My 81 CSR 650 has an intermittent misfire that occurs somewhat randomly when under load on one of the far right (3 or 4) cylinders at between 5-6k rpm. The misfire is really only detectable by a slight lurching/loss of power and audible exhaust difference in the right pipe. The misfire occurs more often when doing a hard pull for acceleration, but also sometimes happens when cruising at highway speeds the same 5-6K rpm range. Once the rpm drops below 5ish or above 6ish, the misfire does not occur.
The only modification on this bike is a vintage Jardine 4-2 race exhaust (open pipes) that I installed a simple dB killer style baffle bolted inside to reduce the drone. This random misfire predates the exhaust modification and carb rejet. Airbox is modified to accept 78 air ducts, but otherwise stock, and leak free. Carbs are stock with the accelerator pump enabled. Can't remember the exact numbers, but I rejetted the pilots to 17.5 (from 15), and the mains to something around 100 (from 92.5 stock). I have slides installed from a 78 carb set, and all slide needles raised one level below max height. I have rebuilt the carbs and synched completely, and use an in-line fuel filter
I've checked float height, the advancer mechanism (and lubed it), timing, new coils wires and plugs, and had the valves checked approximately 4,000 miles ago. This issue has been going on for a while, and occurred for a bit even before the exhaust was installed, so I suspect that this is isn't a fuel issue as everything was stock before that. Perhaps the IC igniter as those have given me much trouble over the years? The charging system is otherwise good with a 2 year old stator, and good voltage regulator. Also to note that the engine runs fine even at the 5-6k rpm range when idling, and all four cylinders have a good fat blue spark idling. Wondering what everyone's thoughts are on the matter.
