The engine is stock.
It came off a 521 since the original engine in my 510 was blown, so I ended up swapping it in. I did add a Weber 32/36 electric choke carburetor. Maybe that might cause the problem? Since I’m not an expert mechanic or anything, the main problem I would think it could be would be timing. The timing marks are slightly different then the ones that were on my original l16, but I doubt that really makes a difference, right?