I would have to say that I am sure that the front bearing is not a issue with leaking. As long as the seal is in place or the shield is not bad you will have no issues with a leaking front bearing ever causing a tune issue. The front bearing can be bad and have a problem with tuning but speaking about the leaking. Think about all the .21 engines that are run on road. They use shields not seals. You can see daylight through the bearing if you look up. The seal from the front is caused by the tight tolerance from the crankshaft in the case as well as the rotation of the bearings and inner retainer. It is moving so fast that there are no leaks. I have seen a bad rear bearing cause needle problems though.And also check the carb for air leaks. This is a cause as well.