially if its a regulated system.