Yeah if the piston is 7 or 8 grams heavier than normal that's enough to throw one of these way off balance... you can also drill holes in the flywheels on the piston side of the crank and smooth it out that way. This can be done using trial and error by stuffing paper towels or rags and coating the tops of the flywheels with grease to keep any shavings from falling down into the case, or you can stick a few rare earth magnets next to where you plan on drilling to collect the shavings.
To do the actual drilling, start with a 5/16 " bit and go about 1/8 " deep and make one drill start on each flywheel right at the top where the crank pin is. Then make another drill start on each side of the ones you made at the center. Keep the holes about the same depth. Once you got your 6 drill starts done and between 1/16 and 1/8"deep, clean away all the shavings then remove the paper towels or rags from the case making sure no shavings drop into the case (this is also where strong magnets help). Now install the studs and reassemble the engine and take it for a test ride. The engine should be smoother. If it still has a lot of vibration you can repeat this process by going 1/16 " deeper into the w center holes, try again and check for improvement. If it's good then you're done, but if not, then go deeper into the 4 holes that are on either side of the center holes. Depending on how much improvement there was originally, you can judge how much more needs to be drilled and / or you can also open the holes up to 3/8" if needed so make sure your holes are far enough apart that you can open them up a size or 2 if needed. You also want the holes to be in the middle of each flywheel so there's enough room to open the holes up to 1/2 " if needed.
I don't recommend this method for beginners and it's extremely important that no shavings fall into the case and that you don't accidentally nick the piston rod so don't try this unless you're confident you can drill the holes into the right part of the flywheels and you can keep the shavings from falling into the case. If shavings fall into the case, the only way to be sure you got all of them out is to split the case and remove the crank. Just one shaving can put a nasty scratch on the cylinder or piston or get trapped in a bearing run in the engine on short order.
If you can keep the shavings out and judge how much needs to be drilled out and when to stop, this is a fairly easy and effective way to balance the crank. It can even be done with the engine installed in the frame if there's enough room in the frame to do it.
The other option would be to buy a Dax crank that's already balanced and replace your old crank with it along with some better quality bearings. The Dax cranks are balanced with the same drill start method and you can see the drill starts with the crank at tdc in the case, I can take a few pics and post them when I go back to the shop in the morning to show what I'm talking about and give an idea of the hole size and depth.
