Setting up the throttle and brake linkages is a very important step when building the DNX408.
The following article will guide you, step-by-step, to ensure you have the throttle linkages set correctly. This will allow you to get the most driveability from your DNX408, with the least strain on your servos.
The first thing you need to do is ensure that your engine is installed correctly and correctly meshed. Any movement that you need to make with your engine will require you to re-set the linkage from the carb to the throttle pivot.
With your engine set in place you should aim to set the carb to open as shown in the pictures below:
The throttle swivel should be set so that when your carb is 50% open the throttle swivel is at 90 degrees as shown above (middle picture). Fully closed and fully open carbs should also be easily achieved without the swivel turning too far.
Have your radio equipment turned on and sub-trims set to 0. Fit the servo horn onto your servo as near vertical as possible; once fitted, use your sub-trim to move the throttle servo position to exactly vertical. Servo reverse should be set so that when you apply throttle on your radio the throttle servo will move the servo arm towards the front of the car.
Attach the linkage between the servo horn and the throttle swivel as shown below:
Adjust the length of the linkage so that when the carb is fully closed, the flat side of the collar is tight-up against the pivot on the servo horn:
Adjust the EPA on your radio so that the carb opens fully and no further than that; any more travel will only result in straining your servo and you will not gain any extra performance.
With the throttle side of the linkages set, install the brake linkage. Adjust the length of this linkage so that the threaded end of the linkage near the servo horn isn’t too far away from the servo horn pivot:
It is typically best to have your radio equipment switched on and the throttle in neutral position. Operate the brake directly on the brake arm by hand where the ball joint attaches and set a gap of around 1mm between the servo horn pivot and the brake washers/fueltube.
The length of fuel tube you put on here is often driver dependent; more fuel tube will have a more progressive feel to the brakes. Different brands of fuel tube also make a difference. With that in mind it’s important that you have the thread close enough to the servo horn pivot to tighten the loc-nut up enough to take away the dead-zone in the brakes. This can be done by threading the brake ball-joint more or less. The picture below shows a large dead-zone gap to the brakes being applied.
Set the EPA so that you don’t strain your servo on brakes, the fuel tubing should be crushed slightly.
The last thing to do is to fit your throttle return spring. With your brakes set, leave the throttle position in neutral and check to see if your throttle return spring is long enough for the length of rod exposed at the end. If it isn’t, gently stretch the spring so that in the neutral throttle position, there is some pre-load on the spring helping to keep the carb closed.
The last thing to check is that when the spring is fitted, and full brakes are applied, the throttle return spring doesn’t become coil-bound, causing excessive strain on the servo. If it does, remove the spring, gently stretch it and re-fit the spring with the loc-nut tightened-up less; again, check the pre-load on the spring to ensure that the carb closes when in neutral throttle position.
Below are pictures of the servo arm working in neutral, full throttle and full brake – make sure that the fuel tube is never crushed too much, neither is the throttle return spring coil-bound on full brakes.