Motorcycle Repair: XS 850 Rejeting for open exhaust, aftermarket air filter, fuel mileage

Question
Terry, I have a 1981 SX 850 Midnight Special. I am planning on running a open exhaust and know i need to re-jet the carb. I am rebuilding the stock carbs so it would be the best time to re-jet.
My question is I need to know what direction to jet and approximate sizes. I plan on using an aftermarket air filter set up also.
Any help would be appreciated.
Thanks, Tom

Answer
Hi Tom,
Several factors come into play when re-jetting the carbs. If you are only changing the air filter and exhaust slightly. (K&N type filter and open baffles) I would first see what the results are with the stock jets, if you are determined to change them I recommend only one size larger jets. (If it had 100 mains go to 105 mains) The exhaust temperature and spark plug condition will be a good indicator of mixture. If it's too lean it will be weak on top-end performance, also it will knock, ping and overheat easily. The exhaust temperature is the most accurate way to determine mixture accuracy, however; it requires special gauges and sensors to monitor closely. The next size larger jets will provide a margin of safety and is unlikely to make the machine too rich. If it's too rich it will not rev as quickly, soot the plugs and get poor fuel mileage.
Regards,
Terry.