I'm assuming that you have checked all the tune-up specifications to make sure the engine is in otherwise proper tune. The premix ratio is in the ballpark and I know of other riders using BelRay MC-1 with no problems. Gas-Gas recommends 95 octane in their bikes and pump gas is in the 91 to 93 octane range. A 80/20 mix of premium pump and 115 unleaded race gas will give you the best performance. The sparkplug should be relatively dry and a milk chocolate color, sometimes a darker gray with certain types of race gas. Gas-Gas recommends NGK BP5RES, RN11YC Champion and Bosch WR9DP plugs. I haven't used Honda filter oil but I would imagine it to be a good product. Twin Air filter oil works well in the Gas-Gas filters as the stock filter is made by Twin Air. Be sure to blot the filter well with paper towels before installation. Excess filter oil can foul plugs and filter oils vary in quality.

You didn't relate your jetting spec.'s or how the engine is responding and I don't know what the ambient conditions are in the riding area you use.

According to the spec sheet I have it should be in the range of 108-main and 33/38-pilot at sea level but conditions vary widely and that should only be a start. You should contact your Gas-Gas dealer to get the correct information and they can supply you with the necessary jets if need be. I would also check your float level in the carb just to be sure. Jim Snell has an excellent article on the Gas-Gas webpage that will answer all your questions. The weeping around the pipe gasket is not unusual per se, probably just meaning that the gasket should be replaced.

These are only some initial suggestions. It's almost impossible to diagnose engine problems by e-mail. It could be that the transmission side crank seal is leaking. It's always best to consult your local Gas-Gas dealer as they have the proper tools, techniques and training to properly diagnose bike problems.

Jon