Motorcycle Repair: 1200 sportster, open pipes, diaphram

Question
QUESTION: hello ,i have a 2000 1200 sportster r runs well upto 60mph with no issues,but then starts to hesitate runs like it is 8 stroking for about 20 seconds or so then starts to clear and start to accelerate up past 70 and beyond,have stripped and checked the carb,jets r all clear, there is no misfire under load you can labour the engine at low speeds without any problems,  believe the carb has been dyno jetted, has open pipes..bike has had the problem since ive owned it,any ideas greatly appreciated,many thanks

ANSWER: What carb are you running?
Mike

---------- FOLLOW-UP ----------

QUESTION: hi mike, its the standard carb, the diaphram is ok, wondering if its the needle at fault

ANSWER: Do you have a stock electronic ignition? Pull your spark plugs and let me know what color they are.
Mike

---------- FOLLOW-UP ----------

QUESTION: its standard ignition, plugs are black,so its on the rich side

Answer
I would start by OHMs testing the coil and plug wires to make sure they are good. If they dont pass then you need to replace them. they can cause the problem you are having. If they are ok, then your carb is going to have to be rejetted
Good luck and happy riding
Mike