If your engine shuts off when you brake, it is usually either your idle screw is set to low, or else you are too rich on the LSN. Do they die immediately when you apply the brake or do they blubber to a stop?

If they die immediately, it is likely that your idle screw is turned too far out. Turn the idle screw in until it holds the carb open enough to keep it running even when the brake is applied. Ideally, you should have your throttle trim and linkages adjusted so that your carb is always resting on the idle stop screw until you apply throttle. You never want to use the throttle trim to adjust your idle speed. All idle speed adjustments should be made with the idle screw while the brake is applied in the model. This ensures that you are resting on the idle screw when you are making the adjustment. This is easy enough to check. Set your throttle trim at center, leave the throttle at neutral, then apply the brake and see if it pushes the carb slide in any: if it does then that is why you are killing the engine when you apply the brake. Adjust your linkages and throttle trim so that your carb is resting on the idle adjust screw and no longer moves when you go from neutral throttle to braking, then adjust the idle stop screw until your engine is idling at the desires speed.

If they stutter and blubber to a stop then you may just be too rich on the LSN. Try leaning out the LSN 1/8th turn at a time until it stops loading up and stalling when you are at idle.