Electrical please help this one is a good one
So I have a 2008 yfz450 that I bought from a friend about two months ago out sat for about 4 years. I cleaned it up carb rebuild, put hmf slip on on and it was running fine, took to pin a small dune trip to Bowie az ran great for two days. The friend that sold it to me showed up and took it for a ride and said it's back firing. So ever since then about three weeks ago it back fires and bogs only when I stand up. If I sit down it runs great but standing immediately causes popping and poor running. I'm at a total loss as to why this is happening. I thought it as the relay above the battery which is under the seat but today I started the bike and pulled the relay bike did the same thing. Messed with the plug goes into the starter switch under the seat between the two fuses same thing. Does anyone know what this could possibly be?