Almost any form of join will alter the electrical characteristics of the cable, specifically, in this case, increase resistance. This will result in a less powerful spark and, especially as the join in the lead ages, an intermittent spark leading to ignition faults.

These cables are in a very hostile environment with heat, abrasion and fuel contamination being commonplace and expected. Joins are weak points and the insulation around them more easily fails Therefore, it’s best to replace a ht lead in its entirety rather than attempt to join it as heat shrink will eventually fail in the environment. It’s a thermoplastic and is designed to change with heat in a similar range to that experienced in the vicinity of the head.
Once the insulation becomes less effective the spark will jump to other areas causing a fault and, critically, a fire risk with any vapour that may be present.

There are some commercial connections designed to work effectively in these environments but they are uncommon and more expensive than a length of cable.