Technical Info – www.CDIBOX.com

Did you check the plugs to see if it is burning too lean with the new CDI? The FIREPLUG CDI has more spark energy than the stock CDI, especially when the 40 year old CDI has got weaker over the years. Jetting is critical since more energy will light things up better & faster. Jetting from the weaker spark original CDI may not be rich enough for the FIREPLUG increased spark energy. Check your jetting!

Are you sure your timing is correct?
The hotter spark from the FIREPLUG CDI will appear to advance the spark timing from the increased spark energy. Typically stock timing marks are correct & should be used as a starting point. With increased spark energy, the timing will appear advanced to the engine. Engines tend to run flat at lower peak RPM with advanced timing. Breaking up at the higher RPM can also be due to advanced timing. Retarding the timing may increase the upper RPM if timing was too advanced initially. Even if you have your timing set to “OEM” marks, the fast guys getting peak performance know that slight adjustments either way can make the difference in a decent running engine and a peak performer. Set your timing where your engine likes it the best!

Is your Engine & Chassis grounded & bonded together?
The engine needs to have a ground wire connecting it to the chassis. Any wire will do typically if it’s connected to a recoil bolt and somewhere on the chassis…preferably where the CDI is bolted down if chassis mounted. This simple bonding wire will eliminate a lot of issues with ignition and lighting since it reduces the chance the ground will be routed through the CDI and cause failure.

Kill-switch / Key-switch or Tether: Is the kill / key-switch or tether causing issues?
The higher voltage from the FIREPLUG CDI also exists on the kill-switch contacts. A switch that is old, or a tether that isn’t being held down as far as it could can cause arcs at upper RPM when the voltage is the highest values, causing a spark-mis. With sled running in dark area, open the hood & look for any sparks or arcs from coils, plug-caps, plug-wires or any other wiring jumping to engine or chassis. Could be as simple as adding a Ground wire from a recoil-bolt to the chassis or you may need to replace a coil, plug caps or spark-plug wire if the insulation has broken down from age or weathering.

Spark-plug caps? Are you running the NGK spark plug caps with the 5K-ohm internal resistors? They use an internal slug of carbon as a resistor to reduce the spark energy and TVI interference Higher energy can cause them to break down & eventually fail. Replacing them with straight-thru copper caps are best for racing or just try newer versions of the NGK caps. Cut the spark-plug wires back ~ 1/4″ on the screw-in cap end to get to the newer copper wire. Removing / replacing caps can push the copper wire back up inside the wire insulation. Cutting the plug-wire insulation back to connect to the good wire can make a big difference.

Spark-plug wires? Automotive spark-plug wires are carbon-core, not copper-wire core.
Automotive spark-plug do not work on small engines. They are carbon-core. Carbon core has significantly high ohms/ft values for small-engine ignition systems and will not work properly. The correct spark-plug wires are the copper-core centers used originally on snowmobiles, lawn-mowers and many other small-engines. You can get them from most small-engine dealers or snowmobile parts catalogs. .

Are you running BR-resistor plugs with the NGK resistor caps?
Nothing wrong with them, but you may be losing some performance. Resistor caps + resistor plugs will reduce the overall spark energy. Try B9ES non-resistor plugs gapped @ 0.020″ gaps. Best luck overall with high-compression / modified engines & race-gas if you close the gaps a bit if breaking up on high-rpm.

One dead cylinder:
Swap coils, plugs & carbs (if twin carbs). See if problem stays with the same side or swaps over following the coils, plugs or carb. Check timing on both cylinders individually. Improper timing can result in late or no firing on cylinder. Check the trigger-coil resistance and compare it with the other to determine if similar values. Replace the spark-plug with new plug. Plugs can appear OK but still will not spark.

FIREPLUG CDI DIAGRAM

HARLEY FIREPLUG CDI WIRING-REV-A