Classic Twin Ignition – Avoiding a Common Installation Issue

At Tri-Spark, we’re always looking to support our customers with practical advice to ensure a smooth installation and reliable performance.

One issue we occasionally see is a Classic Twin ignition returned with a damaged output transistor. In almost every case, this has been caused by incorrect wiring on a negative earth motorcycle.

Why This Happens

The Classic Twin stator is supplied wired for positive earth, as this is the most common configuration on classic British motorcycles from brands like Triumph Motorcycles, Norton Motorcycles and BSA.

However, many bikes have been converted to negative earth over time.

If the ignition module is connected without adjusting for this change in polarity, it can result in damage to the output transistor.

Model-Specific Notes

• For Vincent Motorcycles, we supply a negative earth stator as standard.
• If you have a Velocette motorcycle, please let us know when ordering, as requirements can vary.

How to Avoid the Problem

The good news is that this issue is very easy to prevent with a few simple checks during installation:

• Confirm your bike’s earth polarity before starting
• If the bike is negative earth – swap over the stator terminals (unless a negative earth stator is already supplied)
• Follow the supplied wiring diagram carefully
• Double-check polarity before connecting power

Taking a moment to verify these steps can save time and prevent unnecessary damage.

Reliable Performance When Installed Correctly

When wired correctly, the Classic Twin ignition is extremely reliable and remains one of the most popular upgrades for classic twins and singles.

It delivers improved starting, smoother running, and dependable performance—helping you get the best from your motorcycle.