Cold start device
Home Julian Rogers Home Stepper control Cold start details

This device adjusts the idle speed of the Rover V8 in my TR7. It all stems from my wanting to do away with the mess under the dash which includes the choke.

I started off by removing the choke butterfly from the Holly carb. This doesn’t seem to be necessary, at least not in Cornwall’s climate. To start, you press the accelerator twice to squirt some fuel into the engine and then hit the starter. The engine always fires at once from cold but needs to run at a fast idle to avoid stalling while warming up.

Removing the choke plate must help to increase airflow into the carb (and is reversible). I have seen articles about removing the choke tower to further increase breathing efficiency but I haven’t felt brave enough to do that.

Rather than use the choke cable to set the idle speed, I decided to use a stepper motor to do the job. That way I could automate the process and also have a manual over-ride if I wanted. The stepper motor would be controlled by an Arduino which could also measure air and coolant temperature to set the idle speed.

As an upgrade, the Arduino could monitor the idle speed and operate a closed loop system to stabilise the idle speed at a desired value. The Megajolt ignition I have fitted has four outputs which can be set to trigger at any rmp you care to program in. These could work with idle stabilisation. However, I know that closed loop systems can be tricky to set up including the need to prevent overshoot, “hunting” (constantly cycling between too slow and too fast) etc. So I haven’t tried this one yet.

Back to the hardware, I’ve used a Motor Shield with the Arduino to drive the motor and LM35 sensors for the temperature. These may be a bit on the limit and could be damaged if the engine overheats but so far, they have been ok. The stepper motor has an internally threaded spindle and a threaded rod passes through this so that as the motor turns, the rod is moved in and out. This threaded rod (or lead screw) is connected the idle control lever by a connecting rod and a couple ball joints. The lead screw is fitted with a couple of micro-switches which act as limit switches and to establish an initial position when power is first applied.

A pair of lead screw stepper motors

Next, details of the system…

Cold start device

Cold start device