I don't have that guard and I don't have that issue. Have you done a compression check? Have you checked the carbs out? If the CV boots have a leak it will run like crap at high load as one cylinder won't get enough charge. Could be the plugs too. I doubt it's jetting cause they come pretty decent from the factory. Too rich or too lean on the main could cause this problem, as could needle position. You should be able to see something on the plugs after just a short 30 min ride.